conversations_join · Slack MCP: Risk & Policy

WHAT IT DOES

What conversations_join does on Slack

AI agents use conversations_join 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 conversations_join needs a policy

This action creates a new relationship (user membership in a conversation) and is reversible (the user can leave the conversation). It does not retrieve data (Read), execute arbitrary code (Execute), delete anything (Destructive), or move money (Financial).

From the tool's definition Tool name is 'conversations_join' and description states 'Join an existing conversation.' This modifies the state of the user's participation in a Slack conversation by adding them to it.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

conversations_join Rate-limited

conversations_join 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": {
    "conversations_join": {
      "limits": [
        {
          "counter": "conversations_join_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 Slack, so you can run agents against it safely.

FAQ

Questions about conversations_join

What does the conversations_join tool do? +

Join an existing conversation. 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 conversations_join? +

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

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

Can I rate-limit conversations_join? +

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

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

conversations_join 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.