FetchSandbox MCP

FetchSandbox MCP is a testing tool built for the age of AI-written code. It helps developers and AI coding agents find real failures in integration code — think Stripe webhooks, Paddle subscriptions, Clerk auth — and then prove that a fix actually works. Instead of relying on green CI checks that might pass even when a fix does nothing, FetchSandbox reproduces the bug on your actual code, applies the fix, and verifies the correct end state. The result is a shareable receipt URL that shows the bug was reproduced and the fix holds.

How does it work? You add one MCP server to your IDE (Cursor, Claude Code, or any MCP client) with a single config block. Then you tell your agent something like "Test my Stripe webhook for duplicate deliveries." FetchSandbox surfaces the real failure classes — idempotency issues, out-of-order events, auth bypasses — and runs the scenario against your code. After your agent writes a patch, FetchSandbox re-runs the same scenario to confirm the fix is real. It uses positive and negative controls: a correct fix must pass, and a deliberately broken one must fail. If it can't fail, it won't greenlight the fix.

This tool is ideal for developers and engineering teams who ship integration code frequently and want to avoid production surprises. It's especially useful for teams using AI coding agents that might generate plausible-looking but broken fixes. Use cases include verifying Stripe webhook idempotency, ensuring Clerk session replay doesn't create duplicate users, and testing Paddle subscription lifecycle handling. With 70+ pre-built specs for popular APIs like Twilio, Resend, and WorkOS, FetchSandbox MCP turns vague confidence into concrete proof.