fetch_url · Claude MCP Starter MCP: Risk & Policy

WHAT IT DOES

What fetch_url does on Claude MCP Starter

AI agents invoke fetch_url to trigger actions in Claude MCP Starter. 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 fetch_url needs a policy

This tool actively triggers external HTTP requests (both GET and POST), which constitutes executing an external operation with effects that depend on arguments. POST requests in particular can create or modify remote state. This goes beyond a simple read operation since it can interact with arbitrary external services.

From the tool's definition Perform an HTTP GET or POST request and return the response status code

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fetch_url Rate-limited

fetch_url 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": {
    "fetch_url": {
      "limits": [
        {
          "counter": "fetch_url_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 Claude MCP Starter, so you can run agents against it safely.

FAQ

Questions about fetch_url

What does the fetch_url tool do? +

Perform an HTTP GET or POST request and return the response status code. It is categorised as a Execute tool in the Claude MCP Starter MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on fetch_url? +

Register the Claude MCP Starter MCP server in PolicyLayer and add a rule for fetch_url: 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 Claude MCP Starter. Nothing to install.

What risk level is fetch_url? +

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

Can I rate-limit fetch_url? +

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

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

fetch_url is provided by the Claude MCP Starter MCP server (justdreameritis/claude-mcp-starter). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.