Question 1

How to fix Synapse 1.153.0 failing to start with 'needs attrs!=21.1.0,>=26.1.0, but got attrs==23.2.0' on Ubuntu 24.04?

Accepted Answer

This was caused by an accidental minimum `attrs` version bump in Synapse 1.153.0 and was reverted in Synapse 1.154.0.

Fix by upgrading the Debian/Ubuntu package:

```bash
sudo apt update
sudo apt install matrix-synapse-py3
sudo systemctl restart matrix-synapse
```

If 1.154.0 is not yet available from `packages.matrix.org`, downgrade to the previous working version as a temporary workaround:

```bash
sudo apt install matrix-synapse-py3=1.152.1+noble1
sudo systemctl restart matrix-synapse
```

Avoid manually forcing `attrs` with pip unless you understand the package environment, because the official fix is to upgrade to Synapse 1.154.0 or later.

Question 2

Why does Synapse mautrix bridge encryption fail with 'UnboundLocalError: cannot access local variable now_token' after enabling sticky events?

Accepted Answer

This is a known bug in Synapse v1.150.0 to v1.152.1 where sticky events cause a Python unbound local error in sync handling. The bug prevents bridges like mautrix-signal/WhatsApp from receiving decryption keys. Temporary workaround: downgrade Synapse to v1.149.0. A permanent fix is being tracked in the related GitHub issue #19409.

Question 3

Why does Sliding Sync not send newly requested required_state events if the pos offset stays the same?

Accepted Answer

Known bug in Synapse 1.136.0 (and earlier) where changes to required_state are ignored until other room activity occurs. Workaround: trigger any event in the room (e.g., send a message) to force a sync. Permanent fix is being re-introduced via PR #19792.

Question 4

How do I fix Synapse error "400 - 1 validation error for Mentions" when sending messages in unencrypted rooms?

Accepted Answer

This affects Synapse 1.147.1 with Element Web 1.12.8/1.12.12. Element Web sends `m.mentions: {}` even when nobody is mentioned. Synapse represents event content as `immutabledict`, and Pydantic v2 rejects `immutabledict({})` when validating the `Mentions` model. Encrypted rooms are unaffected because Synapse does not see the plaintext content.

Fix it in Synapse by converting mapping-like input to `dict` before Pydantic validation:

```python
from typing import Any, List
from pydantic import BaseModel, Field, 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
```

Apply this patch to your Synapse package or Docker image, rebuild/redeploy, and restart Synapse. If available, upgrade to an official Synapse release containing the fix for issue #19689. Temporary workaround: use an encrypted room, where the message content is encrypted before Synapse validation.

Question 5

How do I prevent email account enumeration in Synapse's password reset /requestToken flow?

Accepted Answer

Set `request_token_inhibit_3pid_errors: true` in `homeserver.yaml`, then restart Synapse. This makes `/requestToken` endpoints return privacy-preserving responses instead of revealing whether an email address or other third-party identifier is registered.

```yaml
request_token_inhibit_3pid_errors: true
```

Newer Synapse versions containing the fix for issue #18415 default this to `true`. If you prefer the previous usability-focused behavior, explicitly set it to `false`:

```yaml
request_token_inhibit_3pid_errors: false
```

For stricter privacy requirements, you can also handle authentication flows through `matrix-authentication-service`.

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