matrix_send_message

THE RISK

High Risk

Why matrix_send_message needs a policy

This tool sends messages to an external Matrix room, triggering an external operation with real-world communication effects. It is not purely a write to internal data storage — it dispatches outbound messages to third-party recipients. The rate-limit (20/hour) acknowledges potential for abuse. Misuse could lead to spam, phishing, or social engineering attacks via Matrix rooms.

From the tool's definition Send a text (or notice, or HTML-formatted) message to a room

Documented attack patterns abuse exactly the kind of access matrix_send_message gives an agent:

POLICY

How to control matrix_send_message

PolicyLayer is an MCP gateway — it sits between your AI agents and Crow, and nothing reaches the server without passing your rules. This is the rule we recommend for matrix_send_message:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "matrix_send_message": {
      "limits": [
        {
          "counter": "matrix_send_message_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

matrix_send_message stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

Create a free account and register Crow — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

RATE-LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

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Go deeper

High-risk tools in Crow → Every high-risk tool this server exposes, in one view.
All high-risk MCP tools → Every high-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Agent sandbox → Bounding what an agent can execute and where the boundary should sit.
Least privilege for MCP → Granting agents the minimum tool surface the task needs.
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FAQ

Questions about matrix_send_message

What does the matrix_send_message tool do? +

Send a text (or notice, or HTML-formatted) message to a room. Rate-limited: 20/hour. It is categorised as a Execute tool in the Crow MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on matrix_send_message? +

Register the Crow MCP server in PolicyLayer and add a rule for matrix_send_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 Crow. Nothing to install.

What risk level is matrix_send_message? +

matrix_send_message is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit matrix_send_message? +

Yes. Add a rate_limit block to the matrix_send_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 matrix_send_message completely? +

Set action: deny in the PolicyLayer policy for matrix_send_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 matrix_send_message? +

matrix_send_message is provided by the Crow MCP server (kh0pper/crow). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Crow tool call.

Start from Crow, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

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