1.135.0

Linkify GitHub PR ID in changelog
1.135.0rc2
2025-12-17 02:10:27 +00:00 · 2025-08-01 13:12:33 +01:00 · 2025-07-30 12:57:20 +01:00 · 2025-07-30 12:19:34 +01:00 · 2025-07-30 10:43:39 +00:00 · 2025-07-24 08:43:58 +00:00
88 changed files with 4999 additions and 170 deletions
--- a/.github/workflows/docker.yml
+++ b/.github/workflows/docker.yml
@@ -120,7 +120,7 @@ jobs:
        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 # v3.11.1

      - name: Install Cosign
-        uses: sigstore/cosign-installer@398d4b0eeef1380460a10c8013a76f728fb906ac # v3.9.1
+        uses: sigstore/cosign-installer@d58896d6a1865668819e1d91763c7751a165e159 # v3.9.2

      - name: Calculate docker image tag
        uses: docker/metadata-action@902fa8ec7d6ecbf8d84d538b9b233a880e428804 # v5.7.0
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,87 @@
+# Synapse 1.135.0 (2025-08-01)
+
+No significant changes since 1.135.0rc2.
+
+
+
+
+# Synapse 1.135.0rc2 (2025-07-30)
+
+### Bugfixes
+
+- Fix user failing to deactivate with MAS when `/_synapse/mas` is handled by a worker. ([\#18716](https://github.com/element-hq/synapse/issues/18716))
+
+### Internal Changes
+
+- Fix performance regression introduced in [#18238](https://github.com/element-hq/synapse/issues/18238) by adding a cache to `is_server_admin`. ([\#18747](https://github.com/element-hq/synapse/issues/18747))
+
+
+
+
+# Synapse 1.135.0rc1 (2025-07-22)
+
+### Features
+
+- Add `recaptcha_private_key_path` and `recaptcha_public_key_path` config option. ([\#17984](https://github.com/element-hq/synapse/issues/17984), [\#18684](https://github.com/element-hq/synapse/issues/18684))
+- Add plain-text handling for rich-text topics as per [MSC3765](https://github.com/matrix-org/matrix-spec-proposals/pull/3765). ([\#18195](https://github.com/element-hq/synapse/issues/18195))
+- If enabled by the user, server admins will see [soft failed](https://spec.matrix.org/v1.13/server-server-api/#soft-failure) events over the Client-Server API. ([\#18238](https://github.com/element-hq/synapse/issues/18238))
+- Add experimental support for [MSC4277: Harmonizing the reporting endpoints](https://github.com/matrix-org/matrix-spec-proposals/pull/4277). ([\#18263](https://github.com/element-hq/synapse/issues/18263))
+- Add ability to limit amount of media uploaded by a user in a given time period. ([\#18527](https://github.com/element-hq/synapse/issues/18527))
+- Enable workers to write directly to the device lists stream and handle device list updates, reducing load on the main process. ([\#18581](https://github.com/element-hq/synapse/issues/18581))
+- Support arbitrary profile fields. Contributed by @clokep. ([\#18635](https://github.com/element-hq/synapse/issues/18635))
+- Advertise support for Matrix v1.12. ([\#18647](https://github.com/element-hq/synapse/issues/18647))
+- Add an option to issue redactions as an admin user via the [admin redaction endpoint](https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#redact-all-the-events-of-a-user). ([\#18671](https://github.com/element-hq/synapse/issues/18671))
+- Add experimental and incomplete support for [MSC4306: Thread Subscriptions](https://github.com/matrix-org/matrix-spec-proposals/blob/rei/msc_thread_subscriptions/proposals/4306-thread-subscriptions.md). ([\#18674](https://github.com/element-hq/synapse/issues/18674))
+- Include `event_id` when getting state with `?format=event`. Contributed by @tulir @ Beeper. ([\#18675](https://github.com/element-hq/synapse/issues/18675))
+
+### Bugfixes
+
+- Fix CPU and database spinning when retrying sending events to servers whilst at the same time purging those events. ([\#18499](https://github.com/element-hq/synapse/issues/18499))
+- Don't allow creation of tags with names longer than 255 bytes, [as per the spec](https://spec.matrix.org/v1.15/client-server-api/#events-14). ([\#18660](https://github.com/element-hq/synapse/issues/18660))
+- Fix `sliding_sync_connections`-related errors when porting from SQLite to Postgres. ([\#18677](https://github.com/element-hq/synapse/issues/18677))
+- Fix the MAS integration not working when Synapse is started with `--daemonize` or using `synctl`. ([\#18691](https://github.com/element-hq/synapse/issues/18691))
+
+### Improved Documentation
+
+- Document that some config options for the user directory are in violation of the Matrix spec. ([\#18548](https://github.com/element-hq/synapse/issues/18548))
+- Update `rc_delayed_event_mgmt` docs to the actual nesting level. Contributed by @HarHarLinks. ([\#18692](https://github.com/element-hq/synapse/issues/18692))
+
+### Internal Changes
+
+- Add a dedicated internal API for Matrix Authentication Service to Synapse communication. ([\#18520](https://github.com/element-hq/synapse/issues/18520))
+- Allow user registrations to be done on workers. ([\#18552](https://github.com/element-hq/synapse/issues/18552))
+- Remove unnecessary HTTP replication calls. ([\#18564](https://github.com/element-hq/synapse/issues/18564))
+- Refactor `Measure` block metrics to be homeserver-scoped. ([\#18601](https://github.com/element-hq/synapse/issues/18601))
+- Refactor cache metrics to be homeserver-scoped. ([\#18604](https://github.com/element-hq/synapse/issues/18604))
+- Unbreak "Latest dependencies" workflow by using the `--without dev` poetry option instead of removed `--no-dev`. ([\#18617](https://github.com/element-hq/synapse/issues/18617))
+- Update URL Preview code to work with `lxml` 6.0.0+. ([\#18622](https://github.com/element-hq/synapse/issues/18622))
+- Use `markdown-it-py` instead of `commonmark` in the release script. ([\#18637](https://github.com/element-hq/synapse/issues/18637))
+- Fix typing errors with upgraded mypy version. ([\#18653](https://github.com/element-hq/synapse/issues/18653))
+- Add doc comment explaining that config files are shallowly merged. ([\#18664](https://github.com/element-hq/synapse/issues/18664))
+- Minor speed up of insertion into `stream_positions` table. ([\#18672](https://github.com/element-hq/synapse/issues/18672))
+- Remove unused `allow_no_prev_events` option when creating an event. ([\#18676](https://github.com/element-hq/synapse/issues/18676))
+- Clean up `MetricsResource` and Prometheus hacks. ([\#18687](https://github.com/element-hq/synapse/issues/18687))
+- Fix dirty `Cargo.lock` changes appearing after install (`base64`). ([\#18689](https://github.com/element-hq/synapse/issues/18689))
+- Prevent dirty `Cargo.lock` changes from install. ([\#18693](https://github.com/element-hq/synapse/issues/18693))
+- Correct spelling of 'Admin token used' log line. ([\#18697](https://github.com/element-hq/synapse/issues/18697))
+- Reduce log spam when client stops downloading media while it is being streamed to them. ([\#18699](https://github.com/element-hq/synapse/issues/18699))
+
+
+
+### Updates to locked dependencies
+
+* Bump authlib from 1.6.0 to 1.6.1. ([\#18704](https://github.com/element-hq/synapse/issues/18704))
+* Bump base64 from 0.21.7 to 0.22.1. ([\#18666](https://github.com/element-hq/synapse/issues/18666))
+* Bump jsonschema from 4.24.0 to 4.25.0. ([\#18707](https://github.com/element-hq/synapse/issues/18707))
+* Bump lxml from 5.4.0 to 6.0.0. ([\#18631](https://github.com/element-hq/synapse/issues/18631))
+* Bump mypy from 1.13.0 to 1.16.1. ([\#18653](https://github.com/element-hq/synapse/issues/18653))
+* Bump once_cell from 1.19.0 to 1.21.3. ([\#18710](https://github.com/element-hq/synapse/issues/18710))
+* Bump phonenumbers from 9.0.8 to 9.0.9. ([\#18681](https://github.com/element-hq/synapse/issues/18681))
+* Bump ruff from 0.12.2 to 0.12.5. ([\#18683](https://github.com/element-hq/synapse/issues/18683), [\#18705](https://github.com/element-hq/synapse/issues/18705))
+* Bump serde_json from 1.0.140 to 1.0.141. ([\#18709](https://github.com/element-hq/synapse/issues/18709))
+* Bump sigstore/cosign-installer from 3.9.1 to 3.9.2. ([\#18708](https://github.com/element-hq/synapse/issues/18708))
+* Bump types-jsonschema from 4.24.0.20250528 to 4.24.0.20250708. ([\#18682](https://github.com/element-hq/synapse/issues/18682))
+
 # Synapse 1.134.0 (2025-07-15)

 No significant changes since 1.134.0rc1.
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -887,9 +887,9 @@ dependencies = [

 [[package]]
 name = "once_cell"
-version = "1.19.0"
+version = "1.21.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3fdb12b2476b595f9358c5161aa467c2438859caa136dec86c26fdd2efe17b92"
+checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"

 [[package]]
 name = "openssl-probe"
@@ -1355,9 +1355,9 @@ dependencies = [

 [[package]]
 name = "serde_json"
-version = "1.0.140"
+version = "1.0.141"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "20068b6e96dc6c9bd23e01df8827e6c7e1f2fddd43c21810382803c136b99373"
+checksum = "30b9eff21ebe718216c6ec64e1d9ac57087aad11efc64e32002bce4a0d4c03d3"
 dependencies = [
 "itoa",
 "memchr",
--- a/changelog.d/17984.feature
+++ b/changelog.d/17984.feature
@@ -1 +0,0 @@
-Add `recaptcha_private_key_path` and `recaptcha_public_key_path` config option.
--- a/changelog.d/18195.feature
+++ b/changelog.d/18195.feature
@@ -1 +0,0 @@
-Add plain-text handling for rich-text topics as per [MSC3765](https://github.com/matrix-org/matrix-spec-proposals/pull/3765).
--- a/changelog.d/18238.feature
+++ b/changelog.d/18238.feature
@@ -1 +0,0 @@
-If enabled by the user, server admins will see [soft failed](https://spec.matrix.org/v1.13/server-server-api/#soft-failure) events over the Client-Server API.
--- a/changelog.d/18263.feature
+++ b/changelog.d/18263.feature
@@ -1 +0,0 @@
-Add experimental support for [MSC4277](https://github.com/matrix-org/matrix-spec-proposals/pull/4277).
--- a/changelog.d/18499.bugfix
+++ b/changelog.d/18499.bugfix
@@ -1 +0,0 @@
-Fix CPU and database spinning when retrying sending events to servers whilst at the same time purging those events.
--- a/changelog.d/18527.feature
+++ b/changelog.d/18527.feature
@@ -1 +0,0 @@
-Add ability to limit amount uploaded by a user in a given time period.
--- a/changelog.d/18548.doc
+++ b/changelog.d/18548.doc
@@ -1 +0,0 @@
-Document that some config options for the user directory are in violation of the Matrix spec.
--- a/changelog.d/18552.misc
+++ b/changelog.d/18552.misc
@@ -1 +0,0 @@
-Allow user registrations to be done on workers.
--- a/changelog.d/18564.misc
+++ b/changelog.d/18564.misc
@@ -1 +0,0 @@
-Remove unnecessary HTTP replication calls.
--- a/changelog.d/18581.feature
+++ b/changelog.d/18581.feature
@@ -1 +0,0 @@
-Enable workers to write directly to the device lists stream and handle device list updates, reducing load on the main process.
--- a/changelog.d/18601.misc
+++ b/changelog.d/18601.misc
@@ -1 +0,0 @@
-Refactor `Measure` block metrics to be homeserver-scoped.
--- a/changelog.d/18604.misc
+++ b/changelog.d/18604.misc
@@ -1 +0,0 @@
-Refactor cache metrics to be homeserver-scoped.
--- a/changelog.d/18617.misc
+++ b/changelog.d/18617.misc
@@ -1 +0,0 @@
-Unbreak "Latest dependencies" workflow by using the `--without dev` poetry option instead of removed `--no-dev`.
--- a/changelog.d/18622.misc
+++ b/changelog.d/18622.misc
@@ -1 +0,0 @@
-Update URL Preview code to work with `lxml` 6.0.0+.
--- a/changelog.d/18635.feature
+++ b/changelog.d/18635.feature
@@ -1 +0,0 @@
-Support arbitrary profile fields.
--- a/changelog.d/18637.misc
+++ b/changelog.d/18637.misc
@@ -1 +0,0 @@
-Use `markdown-it-py` instead of `commonmark` in the release script.
--- a/changelog.d/18647.feature
+++ b/changelog.d/18647.feature
@@ -1 +0,0 @@
-Advertise support for Matrix v1.12.
--- a/changelog.d/18653.misc
+++ b/changelog.d/18653.misc
@@ -1 +0,0 @@
-Fix typing errors with upgraded mypy version.
--- a/changelog.d/18660.bugfix
+++ b/changelog.d/18660.bugfix
@@ -1 +0,0 @@
-Don't allow creation of tags with names longer than 255 bytes, as per the spec.
--- a/changelog.d/18664.misc
+++ b/changelog.d/18664.misc
@@ -1 +0,0 @@
-Add doc comment explaining that config files are shallowly merged.
--- a/changelog.d/18672.misc
+++ b/changelog.d/18672.misc
@@ -1 +0,0 @@
-Minor speed up of insertion into `stream_positions` table.
--- a/changelog.d/18675.feature
+++ b/changelog.d/18675.feature
@@ -1 +0,0 @@
-Include `event_id` when getting state with `?format=event`. Contributed by @tulir @ Beeper.
--- a/changelog.d/18676.misc
+++ b/changelog.d/18676.misc
@@ -1 +0,0 @@
-Remove unused `allow_no_prev_events` option when creating an event.
--- a/changelog.d/18677.bugfix
+++ b/changelog.d/18677.bugfix
@@ -1 +0,0 @@
-Fix sliding_sync_connections related errors when porting from SQLite to Postgres.
--- a/changelog.d/18684.feature
+++ b/changelog.d/18684.feature
@@ -1 +0,0 @@
-Add `recaptcha_private_key_path` and `recaptcha_public_key_path` config option.
--- a/changelog.d/18687.misc
+++ b/changelog.d/18687.misc
@@ -1 +0,0 @@
-Clean up `MetricsResource` and Prometheus hacks.
--- a/changelog.d/18689.misc
+++ b/changelog.d/18689.misc
@@ -1 +0,0 @@
-Fix dirty `Cargo.lock` changes appearing after install (`base64`).
--- a/changelog.d/18691.bugfix
+++ b/changelog.d/18691.bugfix
@@ -1 +0,0 @@
-Fix the MAS integration not working when Synapse is started with `--daemonize` or using `synctl`.
--- a/changelog.d/18692.doc
+++ b/changelog.d/18692.doc
@@ -1 +0,0 @@
-Update `rc_delayed_event_mgmt` docs to the actual nesting level. Contributed by @HarHarLinks.
--- a/changelog.d/18693.misc
+++ b/changelog.d/18693.misc
@@ -1 +0,0 @@
-Prevent dirty `Cargo.lock` changes from install.
--- a/changelog.d/18697.misc
+++ b/changelog.d/18697.misc
@@ -1 +0,0 @@
-Correct spelling of 'Admin token used' log line.
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,21 @@
+matrix-synapse-py3 (1.135.0) stable; urgency=medium
+
+  * New Synapse release 1.135.0.
+
+ -- Synapse Packaging team <packages@matrix.org>  Fri, 01 Aug 2025 13:12:28 +0100
+
+matrix-synapse-py3 (1.135.0~rc2) stable; urgency=medium
+
+  * New Synapse release 1.135.0rc2.
+
+ -- Synapse Packaging team <packages@matrix.org>  Wed, 30 Jul 2025 12:19:14 +0100
+
+matrix-synapse-py3 (1.135.0~rc1) stable; urgency=medium
+
+  * New Synapse release 1.135.0rc1.
+
+ -- Synapse Packaging team <packages@matrix.org>  Tue, 22 Jul 2025 12:08:37 +0100
+
 matrix-synapse-py3 (1.134.0) stable; urgency=medium

  * New Synapse release 1.134.0.
--- a/docker/configure_workers_and_start.py
+++ b/docker/configure_workers_and_start.py
@@ -178,6 +178,7 @@ WORKERS_CONFIG: Dict[str, Dict[str, Any]] = {
            "^/_matrix/client/(api/v1|r0|v3|unstable)/login$",
            "^/_matrix/client/(api/v1|r0|v3|unstable)/account/3pid$",
            "^/_matrix/client/(api/v1|r0|v3|unstable)/account/whoami$",
+            "^/_matrix/client/(api/v1|r0|v3|unstable)/account/deactivate$",
            "^/_matrix/client/(api/v1|r0|v3|unstable)/devices(/|$)",
            "^/_matrix/client/(r0|v3)/delete_devices$",
            "^/_matrix/client/versions$",
@@ -327,6 +328,15 @@ WORKERS_CONFIG: Dict[str, Dict[str, Any]] = {
        "shared_extra_conf": {},
        "worker_extra_conf": "",
    },
+    "thread_subscriptions": {
+        "app": "synapse.app.generic_worker",
+        "listener_resources": ["client", "replication"],
+        "endpoint_patterns": [
+            "^/_matrix/client/unstable/io.element.msc4306/.*",
+        ],
+        "shared_extra_conf": {},
+        "worker_extra_conf": "",
+    },
 }

 # Templates for sections that may be inserted multiple times in config files
@@ -427,6 +437,7 @@ def add_worker_roles_to_shared_config(
        "to_device",
        "typing",
        "push_rules",
+        "thread_subscriptions",
    }

    # Worker-type specific sharding config. Now a single worker can fulfill multiple
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -1227,7 +1227,7 @@ See also the

 ## Controlling whether a user is shadow-banned

-Shadow-banning is a useful tool for moderating malicious or egregiously abusive users.
+Shadow-banning is a useful tool for moderating malicious or egregiously abusive users. 
 A shadow-banned users receives successful responses to their client-server API requests,
 but the events are not propagated into rooms. This can be an effective tool as it
 (hopefully) takes longer for the user to realise they are being moderated before
@@ -1464,8 +1464,11 @@ _Added in Synapse 1.72.0._

 ## Redact all the events of a user

-This endpoint allows an admin to redact the events of a given user. There are no restrictions on redactions for a 
-local user. By default, we puppet the user who sent the message to redact it themselves. Redactions for non-local users are issued using the admin user, and will fail in rooms where the admin user is not admin/does not have the specified power level to issue redactions. 
+This endpoint allows an admin to redact the events of a given user. There are no restrictions on
+redactions for a local user. By default, we puppet the user who sent the message to redact it themselves.
+Redactions for non-local users are issued using the admin user, and will fail in rooms where the
+admin user is not admin/does not have the specified power level to issue redactions. An option
+is provided to override the default and allow the admin to issue the redactions in all cases.  

