emoji_add · Slack MCP: Risk & Policy

WHAT IT DOES

What emoji_add does on Slack

AI agents use emoji_add 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 emoji_add needs a policy

This tool creates (adds) a new custom emoji resource, which is a reversible write operation. While it modifies Slack workspace configuration, the action can be undone by deleting the emoji. The use of an 'undocumented session endpoint' adds some concern about stability but does not elevate it to Execute (no arbitrary code execution) or Destructive (emoji creation is reversible).

From the tool's definition Tool name 'emoji_add' and description 'Add a custom emoji from a URL' indicate creation of new custom emoji resources in Slack.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

emoji_add Rate-limited

emoji_add 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": {
    "emoji_add": {
      "limits": [
        {
          "counter": "emoji_add_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 emoji_add

What does the emoji_add tool do? +

Add a custom emoji from a URL (undocumented session endpoint). 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 emoji_add? +

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

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

Can I rate-limit emoji_add? +

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

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

emoji_add 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.