Post a new message to a Mattermost channel
AI agents use mattermost_post_message to create or update resources in Mattermost MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mattermost MCP Server environment.
Posting a message is a write operation that modifies the state of a Mattermost workspace by adding content. It is reversible (messages can be deleted), distinguishing it from Destructive actions. The severity is medium because misuse could spam channels, impersonate users, or spread misinformation, but the blast radius is contained to a single workspace and the action is recoverable.
From the tool's definition Tool description states 'Post a new message to a Mattermost channel' — this creates new data (a message) in a reversible manner. The action can be undone by deleting the posted message.
Documented attack patterns abuse exactly the kind of access mattermost_post_message gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mattermost MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for mattermost_post_message:
{
"version": "1",
"default": "deny",
"tools": {
"mattermost_post_message": {
"limits": [
{
"counter": "mattermost_post_message_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}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.
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Post a new message to a Mattermost channel. It is categorised as a Write tool in the Mattermost MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Mattermost MCP Server 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 MCP Server. Nothing to install.
mattermost_post_message is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
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.
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.
mattermost_post_message is provided by the Mattermost MCP Server MCP server (pvev/mattermost-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 9 Mattermost MCP Server tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
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9 Mattermost MCP Server tools catalogued and risk-classified — across an index of 42,500+ MCP servers.