`400 - 1 validation error for Mentions` when sending messages in unencrypted rooms

[pluex1]

Description

When sending a message in an unencrypted room, Synapse returns a 400 error with the following message:

1 validation error for Mentions
Input should be a valid dictionary or instance of Mentions [type=model_type, input_value=immutabledict({}), input_type=immutabledict]
  For further information visit https://errors.pydantic.dev/2.12/v/model_type

Element Web shows the user: "some of your messages could not be sent"

Root Cause

Element Web sends "m.mentions": {} in the request body of every message, even when no one is mentioned. Synapse internally represents event content as immutabledict. When Pydantic v2 tries to validate this immutabledict({}) against the Mentions model, it fails because immutabledict is not recognized as a dict subtype by Pydantic v2.

In encrypted rooms this does not occur, because the content is encrypted before sending and Synapse never sees m.mentions in plaintext.

Expected Behavior

Message is sent successfully.

Actual Behavior

Synapse returns 400 - 1 validation error for Mentions

Environment

Component	Version
Synapse	1.147.1
Element Web	1.12.8 / 1.12.12
Browser	Chrome 144/146/147 (Windows & macOS), Edge 146

• Encrypted rooms: not affected
• Unencrypted rooms: affected

Steps to reproduce

1. Set up Synapse 1.147.1 with Element Web 1.12.8 / 1.12.12
2. Create an unencrypted room
3. Send any message (e.g. "hi")
4. Synapse returns 400, Element Web shows "some of your messages could not be sent"

Homeserver

another homeserver

Synapse Version

1.147.1

Installation Method

Docker (matrixdotorg/synapse)

Database

single postgresql, no, no

Workers

Single process

Platform

runs in kubernetes

Configuration

No response

Relevant log output

synapse.http.server - INFO - PUT - <XForwardedForRequest method='PUT' uri='/_matrix/client/v3/rooms/.../send/m.room.message/...' clientproto='HTTP/1.0' site='8008'>
SynapseError: 400 - 1 validation error for Mentions
  Input should be a valid dictionary or instance of Mentions [type=model_type, input_value=immutabledict({}), input_type=immutabledict]
    For further information visit https://errors.pydantic.dev/2.12/v/model_type

Anything else that would be useful to know?

Element Web sending "m.mentions": {} on every message is a known issue: element-hq/element-web#26345

The fix should be in Synapse: the Mentions Pydantic model should handle immutabledict input by converting it to dict before validation, e.g. using a model_validator with mode='before':

from pydantic import model_validator

class Mentions(BaseModel):
    user_ids: List[str] = Field(default_factory=list)
    room: bool = False

    @model_validator(mode='before')
    @classmethod
    def coerce_immutabledict(cls, v: Any) -> Any:
        if hasattr(v, 'items') and not isinstance(v, dict):
            return dict(v)
        return v

[MadLittleMods]

As far as I can tell, the problem isn't "m.mentions": {} but because it shows up as immutabledict instead of dict when the event is "frozen". It sounds like you're running into the problem that #19634 is aiming to solve.

Events can be "frozen" for a couple of reasons:

1. Are you using a Synapse module that uses the check_event_allowed hook?
2. Are you using the use_frozen_dicts config option? I would suggest turning it off.

Since the fixes haven't shipped yet, the only thing to do would be to manually patch with the same fix, stop using the Synapse module, and don't configure use_frozen_dicts.