mattermost_post_message · Mattermost MCP: Risk & Policy

WHAT IT DOES

What mattermost_post_message does on Mattermost

AI agents use mattermost_post_message to create or update resources in Mattermost — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mattermost environment.

WHY IT NEEDS A POLICY

Why mattermost_post_message needs a policy

This tool creates new messages in Mattermost channels, which is a Write operation—it modifies channel state by adding content. The action is reversible (messages can typically be deleted or edited).

From the tool's definition Tool name 'mattermost_post_message' and description 'Post a new message to a Mattermost channel' indicate message creation. Creates new data (a message) in a channel that is reversible (messages can be edited or deleted).

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

mattermost_post_message Rate-limited

mattermost_post_message stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "mattermost_post_message": {
      "limits": [
        {
          "counter": "mattermost_post_message_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Mattermost, so you can run agents against it safely.

FAQ

Questions about mattermost_post_message

What does the mattermost_post_message tool do? +

Post a new message to a Mattermost channel. It is categorised as a Write tool in the Mattermost MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on mattermost_post_message? +

Register the Mattermost MCP server in PolicyLayer and add a rule for mattermost_post_message: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Mattermost. Nothing to install.

What risk level is mattermost_post_message? +

mattermost_post_message is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit mattermost_post_message? +

Yes. Add a rate_limit block to the mattermost_post_message rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block mattermost_post_message completely? +

Set action: deny in the PolicyLayer policy for mattermost_post_message. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides mattermost_post_message? +

mattermost_post_message is provided by the Mattermost MCP server (@conarti/mattermost-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.