 The API is 
 ```
@@ -1475,7 +1478,7 @@ POST /_synapse/admin/v1/user/$user_id/redact
  "rooms": ["!roomid1", "!roomid2"]
 }
 ```
-If an empty list is provided as the key for `rooms`, all events in all the rooms the user is member of will be redacted, 
+If an empty list is provided as the key for `rooms`, all events in all the rooms the user is member of will be redacted,
 otherwise all the events in the rooms provided in the request will be redacted. 

 The API starts redaction process running, and returns immediately with a JSON body with
@@ -1501,7 +1504,10 @@ The following JSON body parameter must be provided:
 The following JSON body parameters are optional:

 - `reason` - Reason the redaction is being requested, ie "spam", "abuse", etc. This will be included in each redaction event, and be visible to users.
- `limit` - a limit on the number of the user's events to search for ones that can be redacted (events are redacted newest to oldest) in each room, defaults to 1000 if not provided
+- `limit` - a limit on the number of the user's events to search for ones that can be redacted (events are redacted newest to oldest) in each room, defaults to 1000 if not provided.
+- `use_admin` - If set to `true`, the admin user is used to issue the redactions, rather than puppeting the user. Useful
+ when the admin is also the moderator of the rooms that require redactions. Note that the redactions will fail in rooms
+ where the admin does not have the sufficient power level to issue the redactions.  

 _Added in Synapse 1.116.0._

--- a/docs/workers.md
+++ b/docs/workers.md
@@ -238,6 +238,7 @@ information.
    ^/_matrix/client/unstable/im.nheko.summary/summary/.*$
    ^/_matrix/client/(r0|v3|unstable)/account/3pid$
    ^/_matrix/client/(r0|v3|unstable)/account/whoami$
+    ^/_matrix/client/(r0|v3|unstable)/account/deactivate$
    ^/_matrix/client/(r0|v3)/delete_devices$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/devices(/|$)
    ^/_matrix/client/versions$
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 2.1.2 and should not be changed by hand.
+# This file is automatically @generated by Poetry 2.1.1 and should not be changed by hand.

 [[package]]
 name = "annotated-types"
@@ -34,15 +34,15 @@ tests-mypy = ["mypy (>=1.11.1) ; platform_python_implementation == \"CPython\" a

 [[package]]
 name = "authlib"
-version = "1.6.0"
+version = "1.6.1"
 description = "The ultimate Python library in building OAuth and OpenID Connect servers and clients."
 optional = true
 python-versions = ">=3.9"
 groups = ["main"]
-markers = "extra == \"oidc\" or extra == \"jwt\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"jwt\" or extra == \"oidc\""
 files = [
-    {file = "authlib-1.6.0-py2.py3-none-any.whl", hash = "sha256:91685589498f79e8655e8a8947431ad6288831d643f11c55c2143ffcc738048d"},
-    {file = "authlib-1.6.0.tar.gz", hash = "sha256:4367d32031b7af175ad3a323d571dc7257b7099d55978087ceae4a0d88cd3210"},
+    {file = "authlib-1.6.1-py2.py3-none-any.whl", hash = "sha256:e9d2031c34c6309373ab845afc24168fe9e93dc52d252631f52642f21f5ed06e"},
+    {file = "authlib-1.6.1.tar.gz", hash = "sha256:4dffdbb1460ba6ec8c17981a4c67af7d8af131231b5a36a88a1e8c80c111cdfd"},
 ]

 [package.dependencies]
@@ -435,7 +435,7 @@ description = "XML bomb protection for Python stdlib modules"
 optional = true
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "defusedxml-0.7.1-py2.py3-none-any.whl", hash = "sha256:a352e7e428770286cc899e2542b6cdaedb2b4953ff269a210103ec58f6198a61"},
    {file = "defusedxml-0.7.1.tar.gz", hash = "sha256:1bb3032db185915b62d7c6209c5a8792be6a32ab2fedacc84e01b52c51aa3e69"},
@@ -478,7 +478,7 @@ description = "XPath 1.0/2.0/3.0/3.1 parsers and selectors for ElementTree and l
 optional = true
 python-versions = ">=3.7"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "elementpath-4.1.5-py3-none-any.whl", hash = "sha256:2ac1a2fb31eb22bbbf817f8cf6752f844513216263f0e3892c8e79782fe4bb55"},
    {file = "elementpath-4.1.5.tar.gz", hash = "sha256:c2d6dc524b29ef751ecfc416b0627668119d8812441c555d7471da41d4bacb8d"},
@@ -528,7 +528,7 @@ description = "Python wrapper for hiredis"
 optional = true
 python-versions = ">=3.8"
 groups = ["main"]
-markers = "extra == \"redis\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"redis\""
 files = [
    {file = "hiredis-3.2.1-cp310-cp310-macosx_10_15_universal2.whl", hash = "sha256:add17efcbae46c5a6a13b244ff0b4a8fa079602ceb62290095c941b42e9d5dec"},
    {file = "hiredis-3.2.1-cp310-cp310-macosx_10_15_x86_64.whl", hash = "sha256:5fe955cc4f66c57df1ae8e5caf4de2925d43b5efab4e40859662311d1bcc5f54"},
@@ -865,7 +865,7 @@ description = "Jaeger Python OpenTracing Tracer implementation"
 optional = true
 python-versions = ">=3.7"
 groups = ["main"]
-markers = "extra == \"opentracing\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"opentracing\""
 files = [
    {file = "jaeger-client-4.8.0.tar.gz", hash = "sha256:3157836edab8e2c209bd2d6ae61113db36f7ee399e66b1dcbb715d87ab49bfe0"},
 ]
@@ -936,14 +936,14 @@ i18n = ["Babel (>=2.7)"]

 [[package]]
 name = "jsonschema"
-version = "4.24.0"
+version = "4.25.0"
 description = "An implementation of JSON Schema validation for Python"
 optional = false
 python-versions = ">=3.9"
 groups = ["main"]
 files = [
-    {file = "jsonschema-4.24.0-py3-none-any.whl", hash = "sha256:a462455f19f5faf404a7902952b6f0e3ce868f3ee09a359b05eca6673bd8412d"},
-    {file = "jsonschema-4.24.0.tar.gz", hash = "sha256:0b4e8069eb12aedfa881333004bccaec24ecef5a8a6a4b6df142b2cc9599d196"},
+    {file = "jsonschema-4.25.0-py3-none-any.whl", hash = "sha256:24c2e8da302de79c8b9382fee3e76b355e44d2a4364bb207159ce10b517bd716"},
+    {file = "jsonschema-4.25.0.tar.gz", hash = "sha256:e63acf5c11762c0e6672ffb61482bdf57f0876684d8d249c0fe2d730d48bc55f"},
 ]

 [package.dependencies]
@@ -954,7 +954,7 @@ rpds-py = ">=0.7.1"

 [package.extras]
 format = ["fqdn", "idna", "isoduration", "jsonpointer (>1.13)", "rfc3339-validator", "rfc3987", "uri-template", "webcolors (>=1.11)"]
-format-nongpl = ["fqdn", "idna", "isoduration", "jsonpointer (>1.13)", "rfc3339-validator", "rfc3986-validator (>0.1.0)", "uri-template", "webcolors (>=24.6.0)"]
+format-nongpl = ["fqdn", "idna", "isoduration", "jsonpointer (>1.13)", "rfc3339-validator", "rfc3986-validator (>0.1.0)", "rfc3987-syntax (>=1.1.0)", "uri-template", "webcolors (>=24.6.0)"]

 [[package]]
 name = "jsonschema-specifications"
@@ -1003,7 +1003,7 @@ description = "A strictly RFC 4510 conforming LDAP V3 pure Python client library
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"matrix-synapse-ldap3\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"matrix-synapse-ldap3\""
 files = [
    {file = "ldap3-2.9.1-py2.py3-none-any.whl", hash = "sha256:5869596fc4948797020d3f03b7939da938778a0f9e2009f7a072ccf92b8e8d70"},
    {file = "ldap3-2.9.1.tar.gz", hash = "sha256:f3e7fc4718e3f09dda568b57100095e0ce58633bcabbed8667ce3f8fbaa4229f"},
@@ -1019,7 +1019,7 @@ description = "Powerful and Pythonic XML processing library combining libxml2/li
 optional = true
 python-versions = ">=3.8"
 groups = ["main"]
-markers = "extra == \"url-preview\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"url-preview\""
 files = [
    {file = "lxml-6.0.0-cp310-cp310-macosx_10_9_universal2.whl", hash = "sha256:35bc626eec405f745199200ccb5c6b36f202675d204aa29bb52e27ba2b71dea8"},
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@@ -1260,7 +1260,7 @@ description = "An LDAP3 auth provider for Synapse"
 optional = true
 python-versions = ">=3.7"
 groups = ["main"]
-markers = "extra == \"matrix-synapse-ldap3\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"matrix-synapse-ldap3\""
 files = [
    {file = "matrix-synapse-ldap3-0.3.0.tar.gz", hash = "sha256:8bb6517173164d4b9cc44f49de411d8cebdb2e705d5dd1ea1f38733c4a009e1d"},
    {file = "matrix_synapse_ldap3-0.3.0-py3-none-any.whl", hash = "sha256:8b4d701f8702551e98cc1d8c20dbed532de5613584c08d0df22de376ba99159d"},
@@ -1493,7 +1493,7 @@ description = "OpenTracing API for Python. See documentation at http://opentraci
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"opentracing\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"opentracing\""
 files = [
    {file = "opentracing-2.4.0.tar.gz", hash = "sha256:a173117e6ef580d55874734d1fa7ecb6f3655160b8b8974a2a1e98e5ec9c840d"},
 ]
@@ -1699,7 +1699,7 @@ description = "psycopg2 - Python-PostgreSQL Database Adapter"
 optional = true
 python-versions = ">=3.8"
 groups = ["main"]
-markers = "extra == \"postgres\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"postgres\""
 files = [
    {file = "psycopg2-2.9.10-cp310-cp310-win32.whl", hash = "sha256:5df2b672140f95adb453af93a7d669d7a7bf0a56bcd26f1502329166f4a61716"},
    {file = "psycopg2-2.9.10-cp310-cp310-win_amd64.whl", hash = "sha256:c6f7b8561225f9e711a9c47087388a97fdc948211c10a4bccbf0ba68ab7b3b5a"},
@@ -1720,7 +1720,7 @@ description = ".. image:: https://travis-ci.org/chtd/psycopg2cffi.svg?branch=mas
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "platform_python_implementation == \"PyPy\" and (extra == \"postgres\" or extra == \"all\")"
+markers = "platform_python_implementation == \"PyPy\" and (extra == \"all\" or extra == \"postgres\")"
 files = [
    {file = "psycopg2cffi-2.9.0.tar.gz", hash = "sha256:7e272edcd837de3a1d12b62185eb85c45a19feda9e62fa1b120c54f9e8d35c52"},
 ]
@@ -1736,7 +1736,7 @@ description = "A Simple library to enable psycopg2 compatability"
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "platform_python_implementation == \"PyPy\" and (extra == \"postgres\" or extra == \"all\")"
+markers = "platform_python_implementation == \"PyPy\" and (extra == \"all\" or extra == \"postgres\")"
 files = [
    {file = "psycopg2cffi-compat-1.1.tar.gz", hash = "sha256:d25e921748475522b33d13420aad5c2831c743227dc1f1f2585e0fdb5c914e05"},
 ]
@@ -1996,7 +1996,7 @@ description = "A development tool to measure, monitor and analyze the memory beh
 optional = true
 python-versions = ">=3.6"
 groups = ["main"]
-markers = "extra == \"cache-memory\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"cache-memory\""
 files = [
    {file = "Pympler-1.0.1-py3-none-any.whl", hash = "sha256:d260dda9ae781e1eab6ea15bacb84015849833ba5555f141d2d9b7b7473b307d"},
    {file = "Pympler-1.0.1.tar.gz", hash = "sha256:993f1a3599ca3f4fcd7160c7545ad06310c9e12f70174ae7ae8d4e25f6c5d3fa"},
@@ -2056,7 +2056,7 @@ description = "Python implementation of SAML Version 2 Standard"
 optional = true
 python-versions = ">=3.9,<4.0"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "pysaml2-7.5.0-py3-none-any.whl", hash = "sha256:bc6627cc344476a83c757f440a73fda1369f13b6fda1b4e16bca63ffbabb5318"},
    {file = "pysaml2-7.5.0.tar.gz", hash = "sha256:f36871d4e5ee857c6b85532e942550d2cf90ea4ee943d75eb681044bbc4f54f7"},
@@ -2081,7 +2081,7 @@ description = "Extensions to the standard Python datetime module"
 optional = true
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,>=2.7"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "python-dateutil-2.8.2.tar.gz", hash = "sha256:0123cacc1627ae19ddf3c27a5de5bd67ee4586fbdd6440d9748f8abb483d3e86"},
    {file = "python_dateutil-2.8.2-py2.py3-none-any.whl", hash = "sha256:961d03dc3453ebbc59dbdea9e4e11c5651520a876d0f4db161e8674aae935da9"},
@@ -2109,7 +2109,7 @@ description = "World timezone definitions, modern and historical"
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "pytz-2022.7.1-py2.py3-none-any.whl", hash = "sha256:78f4f37d8198e0627c5f1143240bb0206b8691d8d7ac6d78fee88b78733f8c4a"},
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@@ -2408,30 +2408,30 @@ files = [

 [[package]]
 name = "ruff"
-version = "0.12.3"
+version = "0.12.4"
 description = "An extremely fast Python linter and code formatter, written in Rust."
 optional = false
 python-versions = ">=3.7"
 groups = ["dev"]
 files = [
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+    {file = "ruff-0.12.4.tar.gz", hash = "sha256:13efa16df6c6eeb7d0f091abae50f58e9522f3843edb40d56ad52a5a4a4b6873"},
 ]

 [[package]]
@@ -2474,7 +2474,7 @@ description = "Python client for Sentry (https://sentry.io)"
 optional = true
 python-versions = ">=3.6"
 groups = ["main"]
-markers = "extra == \"sentry\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"sentry\""
 files = [
    {file = "sentry_sdk-2.32.0-py2.py3-none-any.whl", hash = "sha256:6cf51521b099562d7ce3606da928c473643abe99b00ce4cb5626ea735f4ec345"},
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@@ -2662,7 +2662,7 @@ description = "Tornado IOLoop Backed Concurrent Futures"
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"opentracing\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"opentracing\""
 files = [
    {file = "threadloop-1.0.2-py2-none-any.whl", hash = "sha256:5c90dbefab6ffbdba26afb4829d2a9df8275d13ac7dc58dccb0e279992679599"},
    {file = "threadloop-1.0.2.tar.gz", hash = "sha256:8b180aac31013de13c2ad5c834819771992d350267bddb854613ae77ef571944"},
@@ -2678,7 +2678,7 @@ description = "Python bindings for the Apache Thrift RPC system"
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"opentracing\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"opentracing\""
 files = [
    {file = "thrift-0.16.0.tar.gz", hash = "sha256:2b5b6488fcded21f9d312aa23c9ff6a0195d0f6ae26ddbd5ad9e3e25dfc14408"},
 ]
@@ -2740,7 +2740,7 @@ description = "Tornado is a Python web framework and asynchronous networking lib
 optional = true
 python-versions = ">=3.9"
 groups = ["main"]
-markers = "extra == \"opentracing\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"opentracing\""
 files = [
    {file = "tornado-6.5-cp39-abi3-macosx_10_9_universal2.whl", hash = "sha256:f81067dad2e4443b015368b24e802d0083fecada4f0a4572fdb72fc06e54a9a6"},
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@@ -2877,7 +2877,7 @@ description = "non-blocking redis client for python"
 optional = true
 python-versions = "*"
 groups = ["main"]
-markers = "extra == \"redis\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"redis\""
 files = [
    {file = "txredisapi-1.4.11-py3-none-any.whl", hash = "sha256:ac64d7a9342b58edca13ef267d4fa7637c1aa63f8595e066801c1e8b56b22d0b"},
    {file = "txredisapi-1.4.11.tar.gz", hash = "sha256:3eb1af99aefdefb59eb877b1dd08861efad60915e30ad5bf3d5bf6c5cedcdbc6"},
@@ -3208,7 +3208,7 @@ description = "An XML Schema validator and decoder"
 optional = true
 python-versions = ">=3.7"
 groups = ["main"]
-markers = "extra == \"saml2\" or extra == \"all\""
+markers = "extra == \"all\" or extra == \"saml2\""
 files = [
    {file = "xmlschema-2.4.0-py3-none-any.whl", hash = "sha256:dc87be0caaa61f42649899189aab2fd8e0d567f2cf548433ba7b79278d231a4a"},
    {file = "xmlschema-2.4.0.tar.gz", hash = "sha256:d74cd0c10866ac609e1ef94a5a69b018ad16e39077bc6393408b40c6babee793"},
@@ -3352,4 +3352,4 @@ url-preview = ["lxml"]
 [metadata]
 lock-version = "2.1"
 python-versions = "^3.9.0"
-content-hash = "a6965a294ca751ec2b5b0b92a050acc9afd4efb3e58550845dd32c60b74a70d1"
+content-hash = "b1a0f4708465fd597d0bc7ebb09443ce0e2613cd58a33387a28036249f26856b"
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -101,7 +101,7 @@ module-name = "synapse.synapse_rust"

 [tool.poetry]
 name = "matrix-synapse"
-version = "1.134.0"
+version = "1.135.0"
 description = "Homeserver for the Matrix decentralised comms protocol"
 authors = ["Matrix.org Team and Contributors <packages@matrix.org>"]
 license = "AGPL-3.0-or-later"
@@ -319,7 +319,7 @@ all = [
 # failing on new releases. Keeping lower bounds loose here means that dependabot
 # can bump versions without having to update the content-hash in the lockfile.
 # This helps prevents merge conflicts when running a batch of dependabot updates.
-ruff = "0.12.3"
+ruff = "0.12.4"
 # Type checking only works with the pydantic.v1 compat module from pydantic v2
 pydantic = "^2"

--- a/schema/synapse-config.schema.yaml
+++ b/schema/synapse-config.schema.yaml
@@ -1,5 +1,5 @@
 $schema: https://element-hq.github.io/synapse/latest/schema/v1/meta.schema.json
-$id: https://element-hq.github.io/synapse/schema/synapse/v1.134/synapse-config.schema.json
+$id: https://element-hq.github.io/synapse/schema/synapse/v1.135/synapse-config.schema.json
 type: object
 properties:
  modules:
--- a/synapse/_pydantic_compat.py
+++ b/synapse/_pydantic_compat.py
@@ -48,6 +48,7 @@ if TYPE_CHECKING or HAS_PYDANTIC_V2:
        conint,
        constr,
        parse_obj_as,
+        root_validator,
        validator,
    )
    from pydantic.v1.error_wrappers import ErrorWrapper
@@ -68,6 +69,7 @@ else:
        conint,
        constr,
        parse_obj_as,
+        root_validator,
        validator,
    )
    from pydantic.error_wrappers import ErrorWrapper
@@ -92,4 +94,5 @@ __all__ = (
    "StrictStr",
    "ValidationError",
    "validator",
+    "root_validator",
 )
--- a/synapse/_scripts/synapse_port_db.py
+++ b/synapse/_scripts/synapse_port_db.py
@@ -136,6 +136,7 @@ BOOLEAN_COLUMNS = {
        "has_known_state",
        "is_encrypted",
    ],
+    "thread_subscriptions": ["subscribed", "automatic"],
    "users": ["shadow_banned", "approved", "locked", "suspended"],
    "un_partial_stated_event_stream": ["rejection_status_changed"],
    "users_who_share_rooms": ["share_private"],
--- a/synapse/api/auth/msc3861_delegated.py
+++ b/synapse/api/auth/msc3861_delegated.py
@@ -369,6 +369,12 @@ class MSC3861DelegatedAuth(BaseAuth):
    async def is_server_admin(self, requester: Requester) -> bool:
        return "urn:synapse:admin:*" in requester.scope

