canvases_create · Slack MCP: Risk & Policy

WHAT IT DOES

What canvases_create does on Slack

AI agents use canvases_create 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 canvases_create needs a policy

This tool creates new canvas objects in Slack, which are collaborative documents. While creation is a Write operation (reversible modification), the severity is high because an AI agent with this capability could create numerous unauthorized canvases, spam workspaces, or inject malicious/misleading content that could be widely viewed.

From the tool's definition Tool name 'canvases_create' and description 'Create a canvas' indicate creation of new data in Slack's canvas feature. The action is reversible (canvases can be deleted), distinguishing it from Destructive operations.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

canvases_create Rate-limited

canvases_create 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": {
    "canvases_create": {
      "limits": [
        {
          "counter": "canvases_create_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 canvases_create

What does the canvases_create tool do? +

Create a canvas. 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 canvases_create? +

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

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

Can I rate-limit canvases_create? +

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

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

canvases_create 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.