Problem

Mattermost integration currently does not support reactions:

1. Send reactions: calling message(action="react") returns:

Message action react not supported for channel mattermost

So agents cannot add emoji reactions to Mattermost posts.

2. Receive reactions: when a human reacts (adds/removes emoji) on a post sent by an agent, the agent does not receive/observe any event/update, so it cannot respond to reactions.

Expected

• message(action="react", channel="mattermost", ...) should add/remove an emoji reaction on the target post (and ideally mirror the semantics of other channels).
• Reaction add/remove on Mattermost posts should be surfaced to agents as incoming events (so workflows can react to them).

Actual

• React action is rejected with the error above.
• Reaction events are not delivered/visible to the agent.

Repro

Send

1. In Mattermost, pick a post.
2. From an agent, call message(action="react") targeting that post.
3. Observe error and no reaction added.

Receive

1. Agent sends a message to Mattermost.
2. Human adds an emoji reaction to that agent post.
3. Agent does not receive any event/notification about the reaction.

Notes / implementation hints (optional)

• Mattermost has a reactions API (POST /api/v4/reactions) and WebSocket event(s) for reaction added/removed.
• It would be great if the Mattermost plugin/provider could map those to OpenClaw message react + reaction event schema.