+    def _is_access_token_the_admin_token(self, token: str) -> bool:
+        admin_token = self._admin_token()
+        if admin_token is None:
+            return False
+        return token == admin_token
+
    async def get_user_by_req(
        self,
        request: SynapseRequest,
@@ -434,7 +440,7 @@ class MSC3861DelegatedAuth(BaseAuth):
            requester = await self.get_user_by_access_token(access_token, allow_expired)

        # Do not record requests from MAS using the virtual `__oidc_admin` user.
-        if access_token != self._admin_token():
+        if not self._is_access_token_the_admin_token(access_token):
            await self._record_request(request, requester)

        if not allow_guest and requester.is_guest:
@@ -470,13 +476,25 @@ class MSC3861DelegatedAuth(BaseAuth):

            raise UnrecognizedRequestError(code=404)

+    def is_request_using_the_admin_token(self, request: SynapseRequest) -> bool:
+        """
+        Check if the request is using the admin token.
+
+        Args:
+            request: The request to check.
+
+        Returns:
+            True if the request is using the admin token, False otherwise.
+        """
+        access_token = self.get_access_token_from_request(request)
+        return self._is_access_token_the_admin_token(access_token)
+
    async def get_user_by_access_token(
        self,
        token: str,
        allow_expired: bool = False,
    ) -> Requester:
-        admin_token = self._admin_token()
-        if admin_token is not None and token == admin_token:
+        if self._is_access_token_the_admin_token(token):
            # XXX: This is a temporary solution so that the admin API can be called by
            # the OIDC provider. This will be removed once we have OIDC client
            # credentials grant support in matrix-authentication-service.
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -104,6 +104,9 @@ from synapse.storage.databases.main.stats import StatsStore
 from synapse.storage.databases.main.stream import StreamWorkerStore
 from synapse.storage.databases.main.tags import TagsWorkerStore
 from synapse.storage.databases.main.task_scheduler import TaskSchedulerWorkerStore
+from synapse.storage.databases.main.thread_subscriptions import (
+    ThreadSubscriptionsWorkerStore,
+)
 from synapse.storage.databases.main.transactions import TransactionWorkerStore
 from synapse.storage.databases.main.ui_auth import UIAuthWorkerStore
 from synapse.storage.databases.main.user_directory import UserDirectoryStore
@@ -132,6 +135,7 @@ class GenericWorkerStore(
    KeyStore,
    RoomWorkerStore,
    DirectoryWorkerStore,
+    ThreadSubscriptionsWorkerStore,
    PushRulesWorkerStore,
    ApplicationServiceTransactionWorkerStore,
    ApplicationServiceWorkerStore,
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -581,3 +581,7 @@ class ExperimentalConfig(Config):

        # MSC4155: Invite filtering
        self.msc4155_enabled: bool = experimental.get("msc4155_enabled", False)
+
+        # MSC4306: Thread Subscriptions
+        # (and MSC4308: sliding sync extension for thread subscriptions)
+        self.msc4306_enabled: bool = experimental.get("msc4306_enabled", False)
--- a/synapse/config/workers.py
+++ b/synapse/config/workers.py
@@ -174,6 +174,10 @@ class WriterLocations:
        default=[MAIN_PROCESS_INSTANCE_NAME],
        converter=_instance_to_list_converter,
    )
+    thread_subscriptions: List[str] = attr.ib(
+        default=["master"],
+        converter=_instance_to_list_converter,
+    )


@attr.s(auto_attribs=True)
--- a/synapse/handlers/admin.py
+++ b/synapse/handlers/admin.py
@@ -358,6 +358,7 @@ class AdminHandler:
        user_id: str,
        rooms: list,
        requester: JsonMapping,
+        use_admin: bool,
        reason: Optional[str],
        limit: Optional[int],
    ) -> str:
@@ -368,6 +369,7 @@ class AdminHandler:
            user_id: the user ID of the user whose events should be redacted
            rooms: the rooms in which to redact the user's events
            requester: the user requesting the events
+            use_admin: whether to use the admin account to issue the redactions
            reason: reason for requesting the redaction, ie spam, etc
            limit: limit on the number of events in each room to redact

@@ -395,6 +397,7 @@ class AdminHandler:
                "rooms": rooms,
                "requester": requester,
                "user_id": user_id,
+                "use_admin": use_admin,
                "reason": reason,
                "limit": limit,
            },
@@ -426,9 +429,17 @@ class AdminHandler:
        user_id = task.params.get("user_id")
        assert user_id is not None

-        # puppet the user if they're ours, otherwise use admin to redact
+        use_admin = task.params.get("use_admin", False)
+
+        # default to puppeting the user unless they are not local or it's been requested to
+        # use the admin user to issue the redactions
+        requester_id = (
+            admin.user.to_string()
+            if use_admin or not self.hs.is_mine_id(user_id)
+            else user_id
+        )
        requester = create_requester(
-            user_id if self.hs.is_mine_id(user_id) else admin.user.to_string(),
+            requester_id,
            authenticated_entity=admin.user.to_string(),
        )

--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -220,6 +220,7 @@ class AuthHandler:
        self._password_localdb_enabled = hs.config.auth.password_localdb_enabled
        self._third_party_rules = hs.get_module_api_callbacks().third_party_event_rules
        self._account_validity_handler = hs.get_account_validity_handler()
+        self._pusher_pool = hs.get_pusherpool()

        # Ratelimiter for failed auth during UIA. Uses same ratelimit config
        # as per `rc_login.failed_attempts`.
@@ -1652,7 +1653,7 @@ class AuthHandler:
        )

        if medium == "email":
-            await self.store.delete_pusher_by_app_id_pushkey_user_id(
+            await self._pusher_pool.remove_pusher(
                app_id="m.email", pushkey=address, user_id=user_id
            )

--- a/synapse/handlers/deactivate_account.py
+++ b/synapse/handlers/deactivate_account.py
@@ -25,6 +25,9 @@ from typing import TYPE_CHECKING, Optional
 from synapse.api.constants import Membership
 from synapse.api.errors import SynapseError
 from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.replication.http.deactivate_account import (
+    ReplicationNotifyAccountDeactivatedServlet,
+)
 from synapse.types import Codes, Requester, UserID, create_requester

 if TYPE_CHECKING:
@@ -44,6 +47,7 @@ class DeactivateAccountHandler:
        self._room_member_handler = hs.get_room_member_handler()
        self._identity_handler = hs.get_identity_handler()
        self._profile_handler = hs.get_profile_handler()
+        self._pusher_pool = hs.get_pusherpool()
        self.user_directory_handler = hs.get_user_directory_handler()
        self._server_name = hs.hostname
        self._third_party_rules = hs.get_module_api_callbacks().third_party_event_rules
@@ -52,10 +56,16 @@ class DeactivateAccountHandler:
        self._user_parter_running = False
        self._third_party_rules = hs.get_module_api_callbacks().third_party_event_rules

+        self._notify_account_deactivated_client = None
+
        # Start the user parter loop so it can resume parting users from rooms where
        # it left off (if it has work left to do).
-        if hs.config.worker.run_background_tasks:
+        if hs.config.worker.worker_app is None:
            hs.get_reactor().callWhenRunning(self._start_user_parting)
+        else:
+            self._notify_account_deactivated_client = (
+                ReplicationNotifyAccountDeactivatedServlet.make_client(hs)
+            )

        self._account_validity_enabled = (
            hs.config.account_validity.account_validity_enabled
@@ -145,7 +155,7 @@ class DeactivateAccountHandler:
        # Most of the pushers will have been deleted when we logged out the
        # associated devices above, but we still need to delete pushers not
        # associated with devices, e.g. email pushers.
-        await self.store.delete_all_pushers_for_user(user_id)
+        await self._pusher_pool.delete_all_pushers_for_user(user_id)

        # Add the user to a table of users pending deactivation (ie.
        # removal from all the rooms they're a member of)
@@ -169,10 +179,6 @@ class DeactivateAccountHandler:
            logger.info("Marking %s as erased", user_id)
            await self.store.mark_user_erased(user_id)

-        # Now start the process that goes through that list and
-        # parts users from rooms (if it isn't already running)
-        self._start_user_parting()
-
        # Reject all pending invites and knocks for the user, so that the
        # user doesn't show up in the "invited" section of rooms' members list.
        await self._reject_pending_invites_and_knocks_for_user(user_id)
@@ -187,18 +193,43 @@ class DeactivateAccountHandler:
        # Remove account data (including ignored users and push rules).
        await self.store.purge_account_data_for_user(user_id)

+        # Remove thread subscriptions for the user
+        await self.store.purge_thread_subscription_settings_for_user(user_id)
+
        # Delete any server-side backup keys
        await self.store.bulk_delete_backup_keys_and_versions_for_user(user_id)

+        # Notify modules and start the room parting process.
+        await self.notify_account_deactivated(user_id, by_admin=by_admin)
+
+        return identity_server_supports_unbinding
+
+    async def notify_account_deactivated(
+        self,
+        user_id: str,
+        by_admin: bool = False,
+    ) -> None:
+        """Notify modules and start the room parting process.
+        Goes through replication if this is not the main process.
+        """
+        if self._notify_account_deactivated_client is not None:
+            await self._notify_account_deactivated_client(
+                user_id=user_id,
+                by_admin=by_admin,
+            )
+            return
+
+        # Now start the process that goes through that list and
+        # parts users from rooms (if it isn't already running)
+        self._start_user_parting()
+
        # Let modules know the user has been deactivated.
        await self._third_party_rules.on_user_deactivation_status_changed(
            user_id,
            True,
-            by_admin,
+            by_admin=by_admin,
        )

-        return identity_server_supports_unbinding
-
    async def _reject_pending_invites_and_knocks_for_user(self, user_id: str) -> None:
        """Reject pending invites and knocks addressed to a given user ID.

--- a/synapse/handlers/thread_subscriptions.py
+++ b/synapse/handlers/thread_subscriptions.py
@@ -0,0 +1,126 @@
+import logging
+from typing import TYPE_CHECKING, Optional
+
+from synapse.api.errors import AuthError, NotFoundError
+from synapse.storage.databases.main.thread_subscriptions import ThreadSubscription
+from synapse.types import UserID
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class ThreadSubscriptionsHandler:
+    def __init__(self, hs: "HomeServer"):
+        self.store = hs.get_datastores().main
+        self.event_handler = hs.get_event_handler()
+        self.auth = hs.get_auth()
+
+    async def get_thread_subscription_settings(
+        self,
+        user_id: UserID,
+        room_id: str,
+        thread_root_event_id: str,
+    ) -> Optional[ThreadSubscription]:
+        """Get thread subscription settings for a specific thread and user.
+        Checks that the thread root is both a real event and also that it is visible
+        to the user.
+
+        Args:
+            user_id: The ID of the user
+            thread_root_event_id: The event ID of the thread root
+
+        Returns:
+            A `ThreadSubscription` containing the active subscription settings or None if not set
+        """
+        # First check that the user can access the thread root event
+        # and that it exists
+        try:
+            event = await self.event_handler.get_event(
+                user_id, room_id, thread_root_event_id
+            )
+            if event is None:
+                raise NotFoundError("No such thread root")
+        except AuthError:
+            raise NotFoundError("No such thread root")
+
+        return await self.store.get_subscription_for_thread(
+            user_id.to_string(), event.room_id, thread_root_event_id
+        )
+
+    async def subscribe_user_to_thread(
+        self,
+        user_id: UserID,
+        room_id: str,
+        thread_root_event_id: str,
+        *,
+        automatic: bool,
+    ) -> Optional[int]:
+        """Sets or updates a user's subscription settings for a specific thread root.
+
+        Args:
+            requester_user_id: The ID of the user whose settings are being updated.
+            thread_root_event_id: The event ID of the thread root.
+            automatic: whether the user was subscribed by an automatic decision by
+                their client.
+
+        Returns:
+            The stream ID for this update, if the update isn't no-opped.
+
+        Raises:
+            NotFoundError if the user cannot access the thread root event, or it isn't
+            known to this homeserver.
+        """
+        # First check that the user can access the thread root event
+        # and that it exists
+        try:
+            event = await self.event_handler.get_event(
+                user_id, room_id, thread_root_event_id
+            )
+            if event is None:
+                raise NotFoundError("No such thread root")
+        except AuthError:
+            logger.info("rejecting thread subscriptions change (thread not accessible)")
+            raise NotFoundError("No such thread root")
+
+        return await self.store.subscribe_user_to_thread(
+            user_id.to_string(),
+            event.room_id,
+            thread_root_event_id,
+            automatic=automatic,
+        )
+
+    async def unsubscribe_user_from_thread(
+        self, user_id: UserID, room_id: str, thread_root_event_id: str
+    ) -> Optional[int]:
+        """Clears a user's subscription settings for a specific thread root.
+
+        Args:
+            requester_user_id: The ID of the user whose settings are being updated.
+            thread_root_event_id: The event ID of the thread root.
+
+        Returns:
+            The stream ID for this update, if the update isn't no-opped.
+
+        Raises:
+            NotFoundError if the user cannot access the thread root event, or it isn't
+            known to this homeserver.
+        """
+        # First check that the user can access the thread root event
+        # and that it exists
+        try:
+            event = await self.event_handler.get_event(
+                user_id, room_id, thread_root_event_id
+            )
+            if event is None:
+                raise NotFoundError("No such thread root")
+        except AuthError:
+            logger.info("rejecting thread subscriptions change (thread not accessible)")
+            raise NotFoundError("No such thread root")
+
+        return await self.store.unsubscribe_user_from_thread(
+            user_id.to_string(),
+            event.room_id,
+            thread_root_event_id,
+        )
--- a/synapse/media/_base.py
+++ b/synapse/media/_base.py
@@ -380,12 +380,13 @@ async def respond_with_multipart_responder(

        try:
            await responder.write_to_consumer(multipart_consumer)
+        except ConsumerRequestedStopError as e:
+            logger.debug("Failed to write to consumer: %s %s", type(e), e)
+            # Unregister the producer, if it has one, so Twisted doesn't complain
+            if request.producer:
+                request.unregisterProducer()
        except Exception as e:
-            # The majority of the time this will be due to the client having gone
-            # away. Unfortunately, Twisted simply throws a generic exception at us
-            # in that case.
            logger.warning("Failed to write to consumer: %s %s", type(e), e)
-
            # Unregister the producer, if it has one, so Twisted doesn't complain
            if request.producer:
                request.unregisterProducer()
@@ -426,12 +427,13 @@ async def respond_with_responder(
        add_file_headers(request, media_type, file_size, upload_name)
        try:
            await responder.write_to_consumer(request)
+        except ConsumerRequestedStopError as e:
+            logger.debug("Failed to write to consumer: %s %s", type(e), e)
+            # Unregister the producer, if it has one, so Twisted doesn't complain
+            if request.producer:
+                request.unregisterProducer()
        except Exception as e:
-            # The majority of the time this will be due to the client having gone
-            # away. Unfortunately, Twisted simply throws a generic exception at us
-            # in that case.
            logger.warning("Failed to write to consumer: %s %s", type(e), e)
-
            # Unregister the producer, if it has one, so Twisted doesn't complain
            if request.producer:
                request.unregisterProducer()
@@ -674,6 +676,10 @@ def _parseparam(s: bytes) -> Generator[bytes, None, None]:
        s = s[end:]


+class ConsumerRequestedStopError(Exception):
+    """A consumer asked us to stop producing"""
+
+
@implementer(interfaces.IPushProducer)
 class ThreadedFileSender:
    """
@@ -751,7 +757,9 @@ class ThreadedFileSender:
        self.wakeup_event.set()

        if not self.deferred.called:
-            self.deferred.errback(Exception("Consumer asked us to stop producing"))
+            self.deferred.errback(
+                ConsumerRequestedStopError("Consumer asked us to stop producing")
+            )

    async def start_read_loop(self) -> None:
        """This is the loop that drives reading/writing"""
--- a/synapse/push/pusherpool.py
+++ b/synapse/push/pusherpool.py
@@ -31,7 +31,10 @@ from synapse.metrics.background_process_metrics import (
 )
 from synapse.push import Pusher, PusherConfig, PusherConfigException
 from synapse.push.pusher import PusherFactory
-from synapse.replication.http.push import ReplicationRemovePusherRestServlet
+from synapse.replication.http.push import (
+    ReplicationDeleteAllPushersForUserRestServlet,
+    ReplicationRemovePusherRestServlet,
+)
 from synapse.types import JsonDict, RoomStreamToken, StrCollection
 from synapse.util.async_helpers import concurrently_execute
 from synapse.util.threepids import canonicalise_email
@@ -78,10 +81,14 @@ class PusherPool:

        # We can only delete pushers on master.
        self._remove_pusher_client = None
+        self._delete_all_pushers_for_user_client = None
        if hs.config.worker.worker_app:
            self._remove_pusher_client = ReplicationRemovePusherRestServlet.make_client(
                hs
            )
+            self._delete_all_pushers_for_user_client = (
+                ReplicationDeleteAllPushersForUserRestServlet.make_client(hs)
+            )

        # Record the last stream ID that we were poked about so we can get
        # changes since then. We set this to the current max stream ID on
@@ -454,6 +461,13 @@ class PusherPool:
                app_id, pushkey, user_id
            )

+    async def delete_all_pushers_for_user(self, user_id: str) -> None:
+        """Deletes all pushers for a user."""
+        if self._delete_all_pushers_for_user_client is not None:
+            await self._delete_all_pushers_for_user_client(user_id=user_id)
+        else:
+            await self.store.delete_all_pushers_for_user(user_id=user_id)
+
    def maybe_stop_pusher(self, app_id: str, pushkey: str, user_id: str) -> None:
        """Stops a pusher with the given app ID and push key if one is running.

