chat_start_stream · Slack MCP: Risk & Policy

WHAT IT DOES

What chat_start_stream does on Slack

AI agents invoke chat_start_stream 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 chat_start_stream needs a policy

The tool performs an action that triggers Slack's message streaming mechanism—an operation whose effects depend on arguments and cannot be undone without further action. This is Execute rather than Write because it initiates an active communication stream (a triggered operation) rather than simply creating/modifying a static piece of data.

From the tool's definition Tool name is 'chat_start_stream' on a Slack MCP server that provides full access to Slack messaging and communication features.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

chat_start_stream Rate-limited

chat_start_stream 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": {
    "chat_start_stream": {
      "limits": [
        {
          "counter": "chat_start_stream_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 chat_start_stream

What does the chat_start_stream tool do? +

chat_start_stream. 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 chat_start_stream? +

Register the Slack MCP server in PolicyLayer and add a rule for chat_start_stream: 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 chat_start_stream? +

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

Can I rate-limit chat_start_stream? +

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

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

chat_start_stream 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.