rtm_start · Slack MCP: Risk & Policy

WHAT IT DOES

What rtm_start does on Slack

AI agents invoke rtm_start to trigger actions in Slack. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why rtm_start needs a policy

RTM_start initiates a real-time messaging session, establishing a WebSocket connection to Slack that opens a channel for executing arbitrary message operations, event monitoring, and interactions. This is an Execute category tool because it triggers an external operation (establishing a real-time connection) whose effects depend on what is done with that connection.

From the tool's definition Tool name 'rtm_start' refers to the Slack RTM (Real-Time Messaging) API start endpoint, which initiates a real-time connection to Slack.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

rtm_start Rate-limited

rtm_start 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.

View as policy code

policy.json

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

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about rtm_start

What does the rtm_start tool do? +

rtm_start. It is categorised as a Execute tool in the Slack MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on rtm_start? +

Register the Slack MCP server in PolicyLayer and add a rule for rtm_start: 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 Slack. Nothing to install.

What risk level is rtm_start? +

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

Can I rate-limit rtm_start? +

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

Set action: deny in the PolicyLayer policy for rtm_start. 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 rtm_start? +

rtm_start is provided by the Slack MCP server (karbassi/slack-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.