--- a/synapse/replication/http/init.py
+++ b/synapse/replication/http/init.py
@@ -23,6 +23,7 @@ from typing import TYPE_CHECKING
 from synapse.http.server import JsonResource
 from synapse.replication.http import (
    account_data,
+    deactivate_account,
    delayed_events,
    devices,
    federation,
@@ -66,3 +67,4 @@ class ReplicationRestResource(JsonResource):
            login.register_servlets(hs, self)
            register.register_servlets(hs, self)
            delayed_events.register_servlets(hs, self)
+            deactivate_account.register_servlets(hs, self)
--- a/synapse/replication/http/deactivate_account.py
+++ b/synapse/replication/http/deactivate_account.py
@@ -0,0 +1,81 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2023 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+# Originally licensed under the Apache License, Version 2.0:
+# <http://www.apache.org/licenses/LICENSE-2.0>.
+#
+# [This file includes modifications made by New Vector Limited]
+#
+#
+
+import logging
+from typing import TYPE_CHECKING, Tuple
+
+from twisted.web.server import Request
+
+from synapse.http.server import HttpServer
+from synapse.replication.http._base import ReplicationEndpoint
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class ReplicationNotifyAccountDeactivatedServlet(ReplicationEndpoint):
+    """Notify that an account has been deactivated.
+
+    Request format:
+
+        POST /_synapse/replication/notify_account_deactivated/:user_id
+
+        {
+            "by_admin": true,
+        }
+
+    """
+
+    NAME = "notify_account_deactivated"
+    PATH_ARGS = ("user_id",)
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+        self.deactivate_account_handler = hs.get_deactivate_account_handler()
+
+    @staticmethod
+    async def _serialize_payload(  # type: ignore[override]
+        user_id: str,
+        by_admin: bool,
+    ) -> JsonDict:
+        """
+        Args:
+            user_id: The user ID which has been deactivated.
+            by_admin: Whether the user was deactivated by an admin.
+        """
+        return {
+            "by_admin": by_admin,
+        }
+
+    async def _handle_request(  # type: ignore[override]
+        self, request: Request, content: JsonDict, user_id: str
+    ) -> Tuple[int, JsonDict]:
+        by_admin = content["by_admin"]
+        await self.deactivate_account_handler.notify_account_deactivated(
+            user_id, by_admin=by_admin
+        )
+        return 200, {}
+
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    ReplicationNotifyAccountDeactivatedServlet(hs).register(http_server)
--- a/synapse/replication/http/push.py
+++ b/synapse/replication/http/push.py
@@ -118,6 +118,39 @@ class ReplicationCopyPusherRestServlet(ReplicationEndpoint):
        return 200, {}


+class ReplicationDeleteAllPushersForUserRestServlet(ReplicationEndpoint):
+    """Deletes all pushers for a user.
+
+    Request format:
+
+        POST /_synapse/replication/delete_all_pushers_for_user/:user_id
+
+        {}
+
+    """
+
+    NAME = "delete_all_pushers_for_user"
+    PATH_ARGS = ("user_id",)
+    CACHE = False
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+
+        self._store = hs.get_datastores().main
+
+    @staticmethod
+    async def _serialize_payload(user_id: str) -> JsonDict:  # type: ignore[override]
+        return {}
+
+    async def _handle_request(  # type: ignore[override]
+        self, request: Request, content: JsonDict, user_id: str
+    ) -> Tuple[int, JsonDict]:
+        await self._store.delete_all_pushers_for_user(user_id)
+
+        return 200, {}
+
+
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
    ReplicationRemovePusherRestServlet(hs).register(http_server)
    ReplicationCopyPusherRestServlet(hs).register(http_server)
+    ReplicationDeleteAllPushersForUserRestServlet(hs).register(http_server)
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -72,7 +72,10 @@ from synapse.replication.tcp.streams import (
    ToDeviceStream,
    TypingStream,
 )
-from synapse.replication.tcp.streams._base import DeviceListsStream
+from synapse.replication.tcp.streams._base import (
+    DeviceListsStream,
+    ThreadSubscriptionsStream,
+)

 if TYPE_CHECKING:
    from synapse.server import HomeServer
@@ -186,6 +189,15 @@ class ReplicationCommandHandler:

                continue

+            if isinstance(stream, ThreadSubscriptionsStream):
+                if (
+                    hs.get_instance_name()
+                    in hs.config.worker.writers.thread_subscriptions
+                ):
+                    self._streams_to_replicate.append(stream)
+
+                continue
+
            if isinstance(stream, DeviceListsStream):
                if hs.get_instance_name() in hs.config.worker.writers.device_lists:
                    self._streams_to_replicate.append(stream)
--- a/synapse/replication/tcp/streams/init.py
+++ b/synapse/replication/tcp/streams/init.py
@@ -41,6 +41,7 @@ from synapse.replication.tcp.streams._base import (
    PushRulesStream,
    ReceiptsStream,
    Stream,
+    ThreadSubscriptionsStream,
    ToDeviceStream,
    TypingStream,
 )
@@ -67,6 +68,7 @@ STREAMS_MAP = {
        ToDeviceStream,
        FederationStream,
        AccountDataStream,
+        ThreadSubscriptionsStream,
        UnPartialStatedRoomStream,
        UnPartialStatedEventStream,
    )
@@ -86,6 +88,7 @@ __all__ = [
    "DeviceListsStream",
    "ToDeviceStream",
    "AccountDataStream",
+    "ThreadSubscriptionsStream",
    "UnPartialStatedRoomStream",
    "UnPartialStatedEventStream",
 ]
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -723,3 +723,46 @@ class AccountDataStream(_StreamFromIdGen):
            heapq.merge(room_rows, global_rows, tag_rows, key=lambda row: row[0])
        )
        return updates, to_token, limited
+
+
+class ThreadSubscriptionsStream(_StreamFromIdGen):
+    """A thread subscription was changed."""
+
+    @attr.s(slots=True, auto_attribs=True)
+    class ThreadSubscriptionsStreamRow:
+        """Stream to inform workers about changes to thread subscriptions."""
+
+        user_id: str
+        room_id: str
+        event_id: str  # The event ID of the thread root
+
+    NAME = "thread_subscriptions"
+    ROW_TYPE = ThreadSubscriptionsStreamRow
+
+    def __init__(self, hs: Any):
+        self.store = hs.get_datastores().main
+        super().__init__(
+            hs.get_instance_name(),
+            self._update_function,
+            self.store._thread_subscriptions_id_gen,
+        )
+
+    async def _update_function(
+        self, instance_name: str, from_token: int, to_token: int, limit: int
+    ) -> StreamUpdateResult:
+        updates = await self.store.get_updated_thread_subscriptions(
+            from_token, to_token, limit
+        )
+        rows = [
+            (
+                stream_id,
+                # These are the args to `ThreadSubscriptionsStreamRow`
+                (user_id, room_id, event_id),
+            )
+            for stream_id, user_id, room_id, event_id in updates
+        ]
+
+        if not rows:
+            return [], to_token, False
+
+        return rows, rows[-1][0], len(updates) == limit
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -1414,7 +1414,7 @@ class RedactUser(RestServlet):
    """
    Redact all the events of a given user in the given rooms or if empty dict is provided
    then all events in all rooms user is member of. Kicks off a background process and
-    returns an id that can be used to check on the progress of the redaction progress
+    returns an id that can be used to check on the progress of the redaction progress.
    """

    PATTERNS = admin_patterns("/user/(?P<user_id>[^/]*)/redact")
@@ -1428,6 +1428,7 @@ class RedactUser(RestServlet):
        rooms: List[StrictStr]
        reason: Optional[StrictStr]
        limit: Optional[StrictInt]
+        use_admin: Optional[StrictBool]

    async def on_POST(
        self, request: SynapseRequest, user_id: str
@@ -1455,8 +1456,12 @@ class RedactUser(RestServlet):
            )
            rooms = current_rooms + banned_rooms

+        use_admin = body.use_admin
+        if not use_admin:
+            use_admin = False
+
        redact_id = await self.admin_handler.start_redact_events(
-            user_id, rooms, requester.serialize(), body.reason, limit
+            user_id, rooms, requester.serialize(), use_admin, body.reason, limit
        )

        return HTTPStatus.OK, {"redact_id": redact_id}
--- a/synapse/rest/client/account.py
+++ b/synapse/rest/client/account.py
@@ -896,23 +896,26 @@ class AccountStatusRestServlet(RestServlet):


 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    ThreepidRestServlet(hs).register(http_server)
+    WhoamiRestServlet(hs).register(http_server)
+
+    if not hs.config.experimental.msc3861.enabled:
+        DeactivateAccountRestServlet(hs).register(http_server)
+
+    # These servlets are only registered on the main process
    if hs.config.worker.worker_app is None:
+        ThreepidBindRestServlet(hs).register(http_server)
+        ThreepidUnbindRestServlet(hs).register(http_server)
+
        if not hs.config.experimental.msc3861.enabled:
            EmailPasswordRequestTokenRestServlet(hs).register(http_server)
-            DeactivateAccountRestServlet(hs).register(http_server)
            PasswordRestServlet(hs).register(http_server)
            EmailThreepidRequestTokenRestServlet(hs).register(http_server)
            MsisdnThreepidRequestTokenRestServlet(hs).register(http_server)
            AddThreepidEmailSubmitTokenServlet(hs).register(http_server)
            AddThreepidMsisdnSubmitTokenServlet(hs).register(http_server)
-    ThreepidRestServlet(hs).register(http_server)
-    if hs.config.worker.worker_app is None:
-        ThreepidBindRestServlet(hs).register(http_server)
-        ThreepidUnbindRestServlet(hs).register(http_server)
-        if not hs.config.experimental.msc3861.enabled:
            ThreepidAddRestServlet(hs).register(http_server)
            ThreepidDeleteRestServlet(hs).register(http_server)
-    WhoamiRestServlet(hs).register(http_server)

-    if hs.config.worker.worker_app is None and hs.config.experimental.msc3720_enabled:
-        AccountStatusRestServlet(hs).register(http_server)
+        if hs.config.experimental.msc3720_enabled:
+            AccountStatusRestServlet(hs).register(http_server)
--- a/synapse/rest/client/thread_subscriptions.py
+++ b/synapse/rest/client/thread_subscriptions.py
@@ -0,0 +1,98 @@
+from http import HTTPStatus
+from typing import Tuple
+
+from synapse._pydantic_compat import StrictBool
+from synapse.api.errors import Codes, NotFoundError, SynapseError
+from synapse.http.server import HttpServer
+from synapse.http.servlet import (
+    RestServlet,
+    parse_and_validate_json_object_from_request,
+)
+from synapse.http.site import SynapseRequest
+from synapse.rest.client._base import client_patterns
+from synapse.server import HomeServer
+from synapse.types import JsonDict, RoomID
+from synapse.types.rest import RequestBodyModel
+
+
+class ThreadSubscriptionsRestServlet(RestServlet):
+    PATTERNS = client_patterns(
+        "/io.element.msc4306/rooms/(?P<room_id>[^/]*)/thread/(?P<thread_root_id>[^/]*)/subscription$",
+        unstable=True,
+        releases=(),
+    )
+    CATEGORY = "Thread Subscriptions requests (unstable)"
+
+    def __init__(self, hs: "HomeServer"):
+        self.auth = hs.get_auth()
+        self.is_mine = hs.is_mine
+        self.store = hs.get_datastores().main
+        self.handler = hs.get_thread_subscriptions_handler()
+
+    class PutBody(RequestBodyModel):
+        automatic: StrictBool
+
+    async def on_GET(
+        self, request: SynapseRequest, room_id: str, thread_root_id: str
+    ) -> Tuple[int, JsonDict]:
+        RoomID.from_string(room_id)
+        if not thread_root_id.startswith("$"):
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST, "Invalid event ID", errcode=Codes.INVALID_PARAM
+            )
+        requester = await self.auth.get_user_by_req(request)
+
+        subscription = await self.handler.get_thread_subscription_settings(
+            requester.user,
+            room_id,
+            thread_root_id,
+        )
+
+        if subscription is None:
+            raise NotFoundError("Not subscribed.")
+
+        return HTTPStatus.OK, {"automatic": subscription.automatic}
+
+    async def on_PUT(
+        self, request: SynapseRequest, room_id: str, thread_root_id: str
+    ) -> Tuple[int, JsonDict]:
+        RoomID.from_string(room_id)
+        if not thread_root_id.startswith("$"):
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST, "Invalid event ID", errcode=Codes.INVALID_PARAM
+            )
+        requester = await self.auth.get_user_by_req(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PutBody)
+
+        await self.handler.subscribe_user_to_thread(
+            requester.user,
+            room_id,
+            thread_root_id,
+            automatic=body.automatic,
+        )
+
+        return HTTPStatus.OK, {}
+
+    async def on_DELETE(
+        self, request: SynapseRequest, room_id: str, thread_root_id: str
+    ) -> Tuple[int, JsonDict]:
+        RoomID.from_string(room_id)
+        if not thread_root_id.startswith("$"):
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST, "Invalid event ID", errcode=Codes.INVALID_PARAM
+            )
+        requester = await self.auth.get_user_by_req(request)
+
+        await self.handler.unsubscribe_user_from_thread(
+            requester.user,
+            room_id,
+            thread_root_id,
+        )
+
+        return HTTPStatus.OK, {}
+
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    if hs.config.experimental.msc4306_enabled:
+        ThreadSubscriptionsRestServlet(hs).register(http_server)
--- a/synapse/rest/synapse/client/init.py
+++ b/synapse/rest/synapse/client/init.py
@@ -30,6 +30,7 @@ from synapse.rest.synapse.client.pick_username import pick_username_resource
 from synapse.rest.synapse.client.rendezvous import MSC4108RendezvousSessionResource
 from synapse.rest.synapse.client.sso_register import SsoRegisterResource
 from synapse.rest.synapse.client.unsubscribe import UnsubscribeResource
+from synapse.rest.synapse.mas import MasResource

 if TYPE_CHECKING:
    from synapse.server import HomeServer
@@ -60,6 +61,7 @@ def build_synapse_client_resource_tree(hs: "HomeServer") -> Mapping[str, Resourc
        from synapse.rest.synapse.client.jwks import JwksResource

        resources["/_synapse/jwks"] = JwksResource(hs)
+        resources["/_synapse/mas"] = MasResource(hs)

