Stabilize support for sending ephemeral events to application services, as per MSC2409 by jason-famedly · Pull Request #19758 · element-hq/synapse

jason-famedly · 2026-05-06T11:41:12Z

MSC2409 was merged into the spec on Oct 28, 2024 and is part of Matrix Spec 1.13

This adds the stable identifiers for MSC2409 while maintaining the existing unstable identifier for backwards compatibility. The existing to_device identifiers borrowed from MSC2409 for MSC4203 are also maintained.

(Includes one drive-by removal of a FIXME comment that was no longer needed after May 16, 2022)

Pull Request Checklist

Pull request is based on the develop branch
Pull request includes a changelog file. The entry should:
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Code style is correct (run the linters)

sandhose · 2026-05-06T12:09:29Z

+                    "ephemeral": ephemeral,
+                    # TODO: Update to stable prefixes once MSC4203 completes FCP merge.
+                    #  Previously, this was part of MSC2409 which is why it has the
+                    #  mismatched unstable identifier


I'm not sure how we handled that in the past, but I feel like we should continue sending the unstable identifier to avoid breaking application services which only support the unstable prefix?

Hmmm. That is fair. Do you think using the change in the configuration setting should be the decider on which identifier should be sent? So that if the config says receive_ephemeral it would provide the stable key, but if it was de.sorunome.msc2409.push_ephemeral it would use the older unstable key? Or do you suppose just sending both each time?

Gave the first option a try in 5c76d3f.

As a note: the existing code did not do the optional exclusion of the key if there was no relevant data. Would you be interested in that here, or push it to later?

but I feel like we should continue sending the unstable identifier to avoid breaking application services which only support the unstable prefix?

The point of the unstable prefixes is that they can change or be removed at any time. They are expected to be broken when a feature moves to being stable. Let's just get rid of the unstable stuff and be done with it.

I really don't want to break existing deployments without prior notice, especially since a single homeserver may use different bridges, some supporting the stable one, some not. This offers a proper migration path

…ugh so the unstable identifier can exist for backwards compatibility

cloudrac3r · 2026-06-07T02:04:55Z

Thank you for moving this forward. 🤍

sandhose

Sorry for not getting back to this earlier. Could you just merge back main in the branch so that I can trigger CI again?

sandhose · 2026-06-16T09:55:42Z

+                    "ephemeral": ephemeral,
+                    # TODO: Update to stable prefixes once MSC4203 completes FCP merge.
+                    #  Previously, this was part of MSC2409 which is why it has the
+                    #  mismatched unstable identifier


I really don't want to break existing deployments without prior notice, especially since a single homeserver may use different bridges, some supporting the stable one, some not. This offers a proper migration path

