Skip to main content
MCP server · Cursor · Claude Code · any MCP client

The MCP that proves your AI's integration fixes work.

Add one MCP server. Your coding agent finds the real failures in your Stripe, Paddle, Clerk & auth code — retries, duplicate webhooks, out-of-order events — fixes them, and proves each fix with a receipt. Bug reproduced, fix verified. Not mocks.

Add the MCPSee how it works →

npx fetchsandbox-mcp@latest · one config block

fix-proof · webhook_duplicate_side_effect✓ proven
✕Before fixseats 5 → 10 → 15 · exit 1
✓After fixseats 5 → 5 → 5 · exit 0
reproduced on your code, then verifiedgreen_allowed: true

The problem

Green CI doesn't mean fixed.

AI writes most integration code now. It compiles, passes review, returns 200 — then breaks in prod on the parts nobody tested: retries, duplicate webhooks, out-of-order events. And when your agent “fixes” it, the test can pass while the fix quietly does nothing.

# the bug: a Stripe retry double-grants seats  5 → 10 → 15
# the agent's “fix”: grants nothing at all  0 → 0 → 0  — and the test went green
# a correct fix: grant once, dedupe the retries  5 → 5 → 5
“Did the bug stop?” is the wrong question. “Is the state exactly what a correct implementation leaves?” is the right one.

How it works

Find → fix → prove. A receipt, not a vibe.

01 · FIND

Find the real failures

Surfaces the failure classes hiding in your integration — idempotency, ordering, auth bypasses, silent drops — on your actual code.

02 · FIX

Fix in your workflow

Your coding agent writes the patch in Cursor, Claude Code, or CI. Or FetchSandbox proposes one. Grounded in the real flow.

03 · PROVE

Prove it holds

Runs the actual scenario against your real code, before and after. A shareable receipt: bug reproduced, fix verified.

Why you can trust the green

A gate that greens a broken fix is worse than no gate.

Most tools tell you a test passed. FetchSandbox tells you the fix is real — and refuses to say so when it isn't.

✓

Measured, never self-reported. Green requires a real buggy → fixed flip on your code. No flip, no green.

✓

Reproduce first, or decline. If we can't reproduce the bug on your real code, we don't green it — we say so.

✓

Exact state, not “symptom stopped.” We assert the correct end state, so an over-suppressing fix that grants nothing gets rejected.

✓

Positive + negative controls. A correct fix must pass; a deliberately broken one must fail. If it can't fail, it isn't a proof.

Real APIs, real state

70+ pre-built specs. The lifecycle, not a mock.

Stateful sandboxes that fire real webhooks, advance real resource state, and cover the failure scenarios your fixture file doesn't — no keys, no accounts, no burning your API quota.

StripePaddlePolarClerkDescopeWorkOSResendTwilioAgentMail+ 59 more
#3 Product of the Day on Product HuntEvery proof is a shareable receiptMCP + CLI — runs where your agent runs

Coming to your pull requestsNext

Every AI-written integration ships with proof it works.

The proof engine, as a check on your PR. It reproduces the failure in an isolated sandbox — never in your runner, never touching your secrets — and blocks the merge when the fix doesn't hold.

✓ FetchSandbox / proof — All checks have passed
✓Stripe · webhook idempotencyseats 5→5→5 · proven
✓Clerk · session replayno duplicate users · proven
•Paddle · subscription lifecyclerunning…

Add it in one config block

Stop trusting green. Start proving it.

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

Cursor · Claude Code · any MCP client

Then say this to your agent

Test my Stripe webhook for duplicate deliveries.
Paddle events arrived out of order and reactivated a paused subscription.
Stripe webhook signature verification fails for valid events.

Name the API and what you're seeing. It reproduces the failure, then hands back a receipt URL you can open.

Open FetchSandboxRead the docs