    # provider-specific SSO bits. Only load these if they are enabled, since they
    # rely on optional dependencies.
--- a/synapse/rest/synapse/mas/init.py
+++ b/synapse/rest/synapse/mas/init.py
@@ -0,0 +1,71 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl_3.0.html>.
+#
+#
+
+
+import logging
+from typing import TYPE_CHECKING
+
+from twisted.web.resource import Resource
+
+from synapse.rest.synapse.mas.devices import (
+    MasDeleteDeviceResource,
+    MasSyncDevicesResource,
+    MasUpdateDeviceDisplayNameResource,
+    MasUpsertDeviceResource,
+)
+from synapse.rest.synapse.mas.users import (
+    MasAllowCrossSigningResetResource,
+    MasDeleteUserResource,
+    MasIsLocalpartAvailableResource,
+    MasProvisionUserResource,
+    MasQueryUserResource,
+    MasReactivateUserResource,
+    MasSetDisplayNameResource,
+    MasUnsetDisplayNameResource,
+)
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
+logger = logging.getLogger(__name__)
+
+
+class MasResource(Resource):
+    """
+    Provides endpoints for MAS to manage user accounts and devices.
+
+    All endpoints are mounted under the path `/_synapse/mas/` and only work
+    using the MAS admin token.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        Resource.__init__(self)
+        self.putChild(b"query_user", MasQueryUserResource(hs))
+        self.putChild(b"provision_user", MasProvisionUserResource(hs))
+        self.putChild(b"is_localpart_available", MasIsLocalpartAvailableResource(hs))
+        self.putChild(b"delete_user", MasDeleteUserResource(hs))
+        self.putChild(b"upsert_device", MasUpsertDeviceResource(hs))
+        self.putChild(b"delete_device", MasDeleteDeviceResource(hs))
+        self.putChild(
+            b"update_device_display_name", MasUpdateDeviceDisplayNameResource(hs)
+        )
+        self.putChild(b"sync_devices", MasSyncDevicesResource(hs))
+        self.putChild(b"reactivate_user", MasReactivateUserResource(hs))
+        self.putChild(b"set_displayname", MasSetDisplayNameResource(hs))
+        self.putChild(b"unset_displayname", MasUnsetDisplayNameResource(hs))
+        self.putChild(
+            b"allow_cross_signing_reset", MasAllowCrossSigningResetResource(hs)
+        )
--- a/synapse/rest/synapse/mas/_base.py
+++ b/synapse/rest/synapse/mas/_base.py
@@ -0,0 +1,47 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl_3.0.html>.
+#
+#
+
+
+from typing import TYPE_CHECKING, cast
+
+from synapse.api.errors import SynapseError
+from synapse.http.server import DirectServeJsonResource
+
+if TYPE_CHECKING:
+    from synapse.app.generic_worker import GenericWorkerStore
+    from synapse.http.site import SynapseRequest
+    from synapse.server import HomeServer
+
+
+class MasBaseResource(DirectServeJsonResource):
+    def __init__(self, hs: "HomeServer"):
+        # Importing this module requires authlib, which is an optional
+        # dependency but required if msc3861 is enabled
+        from synapse.api.auth.msc3861_delegated import MSC3861DelegatedAuth
+
+        DirectServeJsonResource.__init__(self, extract_context=True)
+        auth = hs.get_auth()
+        assert isinstance(auth, MSC3861DelegatedAuth)
+        self.msc3861_auth = auth
+        self.store = cast("GenericWorkerStore", hs.get_datastores().main)
+        self.hostname = hs.hostname
+
+    def assert_request_is_from_mas(self, request: "SynapseRequest") -> None:
+        """Assert that the request is coming from MAS itself, not a regular user.
+
+        Throws a 403 if the request is not coming from MAS.
+        """
+        if not self.msc3861_auth.is_request_using_the_admin_token(request):
+            raise SynapseError(403, "This endpoint must only be called by MAS")
--- a/synapse/rest/synapse/mas/devices.py
+++ b/synapse/rest/synapse/mas/devices.py
@@ -0,0 +1,238 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl_3.0.html>.
+#
+#
+
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Optional, Tuple
+
+from synapse._pydantic_compat import StrictStr
+from synapse.api.errors import NotFoundError
+from synapse.http.servlet import parse_and_validate_json_object_from_request
+from synapse.types import JsonDict, UserID
+from synapse.types.rest import RequestBodyModel
+
+if TYPE_CHECKING:
+    from synapse.http.site import SynapseRequest
+    from synapse.server import HomeServer
+
+
+from ._base import MasBaseResource
+
+logger = logging.getLogger(__name__)
+
+
+class MasUpsertDeviceResource(MasBaseResource):
+    """
+    Endpoint for MAS to create or update user devices.
+
+    Takes a localpart, device ID, and optional display name to create new devices
+    or update existing ones.
+
+    POST /_synapse/mas/upsert_device
+    {"localpart": "alice", "device_id": "DEVICE123", "display_name": "Alice's Phone"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.device_handler = hs.get_device_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        device_id: StrictStr
+        display_name: Optional[StrictStr]
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        inserted = await self.device_handler.upsert_device(
+            user_id=str(user_id),
+            device_id=body.device_id,
+            display_name=body.display_name,
+        )
+
+        return HTTPStatus.CREATED if inserted else HTTPStatus.OK, {}
+
+
+class MasDeleteDeviceResource(MasBaseResource):
+    """
+    Endpoint for MAS to delete user devices.
+
+    Takes a localpart and device ID to remove the specified device from the user's account.
+
+    POST /_synapse/mas/delete_device
+    {"localpart": "alice", "device_id": "DEVICE123"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.device_handler = hs.get_device_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        device_id: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        await self.device_handler.delete_devices(
+            user_id=str(user_id),
+            device_ids=[body.device_id],
+        )
+
+        return HTTPStatus.NO_CONTENT, {}
+
+
+class MasUpdateDeviceDisplayNameResource(MasBaseResource):
+    """
+    Endpoint for MAS to update a device's display name.
+
+    Takes a localpart, device ID, and new display name to update the device's name.
+
+    POST /_synapse/mas/update_device_display_name
+    {"localpart": "alice", "device_id": "DEVICE123", "display_name": "Alice's New Phone"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.device_handler = hs.get_device_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        device_id: StrictStr
+        display_name: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        await self.device_handler.update_device(
+            user_id=str(user_id),
+            device_id=body.device_id,
+            content={"display_name": body.display_name},
+        )
+
+        return HTTPStatus.OK, {}
+
+
+class MasSyncDevicesResource(MasBaseResource):
+    """
+    Endpoint for MAS to synchronize a user's complete device list.
+
+    Takes a localpart and a set of device IDs to ensure the user's device list
+    matches the provided set by adding missing devices and removing extra ones.
+
+    POST /_synapse/mas/sync_devices
+    {"localpart": "alice", "devices": ["DEVICE123", "DEVICE456"]}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.device_handler = hs.get_device_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        devices: set[StrictStr]
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        current_devices = await self.store.get_devices_by_user(user_id=str(user_id))
+        current_devices_list = set(current_devices.keys())
+        target_device_list = set(body.devices)
+
+        to_add = target_device_list - current_devices_list
+        to_delete = current_devices_list - target_device_list
+
+        # Log what we're about to do to make it easier to debug if it stops
+        # mid-way, as this can be a long operation if there are a lot of devices
+        # to delete or to add.
+        if to_add and to_delete:
+            logger.info(
+                "Syncing %d devices for user %s will add %d devices and delete %d devices",
+                len(target_device_list),
+                user_id,
+                len(to_add),
+                len(to_delete),
+            )
+        elif to_add:
+            logger.info(
+                "Syncing %d devices for user %s will add %d devices",
+                len(target_device_list),
+                user_id,
+                len(to_add),
+            )
+        elif to_delete:
+            logger.info(
+                "Syncing %d devices for user %s will delete %d devices",
+                len(target_device_list),
+                user_id,
+                len(to_delete),
+            )
+
+        if to_delete:
+            await self.device_handler.delete_devices(
+                user_id=str(user_id), device_ids=to_delete
+            )
+
+        for device_id in to_add:
+            await self.device_handler.upsert_device(
+                user_id=str(user_id),
+                device_id=device_id,
+            )
+
+        return 200, {}
--- a/synapse/rest/synapse/mas/users.py
+++ b/synapse/rest/synapse/mas/users.py
@@ -0,0 +1,467 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl_3.0.html>.
+#
+#
+
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Any, Optional, Tuple, TypedDict
+
+from synapse._pydantic_compat import StrictBool, StrictStr, root_validator
+from synapse.api.errors import NotFoundError, SynapseError
+from synapse.http.servlet import (
+    parse_and_validate_json_object_from_request,
+    parse_string,
+)
+from synapse.types import JsonDict, UserID, UserInfo, create_requester
+from synapse.types.rest import RequestBodyModel
+
+if TYPE_CHECKING:
+    from synapse.http.site import SynapseRequest
+    from synapse.server import HomeServer
+
+
+from ._base import MasBaseResource
+
+logger = logging.getLogger(__name__)
+
+
+class MasQueryUserResource(MasBaseResource):
+    """
+    Endpoint for MAS to query user information by localpart.
+
+    Takes a localpart parameter and returns user profile data including display name,
+    avatar URL, and account status (suspended/deactivated).
+
+    GET /_synapse/mas/query_user?localpart=alice
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+    class Response(TypedDict):
+        user_id: str
+        display_name: Optional[str]
+        avatar_url: Optional[str]
+        is_suspended: bool
+        is_deactivated: bool
+
+    async def _async_render_GET(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, Response]:
+        self.assert_request_is_from_mas(request)
+
+        localpart = parse_string(request, "localpart", required=True)
+        user_id = UserID(localpart, self.hostname)
+
+        user: Optional[UserInfo] = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        profile = await self.store.get_profileinfo(user_id=user_id)
+
+        return HTTPStatus.OK, self.Response(
+            user_id=user_id.to_string(),
+            display_name=profile.display_name,
+            avatar_url=profile.avatar_url,
+            is_suspended=user.suspended,
+            is_deactivated=user.is_deactivated,
+        )
+
+
+class MasProvisionUserResource(MasBaseResource):
+    """
+    Endpoint for MAS to create or update user accounts and their profile data.
+
+    Takes a localpart and optional profile fields (display name, avatar URL, email addresses).
+    Can create new users or update existing ones by setting or unsetting profile fields.
+
+    POST /_synapse/mas/provision_user
+    {"localpart": "alice", "set_displayname": "Alice", "set_emails": ["alice@example.com"]}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+        self.registration_handler = hs.get_registration_handler()
+        self.identity_handler = hs.get_identity_handler()
+        self.auth_handler = hs.get_auth_handler()
+        self.profile_handler = hs.get_profile_handler()
+        self.clock = hs.get_clock()
+        self.auth = hs.get_auth()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+
+        unset_displayname: StrictBool = False
+        set_displayname: Optional[StrictStr] = None
+
+        unset_avatar_url: StrictBool = False
+        set_avatar_url: Optional[StrictStr] = None
+
+        unset_emails: StrictBool = False
+        set_emails: Optional[list[StrictStr]] = None
+
+        @root_validator(pre=True)
+        def validate_exclusive(cls, values: Any) -> Any:
+            if "unset_displayname" in values and "set_displayname" in values:
+                raise ValueError(
+                    "Cannot specify both unset_displayname and set_displayname"
+                )
+            if "unset_avatar_url" in values and "set_avatar_url" in values:
+                raise ValueError(
+                    "Cannot specify both unset_avatar_url and set_avatar_url"
+                )
+            if "unset_emails" in values and "set_emails" in values:
+                raise ValueError("Cannot specify both unset_emails and set_emails")
+
+            return values
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+
+        localpart = body.localpart
+        user_id = UserID(localpart, self.hostname)
+
+        requester = create_requester(user_id=user_id)
+        existing_user = await self.store.get_user_by_id(user_id=str(user_id))
+        if existing_user is None:
+            created = True
+            await self.registration_handler.register_user(
+                localpart=localpart,
+                default_display_name=body.set_displayname,
+                bind_emails=body.set_emails,
+                by_admin=True,
+            )
+        else:
+            created = False
+            if body.unset_displayname:
+                await self.profile_handler.set_displayname(
+                    target_user=user_id,
+                    requester=requester,
+                    new_displayname="",
+                    by_admin=True,
+                )
+            elif body.set_displayname is not None:
+                await self.profile_handler.set_displayname(
+                    target_user=user_id,
+                    requester=requester,
+                    new_displayname=body.set_displayname,
+                    by_admin=True,
+                )
+
+            new_email_list: Optional[set[str]] = None
+            if body.unset_emails:
+                new_email_list = set()
+            elif body.set_emails is not None:
+                new_email_list = set(body.set_emails)
+
+            if new_email_list is not None:
+                medium = "email"
+                current_threepid_list = await self.store.user_get_threepids(
+                    user_id=user_id.to_string()
+                )
+                current_email_list = {
+                    t.address for t in current_threepid_list if t.medium == medium
+                }
+
+                to_delete = current_email_list - new_email_list
+                to_add = new_email_list - current_email_list
+
+                for address in to_delete:
+                    await self.identity_handler.try_unbind_threepid(
+                        mxid=user_id.to_string(),
+                        medium=medium,
+                        address=address,
+                        id_server=None,
+                    )
+
+                    await self.auth_handler.delete_local_threepid(
+                        user_id=user_id.to_string(),
+                        medium=medium,
+                        address=address,
+                    )
+
+                current_time = self.clock.time_msec()
+                for address in to_add:
+                    await self.auth_handler.add_threepid(
+                        user_id=user_id.to_string(),
+                        medium=medium,
+                        address=address,
+                        validated_at=current_time,
+                    )
+
+        if body.unset_avatar_url:
+            await self.profile_handler.set_avatar_url(
+                target_user=user_id,
+                requester=requester,
+                new_avatar_url="",
+                by_admin=True,
+            )
+        elif body.set_avatar_url is not None:
+            await self.profile_handler.set_avatar_url(
+                target_user=user_id,
+                requester=requester,
+                new_avatar_url=body.set_avatar_url,
+                by_admin=True,
+            )
+
+        return HTTPStatus.CREATED if created else HTTPStatus.OK, {}
+
+
+class MasIsLocalpartAvailableResource(MasBaseResource):
+    """
+    Endpoint for MAS to check if a localpart is available for user registration.
+
+    Takes a localpart parameter and validates its format and availability,
+    checking for conflicts with existing users or application service namespaces.
+
+    GET /_synapse/mas/is_localpart_available?localpart=alice
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+
+        self.registration_handler = hs.get_registration_handler()
+
+    async def _async_render_GET(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+        localpart = parse_string(request, "localpart")
+        if localpart is None:
+            raise SynapseError(400, "Missing localpart")
+
+        await self.registration_handler.check_username(localpart)
+
+        return HTTPStatus.OK, {}
+
+
+class MasDeleteUserResource(MasBaseResource):
+    """
+    Endpoint for MAS to delete/deactivate user accounts.
+
+    Takes a localpart and an erase flag to determine whether to deactivate
+    the account and optionally erase user data for compliance purposes.
+
+    POST /_synapse/mas/delete_user
+    {"localpart": "alice", "erase": true}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+
+        self.deactivate_account_handler = hs.get_deactivate_account_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        erase: StrictBool
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        await self.deactivate_account_handler.deactivate_account(
+            user_id=user_id.to_string(),
+            erase_data=body.erase,
+            requester=create_requester(user_id=user_id),
+        )
+
+        return HTTPStatus.OK, {}
+
+
+class MasReactivateUserResource(MasBaseResource):
+    """
+    Endpoint for MAS to reactivate previously deactivated user accounts.
+
+    Takes a localpart parameter to restore access to deactivated accounts.
+
+    POST /_synapse/mas/reactivate_user
+    {"localpart": "alice"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.deactivate_account_handler = hs.get_deactivate_account_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        await self.deactivate_account_handler.activate_account(user_id=str(user_id))
+
+        return HTTPStatus.OK, {}
+
+
+class MasSetDisplayNameResource(MasBaseResource):
+    """
+    Endpoint for MAS to set a user's display name.
+
+    Takes a localpart and display name to update the user's profile.
+
+    POST /_synapse/mas/set_displayname
+    {"localpart": "alice", "displayname": "Alice"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.profile_handler = hs.get_profile_handler()
+        self.auth_handler = hs.get_auth_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+        displayname: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        requester = create_requester(user_id=user_id)
+
+        await self.profile_handler.set_displayname(
+            target_user=requester.user,
+            requester=requester,
+            new_displayname=body.displayname,
+            by_admin=True,
+        )
+
+        return HTTPStatus.OK, {}
+
+
+class MasUnsetDisplayNameResource(MasBaseResource):
+    """
+    Endpoint for MAS to clear a user's display name.
+
+    Takes a localpart parameter to remove the display name for the specified user.
+
+    POST /_synapse/mas/unset_displayname
+    {"localpart": "alice"}
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.profile_handler = hs.get_profile_handler()
+        self.auth_handler = hs.get_auth_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        requester = create_requester(user_id=user_id)
+
+        await self.profile_handler.set_displayname(
+            target_user=requester.user,
+            requester=requester,
+            new_displayname="",
+            by_admin=True,
+        )
+
+        return HTTPStatus.OK, {}
+
+
+class MasAllowCrossSigningResetResource(MasBaseResource):
+    """
+    Endpoint for MAS to allow cross-signing key reset without user interaction.
+
+    Takes a localpart parameter to temporarily allow cross-signing key replacement
+    without requiring User-Interactive Authentication (UIA).
+
+    POST /_synapse/mas/allow_cross_signing_reset
+    {"localpart": "alice"}
+    """
+
+    REPLACEMENT_PERIOD_MS = 10 * 60 * 1000  # 10 minutes
+
+    def __init__(self, hs: "HomeServer"):
+        MasBaseResource.__init__(self, hs)
+
+        self.auth_handler = hs.get_auth_handler()
+
+    class PostBody(RequestBodyModel):
+        localpart: StrictStr
+
+    async def _async_render_POST(
+        self, request: "SynapseRequest"
+    ) -> Tuple[int, JsonDict]:
+        self.assert_request_is_from_mas(request)
+
+        body = parse_and_validate_json_object_from_request(request, self.PostBody)
+        user_id = UserID(body.localpart, self.hostname)
+
+        # Check the user exists
+        user = await self.store.get_user_by_id(user_id=str(user_id))
+        if user is None:
+            raise NotFoundError("User not found")
+
+        timestamp = (
+            await self.store.allow_master_cross_signing_key_replacement_without_uia(
+                user_id=str(user_id),
+                duration_ms=self.REPLACEMENT_PERIOD_MS,
+            )
+        )
+
+        if timestamp is None:
+            # If there are no cross-signing keys, this is a no-op, but we should log
+            logger.warning(
+                "User %s has no master cross-signing key", user_id.to_string()
+            )
+
+        return HTTPStatus.OK, {}
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -117,6 +117,7 @@ from synapse.handlers.sliding_sync import SlidingSyncHandler
 from synapse.handlers.sso import SsoHandler
 from synapse.handlers.stats import StatsHandler
 from synapse.handlers.sync import SyncHandler
+from synapse.handlers.thread_subscriptions import ThreadSubscriptionsHandler
 from synapse.handlers.typing import FollowerTypingHandler, TypingWriterHandler
 from synapse.handlers.user_directory import UserDirectoryHandler
 from synapse.handlers.worker_lock import WorkerLocksHandler
@@ -789,6 +790,10 @@ class HomeServer(metaclass=abc.ABCMeta):
    def get_timestamp_lookup_handler(self) -> TimestampLookupHandler:
        return TimestampLookupHandler(self)

+    @cache_in_self
+    def get_thread_subscriptions_handler(self) -> ThreadSubscriptionsHandler:
+        return ThreadSubscriptionsHandler(self)
+
    @cache_in_self
    def get_registration_handler(self) -> RegistrationHandler:
        return RegistrationHandler(self)
--- a/synapse/storage/databases/main/init.py
+++ b/synapse/storage/databases/main/init.py
@@ -19,7 +19,6 @@
 # [This file includes modifications made by New Vector Limited]
 #
 #
-
 import logging
 from typing import TYPE_CHECKING, List, Optional, Tuple, Union, cast

@@ -35,6 +34,9 @@ from synapse.storage.database import (
 )
 from synapse.storage.databases.main.sliding_sync import SlidingSyncStore
 from synapse.storage.databases.main.stats import UserSortOrder
+from synapse.storage.databases.main.thread_subscriptions import (
+    ThreadSubscriptionsWorkerStore,
+)
 from synapse.storage.engines import BaseDatabaseEngine
 from synapse.storage.types import Cursor
 from synapse.types import get_domain_from_id
@@ -141,6 +143,7 @@ class DataStore(
    SearchStore,
    TagsStore,
    AccountDataStore,
+    ThreadSubscriptionsWorkerStore,
    PushRulesWorkerStore,
    StreamWorkerStore,
    OpenIdStore,
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -2986,6 +2986,10 @@ class PersistEventsStore:
            # Upsert into the threads table, but only overwrite the value if the
            # new event is of a later topological order OR if the topological
            # ordering is equal, but the stream ordering is later.
+            # (Note by definition that the stream ordering will always be later
+            # unless this is a backfilled event [= negative stream ordering]
+            # because we are only persisting this event now and stream_orderings
+            # are strictly monotonically increasing)
            sql = """
            INSERT INTO threads (room_id, thread_id, latest_event_id, topological_ordering, stream_ordering)
            VALUES (?, ?, ?, ?, ?)
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -673,6 +673,7 @@ class RegistrationWorkerStore(StatsStore, CacheInvalidationWorkerStore):
            desc="delete_account_validity_for_user",
        )

+    @cached(max_entries=100000)
    async def is_server_admin(self, user: UserID) -> bool:
        """Determines if a user is an admin of this homeserver.

@@ -707,6 +708,9 @@ class RegistrationWorkerStore(StatsStore, CacheInvalidationWorkerStore):
            self._invalidate_cache_and_stream(
                txn, self.get_user_by_id, (user.to_string(),)
            )
+            self._invalidate_cache_and_stream(
+                txn, self.is_server_admin, (user.to_string(),)
+            )

        await self.db_pool.runInteraction("set_server_admin", set_server_admin_txn)

@@ -2596,6 +2600,36 @@ class RegistrationWorkerStore(StatsStore, CacheInvalidationWorkerStore):

        await self.db_pool.runInteraction("delete_access_token", f)

+    async def user_set_password_hash(
+        self, user_id: str, password_hash: Optional[str]
+    ) -> None:
+        """
+        NB. This does *not* evict any cache because the one use for this
+            removes most of the entries subsequently anyway so it would be
+            pointless. Use flush_user separately.
+        """
+
+        def user_set_password_hash_txn(txn: LoggingTransaction) -> None:
+            self.db_pool.simple_update_one_txn(
+                txn, "users", {"name": user_id}, {"password_hash": password_hash}
+            )
+            self._invalidate_cache_and_stream(txn, self.get_user_by_id, (user_id,))
+
+        await self.db_pool.runInteraction(
+            "user_set_password_hash", user_set_password_hash_txn
+        )
+
+    async def add_user_pending_deactivation(self, user_id: str) -> None:
+        """
+        Adds a user to the table of users who need to be parted from all the rooms they're
+        in
+        """
+        await self.db_pool.simple_insert(
+            "users_pending_deactivation",
+            values={"user_id": user_id},
+            desc="add_user_pending_deactivation",
+        )
+

 class RegistrationBackgroundUpdateStore(RegistrationWorkerStore):
    def __init__(
@@ -2820,25 +2854,6 @@ class RegistrationStore(RegistrationBackgroundUpdateStore):

        return next_id

-    async def user_set_password_hash(
-        self, user_id: str, password_hash: Optional[str]
-    ) -> None:
-        """
-        NB. This does *not* evict any cache because the one use for this
-            removes most of the entries subsequently anyway so it would be
-            pointless. Use flush_user separately.
-        """
-
-        def user_set_password_hash_txn(txn: LoggingTransaction) -> None:
-            self.db_pool.simple_update_one_txn(
-                txn, "users", {"name": user_id}, {"password_hash": password_hash}
-            )
-            self._invalidate_cache_and_stream(txn, self.get_user_by_id, (user_id,))
-
-        await self.db_pool.runInteraction(
-            "user_set_password_hash", user_set_password_hash_txn
-        )
-
    async def user_set_consent_version(
        self, user_id: str, consent_version: str
    ) -> None:
@@ -2891,17 +2906,6 @@ class RegistrationStore(RegistrationBackgroundUpdateStore):

        await self.db_pool.runInteraction("user_set_consent_server_notice_sent", f)

