Synapse 1.154.0rc1 (2026-05-27)

Features

Add support for MSC4452: Preview URL capabilities API 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)

Fix a bug in MSC4186: Simplified Sliding Sync that could prevent user avatars from showing if the room had an empty name. (#19468, #19791)
Fix access token cache not being invalidated for sessions using refresh tokens. Contributed by @FrenchGithubUser @ Famedly. (#19483)
Fix bug where Synapse would return 400 (M_BAD_JSON) when sending a message with a mentions field and Synapse module check_event_allowed callback registered (frozen event). Contributed by @gaetan-sbt. (#19634)
Fix long-standing but niche bug with /sync where it could attempt to fetch data with flawed invalid future tokens. (#19644)
Fix /sync failing when MSC4354 Sticky Events are enabled and the sync request filters out Ephemeral Data Units (EDUs). (#19787)
Fix packaging for Fedora and EPEL caused by unnecessary bumping attrs minimum version requirement in pyproject.toml file. Contributed by Oleg Girko. (#19789)
Fix merging signatures when a policy server is running under the same server name as Synapse. The bug was re-introduced in v1.153.0rc1 after being fixed earlier in v1.151.0rc1. Contributed by @tulir @ Beeper. (#19797)

Added details about how Synapse syncs the picture claim when update_profile_information setting is true. (#19508)

Port Event.content field to Rust. (#19725)
Prefer close backfill points (absolute distance). (#19748)
Replace unique quarantined_media waiting patterns with standard wait_for_stream_token(...). (#19764)
Improve Synapse logging around when someone encounters We can't get valid state history. so you can correlate everything by event_id. (#19765)
Tidy up Rust RoomVersion structs. (#19766)
Update WorkerLock tests to better stress the WORKER_LOCK_MAX_RETRY_INTERVAL. (#19772)
Refactor MSC4242: State DAG checks behind a single TypeIs helper to avoid scattered isinstance casts. (#19774)
Use StrCollection for prev_state_events. (#19777)
Fix up the construction of events in tests, ahead of the Rust event port. (#19781)