FetchSandbox MCP server

fetchsandbox-mcp

Also on Smithery, npm and the official MCP registry.

A deterministic verification engine for agents, as an MCP server for FetchSandbox.

Your agent writes an integration. This checks whether it actually works — against a sandbox that behaves like the real provider, including the failures: retried webhooks, declined cards, rate limits, auth errors.

When it finds a bug, it can propose a fix and then prove it: the same failure is run against your code before and after the diff. Green only if it reproduced first and stopped after. You get a receipt URL either way.

Install

Same stdio command everywhere. npx fetches the current version, so there's nothing to install.

{
  "mcpServers": {
    "fetchsandbox": {
      "command": "npx",
      "args": ["-y", "fetchsandbox-mcp@latest"]
    }
  }
}

Restart the client afterwards. Anything else that speaks MCP takes the same command and args.

Using it

Describe the problem the way you'd describe it to a colleague. You don't need to name a tool.

Customers are reporting more seats than they bought after a Paddle payment.

Can you find out why?

The agent works through: route the symptom, reproduce it against the provider sandbox, read your code, get a fix, prove the fix on your code. Each step hands back what the next one needs.

One thing worth knowing, because it's easy to get backwards: provefix needs the unfixed** tree. Run it before you write the diff to disk, or there's no bug left to reproduce and no proof to be had.

Accounts

You don't need one to start. Install it, ask a question, and everything runs.

The first time a run produces something worth keeping — a receipt, or a set of findings — you'll get a short code and a link. Signing in takes about twenty seconds and does two things: the evidence behind your receipts stops being archived after 15 days, and the runs from that machine collect in one place. You'll be asked at most once a day, and never once you're signed in.

For CI, or anywhere a browser isn't available, set a key instead:

FETCHSANDBOX_API_KEY=fsk_...

The key is written to ~/.fetchsandbox/credentials.json when you sign in from an editor; the environment variable always wins.

Tools

Start with guide. It picks the right ones for what you asked.

Finding and fixing

Shortened. The full README is on GitHub.