-    async def add_user_pending_deactivation(self, user_id: str) -> None:
-        """
-        Adds a user to the table of users who need to be parted from all the rooms they're
-        in
-        """
-        await self.db_pool.simple_insert(
-            "users_pending_deactivation",
-            values={"user_id": user_id},
-            desc="add_user_pending_deactivation",
-        )
-
    async def validate_threepid_session(
        self, session_id: str, client_secret: str, token: str, current_ts: int
    ) -> Optional[str]:
--- a/synapse/storage/databases/main/thread_subscriptions.py
+++ b/synapse/storage/databases/main/thread_subscriptions.py
@@ -0,0 +1,382 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+import logging
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Tuple,
+    Union,
+    cast,
+)
+
+import attr
+
+from synapse.replication.tcp.streams._base import ThreadSubscriptionsStream
+from synapse.storage.database import (
+    DatabasePool,
+    LoggingDatabaseConnection,
+    LoggingTransaction,
+)
+from synapse.storage.databases.main.cache import CacheInvalidationWorkerStore
+from synapse.storage.util.id_generators import MultiWriterIdGenerator
+from synapse.util.caches.descriptors import cached
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class ThreadSubscription:
+    automatic: bool
+    """
+    whether the subscription was made automatically (as opposed to by manual
+    action from the user)
+    """
+
+
+class ThreadSubscriptionsWorkerStore(CacheInvalidationWorkerStore):
+    def __init__(
+        self,
+        database: DatabasePool,
+        db_conn: LoggingDatabaseConnection,
+        hs: "HomeServer",
+    ):
+        super().__init__(database, db_conn, hs)
+
+        self._can_write_to_thread_subscriptions = (
+            self._instance_name in hs.config.worker.writers.thread_subscriptions
+        )
+
+        self._thread_subscriptions_id_gen: MultiWriterIdGenerator = (
+            MultiWriterIdGenerator(
+                db_conn=db_conn,
+                db=database,
+                notifier=hs.get_replication_notifier(),
+                stream_name="thread_subscriptions",
+                instance_name=self._instance_name,
+                tables=[
+                    ("thread_subscriptions", "instance_name", "stream_id"),
+                ],
+                sequence_name="thread_subscriptions_sequence",
+                writers=hs.config.worker.writers.thread_subscriptions,
+            )
+        )
+
+    def process_replication_rows(
+        self,
+        stream_name: str,
+        instance_name: str,
+        token: int,
+        rows: Iterable[Any],
+    ) -> None:
+        if stream_name == ThreadSubscriptionsStream.NAME:
+            for row in rows:
+                self.get_subscription_for_thread.invalidate(
+                    (row.user_id, row.room_id, row.event_id)
+                )
+
+        super().process_replication_rows(stream_name, instance_name, token, rows)
+
+    def process_replication_position(
+        self, stream_name: str, instance_name: str, token: int
+    ) -> None:
+        if stream_name == ThreadSubscriptionsStream.NAME:
+            self._thread_subscriptions_id_gen.advance(instance_name, token)
+        super().process_replication_position(stream_name, instance_name, token)
+
+    async def subscribe_user_to_thread(
+        self, user_id: str, room_id: str, thread_root_event_id: str, *, automatic: bool
+    ) -> Optional[int]:
+        """Updates a user's subscription settings for a specific thread root.
+
+        If no change would be made to the subscription, does not produce any database change.
+
+        Args:
+            user_id: The ID of the user whose settings are being updated.
+            room_id: The ID of the room the thread root belongs to.
+            thread_root_event_id: The event ID of the thread root.
+            automatic: Whether the subscription was performed automatically by the user's client.
+                Only `False` will overwrite an existing value of automatic for a subscription row.
+
+        Returns:
+            The stream ID for this update, if the update isn't no-opped.
+        """
+        assert self._can_write_to_thread_subscriptions
+
+        def _subscribe_user_to_thread_txn(txn: LoggingTransaction) -> Optional[int]:
+            already_automatic = self.db_pool.simple_select_one_onecol_txn(
+                txn,
+                table="thread_subscriptions",
+                keyvalues={
+                    "user_id": user_id,
+                    "event_id": thread_root_event_id,
+                    "room_id": room_id,
+                    "subscribed": True,
+                },
+                retcol="automatic",
+                allow_none=True,
+            )
+
+            if already_automatic is None:
+                already_subscribed = False
+                already_automatic = True
+            else:
+                already_subscribed = True
+                # convert int (SQLite bool) to Python bool
+                already_automatic = bool(already_automatic)
+
+            if already_subscribed and already_automatic == automatic:
+                # there is nothing we need to do here
+                return None
+
+            stream_id = self._thread_subscriptions_id_gen.get_next_txn(txn)
+
+            values: Dict[str, Optional[Union[bool, int, str]]] = {
+                "subscribed": True,
+                "stream_id": stream_id,
+                "instance_name": self._instance_name,
+                "automatic": already_automatic and automatic,
+            }
+
+            self.db_pool.simple_upsert_txn(
+                txn,
+                table="thread_subscriptions",
+                keyvalues={
+                    "user_id": user_id,
+                    "event_id": thread_root_event_id,
+                    "room_id": room_id,
+                },
+                values=values,
+            )
+
+            txn.call_after(
+                self.get_subscription_for_thread.invalidate,
+                (user_id, room_id, thread_root_event_id),
+            )
+
+            return stream_id
+
+        return await self.db_pool.runInteraction(
+            "subscribe_user_to_thread", _subscribe_user_to_thread_txn
+        )
+
+    async def unsubscribe_user_from_thread(
+        self, user_id: str, room_id: str, thread_root_event_id: str
+    ) -> Optional[int]:
+        """Unsubscribes a user from a thread.
+
+        If no change would be made to the subscription, does not produce any database change.
+
+        Args:
+            user_id: The ID of the user whose settings are being updated.
+            room_id: The ID of the room the thread root belongs to.
+            thread_root_event_id: The event ID of the thread root.
+
+        Returns:
+            The stream ID for this update, if the update isn't no-opped.
+        """
+
+        assert self._can_write_to_thread_subscriptions
+
+        def _unsubscribe_user_from_thread_txn(txn: LoggingTransaction) -> Optional[int]:
+            already_subscribed = self.db_pool.simple_select_one_onecol_txn(
+                txn,
+                table="thread_subscriptions",
+                keyvalues={
+                    "user_id": user_id,
+                    "event_id": thread_root_event_id,
+                    "room_id": room_id,
+                },
+                retcol="subscribed",
+                allow_none=True,
+            )
+
+            if already_subscribed is None or already_subscribed is False:
+                # there is nothing we need to do here
+                return None
+
+            stream_id = self._thread_subscriptions_id_gen.get_next_txn(txn)
+
+            self.db_pool.simple_update_txn(
+                txn,
+                table="thread_subscriptions",
+                keyvalues={
+                    "user_id": user_id,
+                    "event_id": thread_root_event_id,
+                    "room_id": room_id,
+                    "subscribed": True,
+                },
+                updatevalues={
+                    "subscribed": False,
+                    "stream_id": stream_id,
+                    "instance_name": self._instance_name,
+                },
+            )
+
+            txn.call_after(
+                self.get_subscription_for_thread.invalidate,
+                (user_id, room_id, thread_root_event_id),
+            )
+
+            return stream_id
+
+        return await self.db_pool.runInteraction(
+            "unsubscribe_user_from_thread", _unsubscribe_user_from_thread_txn
+        )
+
+    async def purge_thread_subscription_settings_for_user(self, user_id: str) -> None:
+        """
+        Purge all subscriptions for the user.
+        The fact that subscriptions have been purged will not be streamed;
+        all stream rows for the user will in fact be removed.
+        This is intended only for dealing with user deactivation.
+        """
+
+        def _purge_thread_subscription_settings_for_user_txn(
+            txn: LoggingTransaction,
+        ) -> None:
+            self.db_pool.simple_delete_txn(
+                txn,
+                table="thread_subscriptions",
+                keyvalues={"user_id": user_id},
+            )
+            self._invalidate_cache_and_stream(
+                txn, self.get_subscription_for_thread, (user_id,)
+            )
+
+        await self.db_pool.runInteraction(
+            desc="purge_thread_subscription_settings_for_user",
+            func=_purge_thread_subscription_settings_for_user_txn,
+        )
+
+    @cached(tree=True)
+    async def get_subscription_for_thread(
+        self, user_id: str, room_id: str, thread_root_event_id: str
+    ) -> Optional[ThreadSubscription]:
+        """Get the thread subscription for a specific thread and user.
+
+        Args:
+            user_id: The ID of the user
+            room_id: The ID of the room
+            thread_root_event_id: The event ID of the thread root
+
+        Returns:
+            A `ThreadSubscription` dataclass if there is a subscription,
+            or `None` if there is no subscription.
+
+            If there is a row in the table but `subscribed` is `False`,
+            behaves the same as if there was no row at all and returns `None`.
+        """
+        row = await self.db_pool.simple_select_one(
+            table="thread_subscriptions",
+            keyvalues={
+                "user_id": user_id,
+                "room_id": room_id,
+                "event_id": thread_root_event_id,
+                "subscribed": True,
+            },
+            retcols=("automatic",),
+            allow_none=True,
+            desc="get_subscription_for_thread",
+        )
+
+        if row is None:
+            return None
+
+        (automatic_rawbool,) = row
+
+        # convert SQLite integer booleans into real booleans
+        automatic = bool(automatic_rawbool)
+
+        return ThreadSubscription(automatic=automatic)
+
+    def get_max_thread_subscriptions_stream_id(self) -> int:
+        """Get the current maximum stream_id for thread subscriptions.
+
+        Returns:
+            The maximum stream_id
+        """
+        return self._thread_subscriptions_id_gen.get_current_token()
+
+    async def get_updated_thread_subscriptions(
+        self, from_id: int, to_id: int, limit: int
+    ) -> List[Tuple[int, str, str, str]]:
+        """Get updates to thread subscriptions between two stream IDs.
+
+        Args:
+            from_id: The starting stream ID (exclusive)
+            to_id: The ending stream ID (inclusive)
+            limit: The maximum number of rows to return
+
+        Returns:
+            list of (stream_id, user_id, room_id, thread_root_id) tuples
+        """
+
+        def get_updated_thread_subscriptions_txn(
+            txn: LoggingTransaction,
+        ) -> List[Tuple[int, str, str, str]]:
+            sql = """
+                SELECT stream_id, user_id, room_id, event_id
+                FROM thread_subscriptions
+                WHERE ? < stream_id AND stream_id <= ?
+                ORDER BY stream_id ASC
+                LIMIT ?
+            """
+
+            txn.execute(sql, (from_id, to_id, limit))
+            return cast(List[Tuple[int, str, str, str]], txn.fetchall())
+
+        return await self.db_pool.runInteraction(
+            "get_updated_thread_subscriptions",
+            get_updated_thread_subscriptions_txn,
+        )
+
+    async def get_updated_thread_subscriptions_for_user(
+        self, user_id: str, from_id: int, to_id: int, limit: int
+    ) -> List[Tuple[int, str, str]]:
+        """Get updates to thread subscriptions for a specific user.
+
+        Args:
+            user_id: The ID of the user
+            from_id: The starting stream ID (exclusive)
+            to_id: The ending stream ID (inclusive)
+            limit: The maximum number of rows to return
+
+        Returns:
+            A list of (stream_id, room_id, thread_root_event_id) tuples.
+        """
+
+        def get_updated_thread_subscriptions_for_user_txn(
+            txn: LoggingTransaction,
+        ) -> List[Tuple[int, str, str]]:
+            sql = """
+                SELECT stream_id, room_id, event_id
+                FROM thread_subscriptions
+                WHERE user_id = ? AND ? < stream_id AND stream_id <= ?
+                ORDER BY stream_id ASC
+                LIMIT ?
+            """
+
+            txn.execute(sql, (user_id, from_id, to_id, limit))
+            return [(row[0], row[1], row[2]) for row in txn]
+
+        return await self.db_pool.runInteraction(
+            "get_updated_thread_subscriptions_for_user",
+            get_updated_thread_subscriptions_for_user_txn,
+        )
--- a/synapse/storage/databases/main/user_erasure_store.py
+++ b/synapse/storage/databases/main/user_erasure_store.py
@@ -70,8 +70,6 @@ class UserErasureWorkerStore(CacheInvalidationWorkerStore):

        return {u: u in erased_users for u in user_ids}

-
-class UserErasureStore(UserErasureWorkerStore):
    async def mark_user_erased(self, user_id: str) -> None:
        """Indicate that user_id wishes their message history to be erased.

@@ -113,3 +111,7 @@ class UserErasureStore(UserErasureWorkerStore):
            self._invalidate_cache_and_stream(txn, self.is_user_erased, (user_id,))

        await self.db_pool.runInteraction("mark_user_not_erased", f)
+
+
+class UserErasureStore(UserErasureWorkerStore):
+    pass
--- a/synapse/storage/schema/main/delta/92/04_thread_subscriptions.sql
+++ b/synapse/storage/schema/main/delta/92/04_thread_subscriptions.sql
@@ -0,0 +1,59 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+-- Introduce a table for tracking users' subscriptions to threads.
+CREATE TABLE thread_subscriptions (
+  stream_id INTEGER NOT NULL PRIMARY KEY,
+  instance_name TEXT NOT NULL,
+
+  room_id TEXT NOT NULL,
+  event_id TEXT NOT NULL,
+  user_id TEXT NOT NULL,
+
+  subscribed BOOLEAN NOT NULL,
+  automatic BOOLEAN NOT NULL,
+
+  CONSTRAINT thread_subscriptions_fk_users
+    FOREIGN KEY (user_id)
+    REFERENCES users(name),
+
+  CONSTRAINT thread_subscriptions_fk_rooms
+    FOREIGN KEY (room_id)
+    -- When we delete a room, we should already have deleted all the events in that room
+    -- and so there shouldn't be any subscriptions left in that room.
+    -- So the `ON DELETE CASCADE` should be optional, but included anyway for good measure.
+    REFERENCES rooms(room_id) ON DELETE CASCADE,
+
+  CONSTRAINT thread_subscriptions_fk_events
+    FOREIGN KEY (event_id)
+    REFERENCES events(event_id) ON DELETE CASCADE,
+
+  -- This order provides a useful index for:
+  -- 1. foreign key constraint on (room_id)
+  -- 2. foreign key constraint on (room_id, event_id)
+  -- 3. finding the user's settings for a specific thread (as well as enforcing uniqueness)
+  UNIQUE (room_id, event_id, user_id)
+);
+
+-- this provides a useful index for finding a user's own rules,
+-- potentially scoped to a single room
+CREATE INDEX thread_subscriptions_user_room ON thread_subscriptions (user_id, room_id);
+
+-- this provides a useful way for clients to efficiently find new changes to
+-- their subscriptions.
+-- (This is necessary to sync subscriptions between multiple devices.)
+CREATE INDEX thread_subscriptions_by_user ON thread_subscriptions (user_id, stream_id);
+
+-- this provides a useful index for deleting the subscriptions when the underlying
+-- events are removed. This also covers the foreign key constraint on `events`.
+CREATE INDEX thread_subscriptions_by_event ON thread_subscriptions (event_id);
--- a/synapse/storage/schema/main/delta/92/04_thread_subscriptions_seq.sql.postgres
+++ b/synapse/storage/schema/main/delta/92/04_thread_subscriptions_seq.sql.postgres
@@ -0,0 +1,19 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+CREATE SEQUENCE thread_subscriptions_sequence
+  -- Synapse streams start at 2, because the default position is 1
+  -- so any item inserted at position 1 is ignored.
+  -- This is also what existing streams do, except they use `setval(..., 1)`
+  -- which is semantically the same except less obvious.
+  START WITH 2;
--- a/synapse/storage/schema/main/delta/92/05_thread_subscriptions_comments.sql.postgres
+++ b/synapse/storage/schema/main/delta/92/05_thread_subscriptions_comments.sql.postgres
@@ -0,0 +1,18 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+COMMENT ON TABLE thread_subscriptions IS 'Tracks local users that subscribe to threads';
+
+COMMENT ON COLUMN thread_subscriptions.subscribed IS 'Whether the user is subscribed to the thread or not. We track unsubscribed threads because we need to stream the subscription change to the client.';
+
+COMMENT ON COLUMN thread_subscriptions.automatic IS 'True if the user was subscribed to the thread automatically by their client, or false if the client manually requested the subscription.';
--- a/synapse/storage/schema/main/delta/92/06_threads_last_sent_stream_ordering_comments.sql.postgres
+++ b/synapse/storage/schema/main/delta/92/06_threads_last_sent_stream_ordering_comments.sql.postgres
@@ -0,0 +1,24 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+COMMENT ON COLUMN threads.latest_event_id IS
+    'the ID of the event that is latest, ordered by (topological_ordering, stream_ordering)';
+
+COMMENT ON COLUMN threads.topological_ordering IS
+    $$the topological ordering of the thread''s LATEST event.
+Used as the primary way of ordering threads by recency in a room.$$;
+
+COMMENT ON COLUMN threads.stream_ordering IS
+    $$the stream ordering of the thread's LATEST event.
+Used as a tie-breaker for ordering threads by recency in a room, when the topological order is a tie.
+Also used for recency ordering in sliding sync.$$;
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -184,6 +184,12 @@ class MultiWriterIdGenerator(AbstractStreamIdGenerator):

    Note: Only works with Postgres.

+    Warning: Streams using this generator start at ID 2, because ID 1 is always assumed
+        to have been 'seen as persisted'.
+        Unclear if this extant behaviour is desirable for some reason.
+        When creating a new sequence for a new stream,
+        it will be necessary to use `START WITH 2`.
+
    Args:
        db_conn
        db
@@ -269,6 +275,9 @@ class MultiWriterIdGenerator(AbstractStreamIdGenerator):
        self._known_persisted_positions: List[int] = []

        # The maximum stream ID that we have seen been allocated across any writer.
+        # Since this defaults to 1, this means that ID 1 is assumed to have already
+        # been 'seen'. In other words, multi-writer streams start at 2.
+        # Unclear if this is desirable behaviour.
        self._max_seen_allocated_stream_id = 1

        # The maximum position of the local instance. This can be higher than
--- a/synapse/types/init.py
+++ b/synapse/types/init.py
@@ -362,7 +362,8 @@ class RoomID(DomainSpecificString):

@attr.s(slots=True, frozen=True, repr=False)
 class EventID(DomainSpecificString):
-    """Structure representing an event id."""
+    """Structure representing an event ID which is namespaced to a homeserver.
+    Room versions 3 and above are not supported by this grammar."""

