send_message · Claude Sync MCP: Risk & Policy

WHAT IT DOES

What send_message does on Claude Sync

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

WHY IT NEEDS A POLICY

Why send_message needs a policy

This tool creates and stores new data (messages) in a shared system, making it a Write operation. Severity is medium because misuse could flood the message bus, disrupt agent coordination, or send unintended communications to other agents, but it does not delete data, execute arbitrary code, or cause financial impact. The local-first architecture and SQLite persistence confirm the data modification aspect.

From the tool's definition Tool description states 'Send a message to another Claude agent' which creates new data (a message) in the shared message bus with SQLite persistence. The return of message_id confirms persistent storage of the created message.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

send_message Rate-limited

send_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": {
    "send_message": {
      "limits": [
        {
          "counter": "send_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 Claude Sync, so you can run agents against it safely.

FAQ

Questions about send_message

What does the send_message tool do? +

Send a message to another Claude agent. Returns message_id. It is categorised as a Write tool in the Claude Sync MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on send_message? +

Register the Claude Sync MCP server in PolicyLayer and add a rule for 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 Claude Sync. Nothing to install.

What risk level is send_message? +

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

Can I rate-limit send_message? +

Yes. Add a rate_limit block to the 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 send_message completely? +

Set action: deny in the PolicyLayer policy for 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 send_message? +

send_message is provided by the Claude Sync MCP server (the-firmament/claude-sync). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.