Co-authored-by: Quentin Gliech <quenting@element.io>

@jason-famedly

Tested on NetBSD 10 amd64 with 2026Q2 environment. # Synapse 1.156.0 (2026-07-07) ## Features - Expose [MSC4354 Sticky Events](matrix-org/matrix-spec-proposals#4354) over [MSC4186 (Simplified) Sliding Sync](matrix-org/matrix-spec-proposals#4186). ([\#19591](element-hq/synapse#19591)) - Stabilize support for sending ephemeral events to application services, as per [MSC2409](matrix-org/matrix-spec-proposals#2409). Contributed by @jason-famedly @ Famedly. ([\#19758](element-hq/synapse#19758)) - Include `allowed_room_ids` in the `/summary` client-server API response for rooms with restricted join rules, as required by Matrix 1.15. Contributed by @FrenchGithubUser @famedly. ([\#19762](element-hq/synapse#19762)) - [MSC4140: Cancellable delayed events](matrix-org/matrix-spec-proposals#4140): Allow authentication on delayed event management endpoints (such as `/restart`) to bypass ratelimits for unauthenticated requests based on the client IP address. ([\#19794](element-hq/synapse#19794)) - Add new metric `synapse_non_deactivated_user_count` which tracks the number of non-deactivated users in the database, split by `app_service`. ([\#19848](element-hq/synapse#19848)) - The `GET /_matrix/client/unstable/org.matrix.msc1763/retention/configuration` endpoint is now provided when retention is enabled and `experimental_features.msc1763_enabled` is enabled, based on [MSC1763](matrix-org/matrix-spec-proposals#1763). ([\#19853](element-hq/synapse#19853)) - Add experimental support for [MSC4491: Invite reasons in room creation](matrix-org/matrix-spec-proposals#4491). ([\#19874](element-hq/synapse#19874)) # Synapse 1.155.0 (2026-06-16) # Synapse 1.154.0 (2026-06-04) ## Features - Add support for [MSC4452: Preview URL capabilities API](matrix-org/matrix-spec-proposals#4452) which exposes a `io.element.msc4452.preview_url` capability. If `experimental_features.msc4452_enabled` is `true`, the `/_matrix/(client/v1/media|media/v3)/preview_url` endpoint now responds with a 403 status code when the capability is disabled. ([\#19715](element-hq/synapse#19715)) # Synapse 1.153.0 (2026-05-19) ## Features - Make ACLs apply to EDUs per [MSC4163](matrix-org/matrix-spec-proposals#4163). ([\#18475](element-hq/synapse#18475)) - Stabilize [MSC3266: Room summary API](matrix-org/matrix-spec-proposals#3266), removing the experimental config flag `msc3266_enabled`. Contributed by @dasha-uwu. ([\#19720](element-hq/synapse#19720)) - Partial [MSC4311](matrix-org/matrix-spec-proposals#4311) implementation: `m.room.create` is now a required part of stripped `invite_state`/`knock_state` . Contributed by @FrenchGithubUser @famedly. ([\#19722](element-hq/synapse#19722)) - Expose `tombstoned` and `replacement_room` in room details on admin API endpoint `GET /_synapse/admin/v1/rooms/<room_id>`. Contributed by Noah Markert. ([\#19737](element-hq/synapse#19737)) # Synapse 1.152.1 (2026-05-07) # Synapse 1.152.0 (2026-04-28) ## Features - Add a ["Listing quarantined media changes" Admin API](https://element-hq.github.io/synapse/latest/admin_api/media_admin_api.html#listing-quarantined-media-changes) for retrieving a paginated record of when media became (un)quarantined. ([\#19558](element-hq/synapse#19558), [\#19677](element-hq/synapse#19677), [\#19694](element-hq/synapse#19694)) - Advertise [MSC4445](matrix-org/matrix-spec-proposals#4445) sync timeline order in `unstable_features`. ([\#19642](element-hq/synapse#19642)) - Report the Rust compiler version used in the Prometheus metrics. Contributed by Noah Markert. ([\#19643](element-hq/synapse#19643)) - Passthrough 'article' and 'profile' OpenGraph metadata on URL preview requests. ([\#19659](element-hq/synapse#19659)) - Add a way to re-sign local events with a new signing key. ([\#19668](element-hq/synapse#19668)) - Support [MSC4450: Identity Provider selection for User-Interactive Authentication with Legacy Single Sign-On](matrix-org/matrix-spec-proposals#4450). ([\#19693](element-hq/synapse#19693)) - Add experimental support for [MSC4242](matrix-org/matrix-spec-proposals#4242): State DAGs. Excludes federation support. ([\#19424](element-hq/synapse#19424)) - Adds [Admin API](https://element-hq.github.io/synapse/latest/usage/administration/admin_api/index.html) endpoints to list, fetch and delete user reports. ([\#19657](element-hq/synapse#19657)) - Reduce database disk space usage by pruning old rows from `device_lists_changes_in_room`. ([\#19473](element-hq/synapse#19473), [\#19709](element-hq/synapse#19709)) # Synapse 1.151.0 (2026-04-07) ## Features - Add stable support for [MSC4284](matrix-org/matrix-spec-proposals#4284) Policy Servers. ([\#19503](element-hq/synapse#19503)) - Update and stabilize support for [MSC2666](matrix-org/matrix-spec-proposals#2666): Get rooms in common with another user. Contributed by @tulir @ Beeper. ([\#19511](element-hq/synapse#19511)) - Updated experimental support for [MSC4388: Secure out-of-band channel for sign in with QR](matrix-org/matrix-spec-proposals#4388). ([\#19573](element-hq/synapse#19573)) - Stabilize `room_version` and `encryption` fields in the space/room `/hierarchy` API (part of [MSC3266](matrix-org/matrix-spec-proposals#3266)). ([\#19576](element-hq/synapse#19576)) - Introduce a [configuration option](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#matrix_authentication_service) to allow using HTTP/2 over plaintext when Synapse connects to Matrix Authentication Service. ([\#19586](element-hq/synapse#19586)) ## Deprecations and Removals - Remove support for [MSC3852: Expose user agent information on Device](matrix-org/matrix-spec-proposals#3852) as the MSC was closed. ([\#19430](element-hq/synapse#19430)) # Synapse 1.150.0 (2026-03-24) ## Features - Add experimental support for the [MSC4370](matrix-org/matrix-spec-proposals#4370) Federation API `GET /extremities` endpoint. ([\#19314](element-hq/synapse#19314)) - [MSC4140: Cancellable delayed events](matrix-org/matrix-spec-proposals#4140): When persisting a delayed event to the timeline, include its `delay_id` in the event's `unsigned` section in `/sync` responses to the event sender. ([\#19479](element-hq/synapse#19479)) - Expose [MSC4354 Sticky Events](matrix-org/matrix-spec-proposals#4354) over the legacy (v3) /sync API. ([\#19487](element-hq/synapse#19487)) - When Matrix Authentication Service (MAS) integration is enabled, allow MAS to set the user locked status in Synapse. ([\#19554](element-hq/synapse#19554))