    SIGIL = "$"

--- a/tests/replication/tcp/streams/test_thread_subscriptions.py
+++ b/tests/replication/tcp/streams/test_thread_subscriptions.py
@@ -0,0 +1,157 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.replication.tcp.streams._base import (
+    _STREAM_UPDATE_TARGET_ROW_COUNT,
+    ThreadSubscriptionsStream,
+)
+from synapse.server import HomeServer
+from synapse.storage.database import LoggingTransaction
+from synapse.util import Clock
+
+from tests.replication._base import BaseStreamTestCase
+
+
+class ThreadSubscriptionsStreamTestCase(BaseStreamTestCase):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        super().prepare(reactor, clock, hs)
+
+        # Postgres
+        def f(txn: LoggingTransaction) -> None:
+            txn.execute(
+                """
+                ALTER TABLE thread_subscriptions
+                    DROP CONSTRAINT thread_subscriptions_fk_users,
+                    DROP CONSTRAINT thread_subscriptions_fk_rooms,
+                    DROP CONSTRAINT thread_subscriptions_fk_events;
+                """,
+            )
+
+        self.get_success(
+            self.hs.get_datastores().main.db_pool.runInteraction(
+                "disable_foreign_keys", f
+            )
+        )
+
+    def test_thread_subscription_updates(self) -> None:
+        """Test replication with thread subscription updates"""
+        store = self.hs.get_datastores().main
+
+        # Create thread subscription updates
+        updates = []
+        room_id = "!test_room:example.com"
+
+        # Generate several thread subscription updates
+        for i in range(_STREAM_UPDATE_TARGET_ROW_COUNT + 5):
+            thread_root_id = f"$thread_{i}:example.com"
+            self.get_success(
+                store.subscribe_user_to_thread(
+                    "@test_user:example.org",
+                    room_id,
+                    thread_root_id,
+                    automatic=True,
+                )
+            )
+            updates.append(thread_root_id)
+
+        # Also add one in a different room
+        other_room_id = "!other_room:example.com"
+        other_thread_root_id = "$other_thread:example.com"
+        self.get_success(
+            store.subscribe_user_to_thread(
+                "@test_user:example.org",
+                other_room_id,
+                other_thread_root_id,
+                automatic=False,
+            )
+        )
+
+        # Not yet connected: no rows should yet have been received
+        self.assertEqual([], self.test_handler.received_rdata_rows)
+
+        # Now reconnect to pull the updates
+        self.reconnect()
+        self.replicate()
+
+        # We should have received all the expected rows in the right order
+        # Filter the updates to only include thread subscription changes
+        received_rows = [
+            upd
+            for upd in self.test_handler.received_rdata_rows
+            if upd[0] == ThreadSubscriptionsStream.NAME
+        ]
+
+        # Verify all the thread subscription updates
+        for thread_id in updates:
+            (stream_name, token, row) = received_rows.pop(0)
+            self.assertEqual(stream_name, ThreadSubscriptionsStream.NAME)
+            self.assertIsInstance(row, ThreadSubscriptionsStream.ROW_TYPE)
+            self.assertEqual(row.user_id, "@test_user:example.org")
+            self.assertEqual(row.room_id, room_id)
+            self.assertEqual(row.event_id, thread_id)
+
+        # Verify the last update in the different room
+        (stream_name, token, row) = received_rows.pop(0)
+        self.assertEqual(stream_name, ThreadSubscriptionsStream.NAME)
+        self.assertIsInstance(row, ThreadSubscriptionsStream.ROW_TYPE)
+        self.assertEqual(row.user_id, "@test_user:example.org")
+        self.assertEqual(row.room_id, other_room_id)
+        self.assertEqual(row.event_id, other_thread_root_id)
+
+        self.assertEqual([], received_rows)
+
+    def test_multiple_users_thread_subscription_updates(self) -> None:
+        """Test replication with thread subscription updates for multiple users"""
+        store = self.hs.get_datastores().main
+        room_id = "!test_room:example.com"
+        thread_root_id = "$thread_root:example.com"
+
+        # Create updates for multiple users
+        users = ["@user1:example.com", "@user2:example.com", "@user3:example.com"]
+        for user_id in users:
+            self.get_success(
+                store.subscribe_user_to_thread(
+                    user_id, room_id, thread_root_id, automatic=True
+                )
+            )
+
+        # Check no rows have been received yet
+        self.replicate()
+        self.assertEqual([], self.test_handler.received_rdata_rows)
+
+        # Not yet connected: no rows should yet have been received
+        self.reconnect()
+        self.replicate()
+
+        # We should have received all the expected rows
+        # Filter the updates to only include thread subscription changes
+        received_rows = [
+            upd
+            for upd in self.test_handler.received_rdata_rows
+            if upd[0] == ThreadSubscriptionsStream.NAME
+        ]
+
+        # Should have one update per user
+        self.assertEqual(len(received_rows), len(users))
+
+        # Verify all updates
+        for i, user_id in enumerate(users):
+            (stream_name, token, row) = received_rows[i]
+            self.assertEqual(stream_name, ThreadSubscriptionsStream.NAME)
+            self.assertIsInstance(row, ThreadSubscriptionsStream.ROW_TYPE)
+            self.assertEqual(row.user_id, user_id)
+            self.assertEqual(row.room_id, room_id)
+            self.assertEqual(row.event_id, thread_root_id)
--- a/tests/rest/admin/test_user.py
+++ b/tests/rest/admin/test_user.py
@@ -5667,6 +5667,54 @@ class UserRedactionTestCase(unittest.HomeserverTestCase):
                    matched.append(event_id)
        self.assertEqual(len(matched), len(originals))

+    def test_use_admin_param_for_redactions(self) -> None:
+        """
+        Test that if the `use_admin` param is set to true, the admin user is used to issue
+        the redactions and that they succeed in a room where the admin user has sufficient
+        power to issue redactions
+        """
+
+        originals = []
+        join = self.helper.join(self.rm1, self.bad_user, tok=self.bad_user_tok)
+        originals.append(join["event_id"])
+        for i in range(15):
+            event = {"body": f"hello{i}", "msgtype": "m.text"}
+            res = self.helper.send_event(
+                self.rm1, "m.room.message", event, tok=self.bad_user_tok
+            )
+            originals.append(res["event_id"])
+
+        # redact messages
+        channel = self.make_request(
+            "POST",
+            f"/_synapse/admin/v1/user/{self.bad_user}/redact",
+            content={"rooms": [self.rm1], "use_admin": True},
+            access_token=self.admin_tok,
+        )
+        self.assertEqual(channel.code, 200)
+
+        # messages are redacted, and redactions are issued by the admin user
+        filter = json.dumps({"types": [EventTypes.Redaction]})
+        channel = self.make_request(
+            "GET",
+            f"rooms/{self.rm1}/messages?filter={filter}&limit=50",
+            access_token=self.admin_tok,
+        )
+        self.assertEqual(channel.code, 200)
+
+        matches = []
+        for event in channel.json_body["chunk"]:
+            for event_id in originals:
+                if event["type"] == "m.room.redaction" and event["redacts"] == event_id:
+                    matches.append((event_id, event))
+        # we redacted 16 messages
+        self.assertEqual(len(matches), 16)
+
+        for redaction_tuple in matches:
+            redaction = redaction_tuple[1]
+            if redaction["sender"] != self.admin:
+                self.fail("Redaction was not issued by admin account")
+

 class UserRedactionBackgroundTaskTestCase(BaseMultiWorkerStreamTestCase):
    servlets = [
--- a/tests/rest/client/test_relations.py
+++ b/tests/rest/client/test_relations.py
@@ -1181,7 +1181,7 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):
                bundled_aggregations,
            )

-        self._test_bundled_aggregations(RelationTypes.REFERENCE, assert_annotations, 8)
+        self._test_bundled_aggregations(RelationTypes.REFERENCE, assert_annotations, 7)

    def test_thread(self) -> None:
        """
@@ -1226,21 +1226,21 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):

        # The "user" sent the root event and is making queries for the bundled
        # aggregations: they have participated.
-        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(True), 9)
+        self._test_bundled_aggregations(RelationTypes.THREAD, _gen_assert(True), 7)
        # The "user2" sent replies in the thread and is making queries for the
        # bundled aggregations: they have participated.
        #
        # Note that this re-uses some cached values, so the total number of
        # queries is much smaller.
        self._test_bundled_aggregations(
-            RelationTypes.THREAD, _gen_assert(True), 6, access_token=self.user2_token
+            RelationTypes.THREAD, _gen_assert(True), 4, access_token=self.user2_token
        )

        # A user with no interactions with the thread: they have not participated.
        user3_id, user3_token = self._create_user("charlie")
        self.helper.join(self.room, user=user3_id, tok=user3_token)
        self._test_bundled_aggregations(
-            RelationTypes.THREAD, _gen_assert(False), 6, access_token=user3_token
+            RelationTypes.THREAD, _gen_assert(False), 4, access_token=user3_token
        )

    def test_thread_with_bundled_aggregations_for_latest(self) -> None:
@@ -1287,7 +1287,7 @@ class BundledAggregationsTestCase(BaseRelationsTestCase):
                bundled_aggregations["latest_event"].get("unsigned"),
            )

-        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 9)
+        self._test_bundled_aggregations(RelationTypes.THREAD, assert_thread, 7)

