When Mattermost Agents Looked Silent, the Real Cause Was `thread_replies_disabled`

When Mattermost Agents Looked Silent, the Real Cause Was thread_replies_disabled While operating OpenClaw agents on Mattermost, we hit an incident where several agents across multiple nodes appeared to be completely silent in direct messages. The obvious suspects were connectivity issues, model failures, or expired credentials. None of those turned out to be the root cause. The real problem was more subtle: thread replies were disabled on the Mattermost server, but the agent output still contained [[reply_to_current]] . OpenClaw therefore tried to post the response as a thread reply, and Mattermost rejected it with an HTTP 400 error. From the user side, it looked like the agent never answered. Internally, the reply existed but failed during delivery. What the logs actually said The key error was: thread_replies_disabled: replying to threads is disabled on this server That immediately reframed the incident. The issue was not that the LLM failed to generate a response. The issue was that