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Test the API integrations Cursor wrote before production

Cursor writes the integration in minutes. Wire FetchSandbox MCP into Cursor to run the workflow, catch retry and idempotency bugs, and prove the fix.

2026-08-04FetchSandbox Engineering

Cursor is good at writing integration code. Ask it for a Stripe webhook handler, a Descope auth middleware, or a Resend send-and-track flow and you get working code in a couple of minutes.

The problem is that "it compiles and returns 200" is not the same as "it handles the lifecycle." The code Cursor writes almost always passes the happy path. What it gets wrong is what happens on the second delivery: retries, duplicate events, forged tokens, out-of-order webhooks. You do not see those in the editor, and you usually do not see them until production.

What Cursor can't check on its own

Cursor reads your files and the API docs. It cannot run the provider's real lifecycle against your handler, so it can't tell you whether a retried webhook reprocesses an order or whether a decoded-but-unverified JWT sails through your auth check. It will confidently write code that looks right for all of those.

That gap is exactly what FetchSandbox fills. It's an MCP server, so Cursor can call it directly.

Wire it into Cursor

Add FetchSandbox to ~/.cursor/mcp.json (or .cursor/mcp.json in the project):

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

Now Cursor has tools to run curated provider workflows, reproduce a failure, and prove a fix — without you standing up a real Stripe or Descope account.

The failure worth proving first

Here's the classic one. Cursor writes a webhook handler that dedups on the delivery header instead of the event ID:

@app.post("/stripe-webhook")
async def webhook(request: Request):
    delivery_id = request.headers.get("stripe-webhook-id", "")
    if delivery_id in seen:
        return {"ok": True}
    seen.add(delivery_id)
    mark_order_paid(event["data"]["object"])  # runs again on every retry

Stripe assigns a new delivery id to every retry of the same event. The dedup check never matches, so a retried payment_intent.succeeded marks the order paid twice. The fix is to dedup on event["id"], which is stable across retries.

You can't code-review your way to confidence on this. You have to run the retry.

The flow in Cursor

  1. Ask Cursor to list the FetchSandbox workflows for your provider.
  2. Have it run the webhook workflow with retries turned on and watch the run trace.
  3. When the order gets marked paid twice, ask Cursor to find and fix the bug.
  4. Ask FetchSandbox to prove the fix — it reproduces the bug on your real code, applies the fix, and reruns. Green only comes from a measured buggy → fixed flip, never a self-report.
  5. Drop the receipt URL in your PR as evidence the retry no longer double-charges.

That last step is the point. Instead of "Cursor says it fixed it," you get a run trace that shows the bug happened before and doesn't happen after, on your actual handler.

Where to start

The finish line was never "Cursor wrote the integration." It's "the retry doesn't double-charge, the forged token gets rejected, and there's a receipt that proves it."