openclaw - 💡(How to fix) Fix Mattermost: All inbound messages get System: prefix, Discord doesn't [1 participants]

Bug Description

Every inbound message from Mattermost is wrapped with a System: prefix before being submitted to the model. Discord (same agent, same session, same model) does not have this issue.

Mattermost (broken) — first line of every prompt.submitted:

System: [2026-04-26 05:26:13 GMT+8] Mattermost message in #openclaw from @jeyo: /new

Discord (correct) — no user message wrapping:

System: [2026-04-26 04:47:53 GMT+8] Model override not allowed for this agent; reverted to glm/glm-4.7.

(Only system events appear as System: lines; actual user messages arrive cleanly without the System: prefix)

Root Cause (confirmed via trajectory analysis)

Examining *.trajectory.jsonl entries with type: "prompt.submitted":

• Mattermost: prompt string starts with System: [timestamp] Mattermost message in #channel from @user: ... — the user's actual message is wrapped inside a system event line
• Discord: prompt string contains untrusted metadata (sender info, conversation context) but the actual user message content is NOT prefixed with System:

This has been observed across all OpenClaw versions including 2026.4.24. The issue is in the Mattermost channel plugin's inbound processing logic.

Impact

• All Mattermost user messages are treated as system directives by the model
• Sender identity and message content are mixed into the system prompt block rather than being presented as a proper user message
• This corrupts the intended conversation flow

Environment

• OpenClaw 2026.4.24, macOS (Darwin 25.4.0)
• Mattermost channel plugin, MiniMax-M2.7 (200k ctx)
• Both Mattermost and Discord connected to same agent — only Mattermost affected

Expected Behavior

Mattermost messages should be submitted as user-role messages without the System: prefix, consistent with Discord behavior.