chat_post_ephemeral · Slack MCP: Risk & Policy

WHAT IT DOES

What chat_post_ephemeral does on Slack

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

WHY IT NEEDS A POLICY

Why chat_post_ephemeral needs a policy

Posting ephemeral (temporary) messages to Slack channels or users modifies communication state and creates new data artifacts, fitting the Write category. Severity is medium because ephemeral messages are temporary and limited in scope, though misuse could spam users or leak sensitive information to unintended recipients.

From the tool's definition Tool name 'chat_post_ephemeral' indicates posting messages to Slack; 'ephemeral' suggests temporary messages visible only to specific users. The server description confirms it provides 'full access to Slack: messages' via 220 tools.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

chat_post_ephemeral Rate-limited

chat_post_ephemeral 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": {
    "chat_post_ephemeral": {
      "limits": [
        {
          "counter": "chat_post_ephemeral_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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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_post_ephemeral

What does the chat_post_ephemeral tool do? +

chat_post_ephemeral. It is categorised as a Write tool in the Slack MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on chat_post_ephemeral? +

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

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

Can I rate-limit chat_post_ephemeral? +

Yes. Add a rate_limit block to the chat_post_ephemeral 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_post_ephemeral completely? +

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

chat_post_ephemeral 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.