jason-famedly added 4 commits May 6, 2026 06:08

drive-by removal of no longer pertaining FIXME comment

53c5b50

stablize msc2409 while leaving msc4203 intact

d72754f

mention the to_devices portion was broken off near the msc2409 setting

fd9a8c0

changelog

b939211

jason-famedly marked this pull request as ready for review May 6, 2026 11:58

jason-famedly requested a review from a team as a code owner May 6, 2026 11:58

sandhose reviewed May 6, 2026

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separate the old setting from the new proper setting and pass it thro…

5c76d3f

…ugh so the unstable identifier can exist for backwards compatibility

jason-famedly requested a review from sandhose May 6, 2026 15:08

MadLittleMods added the A-Application-Service label May 6, 2026

sandhose approved these changes Jun 16, 2026

View reviewed changes

jason-famedly and others added 2 commits June 16, 2026 05:07

Merge branch 'develop' into jason/stabilize-msc2409

2ea63b9

Update changelog.d/19758.feature

1cd0532

Co-authored-by: Quentin Gliech <quenting@element.io>

sandhose enabled auto-merge (squash) June 16, 2026 10:59

sandhose merged commit 36822e3 into element-hq:develop Jun 16, 2026
41 checks passed

This was referenced Jul 13, 2026

Honor de.sorunome.msc2409.push_ephemeral flag in appservice handler #19928

Merged

legacy de.sorunome.msc2409.push_ephemeral flag no longer honored in appservice handler #19927

Closed

jason-famedly mentioned this pull request Jul 31, 2026

Declare support for Matrix v1.12 and v1.13 #18118

Open

MadLittleMods mentioned this pull request Aug 3, 2026

Stabilise MSC2409 support: send ephemeral events to appservices #18501

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Stabilize support for sending ephemeral events to application services, as per MSC2409 - #19758

Stabilize support for sending ephemeral events to application services, as per MSC2409#19758
sandhose merged 7 commits into
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