cargo_fmt · Cargo MCP Server MCP: Risk & Policy

WHAT IT DOES

What cargo_fmt does on Cargo MCP Server

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

WHY IT NEEDS A POLICY

Why cargo_fmt needs a policy

Formatting modifies source code files in place by rewriting their content according to style rules. This is reversible (version control can restore originals) and does not execute arbitrary code or delete data, so Write is the appropriate category. Misuse could cause unwanted code reformatting across a project, hence medium severity.

From the tool's definition Format Rust code using rustfmt

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

cargo_fmt Rate-limited

cargo_fmt 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": {
    "cargo_fmt": {
      "limits": [
        {
          "counter": "cargo_fmt_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 Cargo MCP Server, so you can run agents against it safely.

FAQ

Questions about cargo_fmt

What does the cargo_fmt tool do? +

Format Rust code using rustfmt. It is categorised as a Write tool in the Cargo 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 cargo_fmt? +

Register the Cargo MCP Server MCP server in PolicyLayer and add a rule for cargo_fmt: 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 Cargo MCP Server. Nothing to install.

What risk level is cargo_fmt? +

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

Can I rate-limit cargo_fmt? +

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

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

cargo_fmt is provided by the Cargo MCP Server MCP server (seemethere/cargo-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.