    def test_nested_thread(self) -> None:
        """
--- a/tests/rest/client/test_thread_subscriptions.py
+++ b/tests/rest/client/test_thread_subscriptions.py
@@ -0,0 +1,256 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+from http import HTTPStatus
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.rest import admin
+from synapse.rest.client import login, profile, room, thread_subscriptions
+from synapse.server import HomeServer
+from synapse.types import JsonDict
+from synapse.util import Clock
+
+from tests import unittest
+
+PREFIX = "/_matrix/client/unstable/io.element.msc4306/rooms"
+
+
+class ThreadSubscriptionsTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        admin.register_servlets_for_client_rest_resource,
+        login.register_servlets,
+        profile.register_servlets,
+        room.register_servlets,
+        thread_subscriptions.register_servlets,
+    ]
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        config["experimental_features"] = {"msc4306_enabled": True}
+        return config
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.user_id = self.register_user("user", "password")
+        self.token = self.login("user", "password")
+        self.other_user_id = self.register_user("other_user", "password")
+        self.other_token = self.login("other_user", "password")
+
+        # Create a room and send a message to use as a thread root
+        self.room_id = self.helper.create_room_as(self.user_id, tok=self.token)
+        self.helper.join(self.room_id, self.other_user_id, tok=self.other_token)
+        response = self.helper.send(self.room_id, body="Root message", tok=self.token)
+        self.root_event_id = response["event_id"]
+
+        # Send a message in the thread
+        self.helper.send_event(
+            room_id=self.room_id,
+            type="m.room.message",
+            content={
+                "body": "Thread message",
+                "msgtype": "m.text",
+                "m.relates_to": {
+                    "rel_type": "m.thread",
+                    "event_id": self.root_event_id,
+                },
+            },
+            tok=self.token,
+        )
+
+    def test_get_thread_subscription_unsubscribed(self) -> None:
+        """Test retrieving thread subscription when not subscribed."""
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_get_thread_subscription_nonexistent_thread(self) -> None:
+        """Test retrieving subscription settings for a nonexistent thread."""
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/$nonexistent:example.org/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_get_thread_subscription_no_access(self) -> None:
+        """Test that a user can't get thread subscription for a thread they can't access."""
+        self.register_user("no_access", "password")
+        no_access_token = self.login("no_access", "password")
+
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=no_access_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_subscribe_manual_then_automatic(self) -> None:
+        """Test subscribing to a thread, first a manual subscription then an automatic subscription.
+        The manual subscription wins over the automatic one."""
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {
+                "automatic": False,
+            },
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the subscription was saved
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertEqual(channel.json_body, {"automatic": False})
+
+        # Now also register an automatic subscription; it should not
+        # override the manual subscription
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {"automatic": True},
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the manual subscription was not overridden
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertEqual(channel.json_body, {"automatic": False})
+
+    def test_subscribe_automatic_then_manual(self) -> None:
+        """Test subscribing to a thread, first an automatic subscription then a manual subscription.
+        The manual subscription wins over the automatic one."""
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {
+                "automatic": True,
+            },
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the subscription was saved
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertEqual(channel.json_body, {"automatic": True})
+
+        # Now also register a manual subscription
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {"automatic": False},
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the manual subscription was not overridden
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertEqual(channel.json_body, {"automatic": False})
+
+    def test_unsubscribe(self) -> None:
+        """Test subscribing to a thread, then unsubscribing."""
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {
+                "automatic": True,
+            },
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the subscription was saved
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertEqual(channel.json_body, {"automatic": True})
+
+        # Now also register a manual subscription
+        channel = self.make_request(
+            "DELETE",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK)
+
+        # Assert the manual subscription was not overridden
+        channel = self.make_request(
+            "GET",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_set_thread_subscription_nonexistent_thread(self) -> None:
+        """Test setting subscription settings for a nonexistent thread."""
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/$nonexistent:example.org/subscription",
+            {"automatic": True},
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_set_thread_subscription_no_access(self) -> None:
+        """Test that a user can't set thread subscription for a thread they can't access."""
+        self.register_user("no_access2", "password")
+        no_access_token = self.login("no_access2", "password")
+
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            {"automatic": True},
+            access_token=no_access_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.NOT_FOUND)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    def test_invalid_body(self) -> None:
+        """Test that sending invalid subscription settings is rejected."""
+        channel = self.make_request(
+            "PUT",
+            f"{PREFIX}/{self.room_id}/thread/{self.root_event_id}/subscription",
+            # non-boolean `automatic`
+            {"automatic": "true"},
+            access_token=self.token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.BAD_REQUEST)
--- a/tests/rest/synapse/mas/init.py
+++ b/tests/rest/synapse/mas/init.py
@@ -0,0 +1,12 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
--- a/tests/rest/synapse/mas/_base.py
+++ b/tests/rest/synapse/mas/_base.py
@@ -0,0 +1,43 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+from twisted.web.resource import Resource
+
+from synapse.rest.synapse.client import build_synapse_client_resource_tree
+from synapse.types import JsonDict
+
+from tests import unittest
+
+
+class BaseTestCase(unittest.HomeserverTestCase):
+    SHARED_SECRET = "shared_secret"
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        config["enable_registration"] = False
+        config["experimental_features"] = {
+            "msc3861": {
+                "enabled": True,
+                "issuer": "https://example.com",
+                "client_id": "dummy",
+                "client_auth_method": "client_secret_basic",
+                "client_secret": "dummy",
+                "admin_token": self.SHARED_SECRET,
+            }
+        }
+        return config
+
+    def create_resource_dict(self) -> dict[str, Resource]:
+        base = super().create_resource_dict()
+        base.update(build_synapse_client_resource_tree(self.hs))
+        return base
--- a/tests/rest/synapse/mas/test_devices.py
+++ b/tests/rest/synapse/mas/test_devices.py
@@ -0,0 +1,693 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.server import HomeServer
+from synapse.types import UserID
+from synapse.util import Clock
+
+from tests.unittest import skip_unless
+from tests.utils import HAS_AUTHLIB
+
+from ._base import BaseTestCase
+
+
+@skip_unless(HAS_AUTHLIB, "requires authlib")
+class MasUpsertDeviceResource(BaseTestCase):
+    def prepare(
+        self, reactor: MemoryReactor, clock: Clock, homeserver: HomeServer
+    ) -> None:
+        # Create a user for testing
+        self.alice_user_id = UserID("alice", "test")
+        self.get_success(
+            homeserver.get_registration_handler().register_user(
+                localpart=self.alice_user_id.localpart,
+            )
+        )
+
+    def test_other_token(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token="other_token",
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 403, channel.json_body)
+        self.assertEqual(
+            channel.json_body["error"], "This endpoint must only be called by MAS"
+        )
+
+    def test_upsert_device(self) -> None:
+        store = self.hs.get_datastores().main
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        # This created a new device, hence the 201 status code
+        self.assertEqual(channel.code, 201, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the device exists
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+        self.assertEqual(device["device_id"], "DEVICE1")
+        self.assertIsNone(device["display_name"])
+
+    def test_update_existing_device(self) -> None:
+        store = self.hs.get_datastores().main
+        device_handler = self.hs.get_device_handler()
+
+        # Create an initial device
+        self.get_success(
+            device_handler.upsert_device(
+                user_id=str(self.alice_user_id),
+                device_id="DEVICE1",
+                display_name="Old Name",
+            )
+        )
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+                "display_name": "New Name",
+            },
+        )
+
+        # This updated an existing device, hence the 200 status code
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the device was updated
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+        self.assertEqual(device["display_name"], "New Name")
+
+    def test_upsert_device_with_display_name(self) -> None:
+        store = self.hs.get_datastores().main
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+                "display_name": "Alice's Phone",
+            },
+        )
+
+        self.assertEqual(channel.code, 201, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the device exists with correct display name
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+        self.assertEqual(device["display_name"], "Alice's Phone")
+
+    def test_upsert_device_missing_localpart(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_upsert_device_missing_device_id(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_upsert_device_nonexistent_user(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/upsert_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "nonexistent",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        # We get a 404 here as the user doesn't exist
+        self.assertEqual(channel.code, 404, channel.json_body)
+
+
+@skip_unless(HAS_AUTHLIB, "requires authlib")
+class MasDeleteDeviceResource(BaseTestCase):
+    def prepare(
+        self, reactor: MemoryReactor, clock: Clock, homeserver: HomeServer
+    ) -> None:
+        # Create a user and device for testing
+        self.alice_user_id = UserID("alice", "test")
+        self.get_success(
+            homeserver.get_registration_handler().register_user(
+                localpart=self.alice_user_id.localpart,
+            )
+        )
+
+        # Create a device
+        device_handler = homeserver.get_device_handler()
+        self.get_success(
+            device_handler.upsert_device(
+                user_id=str(self.alice_user_id),
+                device_id="DEVICE1",
+                display_name="Test Device",
+            )
+        )
+
+    def test_other_token(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token="other_token",
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 403, channel.json_body)
+        self.assertEqual(
+            channel.json_body["error"], "This endpoint must only be called by MAS"
+        )
+
+    def test_delete_device(self) -> None:
+        store = self.hs.get_datastores().main
+
+        # Verify device exists before deletion
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 204)
+
+        # Verify the device no longer exists
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        self.assertIsNone(device)
+
+    def test_delete_nonexistent_device(self) -> None:
+        # Deleting a non-existent device should be idempotent
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "NONEXISTENT",
+            },
+        )
+
+        self.assertEqual(channel.code, 204)
+
+    def test_delete_device_missing_localpart(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_delete_device_missing_device_id(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_delete_device_nonexistent_user(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/delete_device",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "nonexistent",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        # Should fail on a non-existent user
+        self.assertEqual(channel.code, 404, channel.json_body)
+
+
+@skip_unless(HAS_AUTHLIB, "requires authlib")
+class MasUpdateDeviceDisplayNameResource(BaseTestCase):
+    def prepare(
+        self, reactor: MemoryReactor, clock: Clock, homeserver: HomeServer
+    ) -> None:
+        # Create a user and device for testing
+        self.alice_user_id = UserID("alice", "test")
+        self.get_success(
+            homeserver.get_registration_handler().register_user(
+                localpart=self.alice_user_id.localpart,
+            )
+        )
+
+        # Create a device
+        device_handler = homeserver.get_device_handler()
+        self.get_success(
+            device_handler.upsert_device(
+                user_id=str(self.alice_user_id),
+                device_id="DEVICE1",
+                display_name="Old Name",
+            )
+        )
+
+    def test_other_token(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token="other_token",
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+                "display_name": "New Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 403, channel.json_body)
+        self.assertEqual(
+            channel.json_body["error"], "This endpoint must only be called by MAS"
+        )
+
+    def test_update_device_display_name(self) -> None:
+        store = self.hs.get_datastores().main
+
+        # Verify initial display name
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+        self.assertEqual(device["display_name"], "Old Name")
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+                "display_name": "Updated Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the display name was updated
+        device = self.get_success(store.get_device(str(self.alice_user_id), "DEVICE1"))
+        assert device is not None
+        self.assertEqual(device["display_name"], "Updated Name")
+
+    def test_update_nonexistent_device(self) -> None:
+        # Updating a non-existent device should fail
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "NONEXISTENT",
+                "display_name": "New Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 404, channel.json_body)
+
+    def test_update_device_display_name_missing_localpart(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "device_id": "DEVICE1",
+                "display_name": "New Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_update_device_display_name_missing_device_id(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "display_name": "New Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_update_device_display_name_missing_display_name(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "device_id": "DEVICE1",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_update_device_display_name_nonexistent_user(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/update_device_display_name",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "nonexistent",
+                "device_id": "DEVICE1",
+                "display_name": "New Name",
+            },
+        )
+
+        self.assertEqual(channel.code, 404, channel.json_body)
+
+
+@skip_unless(HAS_AUTHLIB, "requires authlib")
+class MasSyncDevicesResource(BaseTestCase):
+    def prepare(
+        self, reactor: MemoryReactor, clock: Clock, homeserver: HomeServer
+    ) -> None:
+        # Create a user for testing
+        self.alice_user_id = UserID("alice", "test")
+        self.get_success(
+            homeserver.get_registration_handler().register_user(
+                localpart=self.alice_user_id.localpart,
+            )
+        )
+
+        # Create some initial devices
+        device_handler = homeserver.get_device_handler()
+        for device_id in ["DEVICE1", "DEVICE2", "DEVICE3"]:
+            self.get_success(
+                device_handler.upsert_device(
+                    user_id=str(self.alice_user_id),
+                    device_id=device_id,
+                    display_name=f"Device {device_id}",
+                )
+            )
+
+    def test_other_token(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token="other_token",
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1", "DEVICE2"],
+            },
+        )
+
+        self.assertEqual(channel.code, 403, channel.json_body)
+        self.assertEqual(
+            channel.json_body["error"], "This endpoint must only be called by MAS"
+        )
+
+    def test_sync_devices_no_changes(self) -> None:
+        # Sync with the same devices that already exist
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1", "DEVICE2", "DEVICE3"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify all devices still exist
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(set(devices.keys()), {"DEVICE1", "DEVICE2", "DEVICE3"})
+
+    def test_sync_devices_add_only(self) -> None:
+        # Sync with additional devices
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1", "DEVICE2", "DEVICE3", "DEVICE4", "DEVICE5"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify new devices were added
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(
+            set(devices.keys()), {"DEVICE1", "DEVICE2", "DEVICE3", "DEVICE4", "DEVICE5"}
+        )
+
+    def test_sync_devices_delete_only(self) -> None:
+        # Sync with fewer devices
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify devices were deleted
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(set(devices.keys()), {"DEVICE1"})
+
+    def test_sync_devices_add_and_delete(self) -> None:
+        # Sync with a mix of additions and deletions
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1", "DEVICE4", "DEVICE5"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the correct devices exist
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(set(devices.keys()), {"DEVICE1", "DEVICE4", "DEVICE5"})
+
+    def test_sync_devices_empty_list(self) -> None:
+        # Sync with empty device list (delete all devices)
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": [],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify all devices were deleted
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(devices, {})
+
+    def test_sync_devices_for_new_user(self) -> None:
+        # Test syncing devices for a user that doesn't have any devices yet
+        bob_user_id = UserID("bob", "test")
+        self.get_success(
+            self.hs.get_registration_handler().register_user(
+                localpart=bob_user_id.localpart,
+            )
+        )
+
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "bob",
+                "devices": ["DEVICE1", "DEVICE2"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify devices were created
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(bob_user_id)))
+        self.assertEqual(set(devices.keys()), {"DEVICE1", "DEVICE2"})
+
+    def test_sync_devices_missing_localpart(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "devices": ["DEVICE1", "DEVICE2"],
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_sync_devices_missing_devices(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_sync_devices_invalid_devices_type(self) -> None:
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": "not_a_list",
+            },
+        )
+
+        self.assertEqual(channel.code, 400, channel.json_body)
+
+    def test_sync_devices_nonexistent_user(self) -> None:
+        # Test syncing devices for a user that doesn't exist
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "nonexistent",
+                "devices": ["DEVICE1", "DEVICE2"],
+            },
+        )
+
+        self.assertEqual(channel.code, 404, channel.json_body)
+
+    def test_sync_devices_duplicate_device_ids(self) -> None:
+        # Test syncing with duplicate device IDs (sets should handle this)
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/sync_devices",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": "alice",
+                "devices": ["DEVICE1", "DEVICE1", "DEVICE2"],
+            },
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+
+        # Verify the correct devices exist (duplicates should be handled)
+        store = self.hs.get_datastores().main
+        devices = self.get_success(store.get_devices_by_user(str(self.alice_user_id)))
+        self.assertEqual(sorted(devices.keys()), ["DEVICE1", "DEVICE2"])
--- a/tests/rest/synapse/mas/test_users.py
+++ b/tests/rest/synapse/mas/test_users.py
--- a/tests/storage/test_thread_subscriptions.py
+++ b/tests/storage/test_thread_subscriptions.py
@@ -0,0 +1,272 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+
+from typing import Optional
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.server import HomeServer
+from synapse.storage.database import LoggingTransaction
+from synapse.storage.engines.sqlite import Sqlite3Engine
+from synapse.util import Clock
+
+from tests import unittest
+
+
+class ThreadSubscriptionsTestCase(unittest.HomeserverTestCase):
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = self.hs.get_datastores().main
+        self.user_id = "@user:test"
+        self.room_id = "!room:test"
+        self.thread_root_id = "$thread_root:test"
+        self.other_thread_root_id = "$other_thread_root:test"
+
+        # Disable foreign key checks for testing
+        # This allows us to insert test data without having to create actual events
+        db_pool = self.store.db_pool
+        if isinstance(db_pool.engine, Sqlite3Engine):
+            self.get_success(
+                db_pool.execute("disable_foreign_keys", "PRAGMA foreign_keys = OFF;")
+            )
+        else:
+            # Postgres
+            def f(txn: LoggingTransaction) -> None:
+                txn.execute(
+                    """
+                    ALTER TABLE thread_subscriptions
+                        DROP CONSTRAINT thread_subscriptions_fk_users,
+                        DROP CONSTRAINT thread_subscriptions_fk_rooms,
+                        DROP CONSTRAINT thread_subscriptions_fk_events;
+                    """,
+                )
+
+            self.get_success(db_pool.runInteraction("disable_foreign_keys", f))
+
+        # Create rooms and events in the db to satisfy foreign key constraints
+        self.get_success(db_pool.simple_insert("rooms", {"room_id": self.room_id}))
+
+        self.get_success(
+            db_pool.simple_insert(
+                "events",
+                {
+                    "event_id": self.thread_root_id,
+                    "room_id": self.room_id,
+                    "topological_ordering": 1,
+                    "stream_ordering": 1,
+                    "type": "m.room.message",
+                    "depth": 1,
+                    "processed": True,
+                    "outlier": False,
+                },
+            )
+        )
+
+        self.get_success(
+            db_pool.simple_insert(
+                "events",
+                {
+                    "event_id": self.other_thread_root_id,
+                    "room_id": self.room_id,
+                    "topological_ordering": 2,
+                    "stream_ordering": 2,
+                    "type": "m.room.message",
+                    "depth": 2,
+                    "processed": True,
+                    "outlier": False,
+                },
+            )
+        )
+
+        # Create the user
+        self.get_success(
+            db_pool.simple_insert("users", {"name": self.user_id, "is_guest": 0})
+        )
+
+    def _subscribe(
+        self,
+        thread_root_id: str,
+        *,
+        automatic: bool,
+        room_id: Optional[str] = None,
+        user_id: Optional[str] = None,
+    ) -> Optional[int]:
+        if user_id is None:
+            user_id = self.user_id
+
+        if room_id is None:
+            room_id = self.room_id
+
+        return self.get_success(
+            self.store.subscribe_user_to_thread(
+                user_id,
+                room_id,
+                thread_root_id,
+                automatic=automatic,
+            )
+        )
+
+    def _unsubscribe(
+        self,
+        thread_root_id: str,
+        room_id: Optional[str] = None,
+        user_id: Optional[str] = None,
+    ) -> Optional[int]:
+        if user_id is None:
+            user_id = self.user_id
+
+        if room_id is None:
+            room_id = self.room_id
+
+        return self.get_success(
+            self.store.unsubscribe_user_from_thread(
+                user_id,
+                room_id,
+                thread_root_id,
+            )
+        )
+
+    def test_set_and_get_thread_subscription(self) -> None:
+        """Test basic setting and getting of thread subscriptions."""
+        # Initial state: no subscription
+        subscription = self.get_success(
+            self.store.get_subscription_for_thread(
+                self.user_id, self.room_id, self.thread_root_id
+            )
+        )
+        self.assertIsNone(subscription)
+
+        # Subscribe
+        self._subscribe(
+            self.thread_root_id,
+            automatic=True,
+        )
+
+        # Assert subscription went through
+        subscription = self.get_success(
+            self.store.get_subscription_for_thread(
+                self.user_id, self.room_id, self.thread_root_id
+            )
+        )
+        self.assertIsNotNone(subscription)
+        self.assertTrue(subscription.automatic)  # type: ignore
+
+        # Now make it a manual subscription
+        self._subscribe(
+            self.thread_root_id,
+            automatic=False,
+        )
+
+        # Assert the manual subscription overrode the automatic one
+        subscription = self.get_success(
+            self.store.get_subscription_for_thread(
+                self.user_id, self.room_id, self.thread_root_id
+            )
+        )
+        self.assertFalse(subscription.automatic)  # type: ignore
+
+    def test_purge_thread_subscriptions_for_user(self) -> None:
+        """Test purging all thread subscription settings for a user."""
+        # Set subscription settings for multiple threads
+        self._subscribe(self.thread_root_id, automatic=True)
+        self._subscribe(self.other_thread_root_id, automatic=False)
+
+        subscriptions = self.get_success(
+            self.store.get_updated_thread_subscriptions_for_user(
+                self.user_id,
+                from_id=0,
+                to_id=50,
+                limit=50,
+            )
+        )
+        min_id = min(id for (id, _, _) in subscriptions)
+        self.assertEqual(
+            subscriptions,
+            [
+                (min_id, self.room_id, self.thread_root_id),
+                (min_id + 1, self.room_id, self.other_thread_root_id),
+            ],
+        )
+
+        # Purge all settings for the user
+        self.get_success(
+            self.store.purge_thread_subscription_settings_for_user(self.user_id)
+        )
+
+        # Check user has no subscriptions
+        subscriptions = self.get_success(
+            self.store.get_updated_thread_subscriptions_for_user(
+                self.user_id,
+                from_id=0,
+                to_id=50,
+                limit=50,
+            )
+        )
+        self.assertEqual(subscriptions, [])
+
+    def test_get_updated_thread_subscriptions(self) -> None:
+        """Test getting updated thread subscriptions since a stream ID."""
+
+        stream_id1 = self._subscribe(self.thread_root_id, automatic=False)
+        stream_id2 = self._subscribe(self.other_thread_root_id, automatic=True)
+        assert stream_id1 is not None
+        assert stream_id2 is not None
+
+        # Get updates since initial ID (should include both changes)
+        updates = self.get_success(
+            self.store.get_updated_thread_subscriptions(0, stream_id2, 10)
+        )
+        self.assertEqual(len(updates), 2)
+
+        # Get updates since first change (should include only the second change)
+        updates = self.get_success(
+            self.store.get_updated_thread_subscriptions(stream_id1, stream_id2, 10)
+        )
+        self.assertEqual(
+            updates,
+            [(stream_id2, self.user_id, self.room_id, self.other_thread_root_id)],
+        )
+
+    def test_get_updated_thread_subscriptions_for_user(self) -> None:
+        """Test getting updated thread subscriptions for a specific user."""
+        other_user_id = "@other_user:test"
+
+        # Set thread subscription for main user
+        stream_id1 = self._subscribe(self.thread_root_id, automatic=True)
+        assert stream_id1 is not None
+
+        # Set thread subscription for other user
+        stream_id2 = self._subscribe(
+            self.other_thread_root_id,
+            automatic=True,
+            user_id=other_user_id,
+        )
+        assert stream_id2 is not None
+
+        # Get updates for main user
+        updates = self.get_success(
+            self.store.get_updated_thread_subscriptions_for_user(
+                self.user_id, 0, stream_id2, 10
+            )
+        )
+        self.assertEqual(updates, [(stream_id1, self.room_id, self.thread_root_id)])
+
+        # Get updates for other user
+        updates = self.get_success(
+            self.store.get_updated_thread_subscriptions_for_user(
+                other_user_id, 0, max(stream_id1, stream_id2), 10
+            )
+        )
+        self.assertEqual(
+            updates, [(stream_id2, self.room_id, self.other_thread_root_id)]
+        )
Author	SHA1	Message	Date
Andrew Morgan	edac7a471f	1.135.0	2025-08-01 13:12:33 +01:00
Andrew Morgan	caf5f0110e	Linkify GitHub PR ID in changelog	2025-07-30 12:57:20 +01:00
Andrew Morgan	c823d2e98a	1.135.0rc2	2025-07-30 12:19:34 +01:00
Andrew Morgan	7ae7468159	Improve performance of `is_server_admin` by adding a cache (#18747 ) Fixes https://github.com/element-hq/synapse/issues/18738	2025-07-30 10:43:39 +00:00
Quentin Gliech	61e79a4cdf	Fix deactivation running off the main process (#18716 ) Best reviewed commit by commit. With the new dedicated MAS API (https://github.com/element-hq/synapse/pull/18520), it's possible that deactivation starts off the main process, which was not possible because of a few calls. I basically looked at everything that the deactivation handler was doing, reviewed whether it could run on workers or not, and find a workaround when possible --------- Co-authored-by: Eric Eastwood <erice@element.io>	2025-07-24 08:43:58 +00:00
Olivier 'reivilibre	9ecf192089	Remove stray dev log	2025-07-22 13:54:32 +01:00
Olivier 'reivilibre	6838a1020b	Tweak changelog again	2025-07-22 12:47:12 +01:00
Olivier 'reivilibre	a77befcc29	Tweak changelog	2025-07-22 12:16:35 +01:00
Olivier 'reivilibre	cedb8cd045	1.135.0rc1	2025-07-22 12:10:57 +01:00
dependabot[bot]	bb84121553	Bump authlib from 1.6.0 to 1.6.1 (#18704 )	2025-07-22 11:57:09 +01:00
dependabot[bot]	7de4fdf61a	Bump ruff from 0.12.3 to 0.12.4 (#18705 )	2025-07-22 11:57:00 +01:00
dependabot[bot]	8fc9aa70a5	Bump jsonschema from 4.24.0 to 4.25.0 (#18707 )	2025-07-22 11:56:43 +01:00
dependabot[bot]	3db73b974f	Bump sigstore/cosign-installer from 3.9.1 to 3.9.2 (#18708 )	2025-07-22 11:56:32 +01:00
dependabot[bot]	c51bd89c3b	Bump serde_json from 1.0.140 to 1.0.141 (#18709 )	2025-07-22 11:55:38 +01:00
dependabot[bot]	7de9ac01a0	Bump once_cell from 1.19.0 to 1.21.3 (#18710 )	2025-07-22 11:55:28 +01:00
Devon Hudson	4e118aecd0	Reduce log spam when client stops downloading media while it is being streamed to them (`ConsumerStopProducingError`) (#18699 ) The case where a consumer stops downloading media that is currently being streamed is now able to be handled explicitly. That scenario isn't really an error, it is expected behaviour. This PR adds a custom exception which allows us to drop the log level for this specific case from `WARNING` to `INFO`. ### Pull Request Checklist <!-- Please read https://element-hq.github.io/synapse/latest/development/contributing_guide.html before submitting your pull request --> * [X] Pull request is based on the develop branch * [X] Pull request includes a [changelog file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog). The entry should: - Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.". - Use markdown where necessary, mostly for `code blocks`. - End with either a period (.) or an exclamation mark (!). - Start with a capital letter. - Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry. * [X] [Code style](https://element-hq.github.io/synapse/latest/code_style.html) is correct (run the [linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)) --------- Co-authored-by: Eric Eastwood <erice@element.io>	2025-07-21 20:11:46 +00:00
Shay	11a11414c5	Add an option to issue redactions as admin user on admin redaction endpoint (#18671 ) Currently the [admin redaction endpoint](https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#redact-all-the-events-of-a-user) defaults to puppeting the user being redacted. This PR adds an optional param `use_admin`, which when provided issues the redactions as the admin user instead.	2025-07-21 16:40:45 +00:00
Quentin Gliech	8a4e2e826d	Dedicated MAS API (#18520 ) This introduces a dedicated API for MAS to consume. Companion PR on the MAS side: element-hq/matrix-authentication-service#4801 This has a few advantages over the previous admin API: - it works on workers (this will be documented once we stabilise MSC3861 as a whole) - it is more efficient because more focused - it propagates trace contexts from MAS - it is only accessible to MAS (through the shared secret) and will let us remove the weird hack that made this token 'admin' with a ghost '@__oidc_admin:' user The next MAS version should support it, but will be opt-in. The version after that should use this new API by default --------- Co-authored-by: Eric Eastwood <erice@element.io>	2025-07-21 16:17:43 +00:00
reivilibre	875269eb53	Add experimental and incomplete support for MSC4306: Thread Subscriptions. (#18674 ) Implements: [MSC4306](https://github.com/matrix-org/matrix-spec-proposals/blob/rei/msc_thread_subscriptions/proposals/4306-thread-subscriptions.md) (partially) What's missing: - Changes to push rules Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>	2025-07-21 15:54:28 +01:00
				`@@ -1 +0,0 @@`
				Add `recaptcha_private_key_path` and `recaptcha_public_key_path` config option.
				`@@ -1 +0,0 @@`
				`Add plain-text handling for rich-text topics as per [MSC3765](https://github.com/matrix-org/matrix-spec-proposals/pull/3765).`
				`@@ -1 +0,0 @@`
				`If enabled by the user, server admins will see [soft failed](https://spec.matrix.org/v1.13/server-server-api/#soft-failure) events over the Client-Server API.`
				`@@ -1 +0,0 @@`
				`Add experimental support for [MSC4277](https://github.com/matrix-org/matrix-spec-proposals/pull/4277).`
				`@@ -1 +0,0 @@`
				`Fix CPU and database spinning when retrying sending events to servers whilst at the same time purging those events.`
				`@@ -1 +0,0 @@`
				`Add ability to limit amount uploaded by a user in a given time period.`
				`@@ -1 +0,0 @@`
				`Document that some config options for the user directory are in violation of the Matrix spec.`
				`@@ -1 +0,0 @@`
				`Allow user registrations to be done on workers.`
				`@@ -1 +0,0 @@`
				`Enable workers to write directly to the device lists stream and handle device list updates, reducing load on the main process.`