chat_unfurl · Slack MCP: Risk & Policy

WHAT IT DOES

What chat_unfurl does on Slack

AI agents use chat_unfurl 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_unfurl needs a policy

Based on Slack terminology, 'unfurl' refers to expanding or enriching link previews and message metadata—a write operation that modifies message presentation. While the empty description introduces uncertainty, the tool name and context of a Slack integration strongly indicate it creates or modifies message content/formatting rather than reading, executing code, destroying data, or moving money.

From the tool's definition Tool name 'chat_unfurl' suggests modifying how messages or links are displayed/rendered in Slack chat. Unfurling typically involves adding preview metadata or formatting to messages, which is a reversible modification.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

chat_unfurl Rate-limited

chat_unfurl 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_unfurl": {
      "limits": [
        {
          "counter": "chat_unfurl_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 chat_unfurl

What does the chat_unfurl tool do? +

chat_unfurl. 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_unfurl? +

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

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

Can I rate-limit chat_unfurl? +

Yes. Add a rate_limit block to the chat_unfurl 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_unfurl completely? +

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

chat_unfurl 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.