conversations_add_message · Slack MCP Server MCP

WHAT IT DOES

What conversations_add_message does on Slack MCP Server

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

WHY IT NEEDS A POLICY

Why conversations_add_message needs a policy

This tool creates and adds new messages to Slack channels, threads, and DMs, which is a reversible write operation. It does not delete, destroy, or move money. While posting messages could cause reputational harm or spread misinformation if misused by an AI agent, the impact is limited to message creation rather than data destruction or financial consequences.

From the tool's definition Tool description states 'Posts a message to a channel, thread, or DM' and 'Supports markdown and plain text' — these are write operations that create new data in Slack.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

conversations_add_message Rate-limited

conversations_add_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": {
    "conversations_add_message": {
      "limits": [
        {
          "counter": "conversations_add_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 Slack MCP Server, so you can run agents against it safely.

FAQ

Questions about conversations_add_message

What does the conversations_add_message tool do? +

Posts a message to a channel, thread, or DM. Supports markdown and plain text. NOTE: Disabled by default for safety - set SLACK_MCP_ADD_MESSAGE_TOOL environment variable to enable. It is categorised as a Write tool in the Slack MCP Server 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_add_message? +

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

What risk level is conversations_add_message? +

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

Can I rate-limit conversations_add_message? +

Yes. Add a rate_limit block to the conversations_add_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 conversations_add_message completely? +

Set action: deny in the PolicyLayer policy for conversations_add_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 conversations_add_message? +

conversations_add_message is provided by the Slack MCP Server MCP server (oregpt/agenticledger_mcp_slack). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.