ruff_check · MCP File System Server MCP: Risk & Policy

WHAT IT DOES

What ruff_check does on MCP File System Server

AI agents invoke ruff_check to trigger actions in MCP File System Server. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why ruff_check needs a policy

This tool runs an external process (ruff linter) on files, making it an Execute category action. While ruff is read-only in nature (it only checks code without modifying it by default), it still executes an external command whose behavior depends on arguments and configuration.

From the tool's definition "Run ruff linter on specified files" — actively executes an external linter process against specified files

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

ruff_check Rate-limited

ruff_check stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "ruff_check": {
      "limits": [
        {
          "counter": "ruff_check_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches MCP File System Server, so you can run agents against it safely.

FAQ

Questions about ruff_check

What does the ruff_check tool do? +

Run ruff linter on specified files. Useful to check that nothing was broken. It is categorised as a Execute tool in the MCP File System Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on ruff_check? +

Register the MCP File System Server MCP server in PolicyLayer and add a rule for ruff_check: 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 MCP File System Server. Nothing to install.

What risk level is ruff_check? +

ruff_check is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit ruff_check? +

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

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

ruff_check is provided by the MCP File System Server MCP server (kvas-it/mcp-server-fs). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.