The argument is always the same. You sent the work on Tuesday. The client says the folder was empty, or the version was wrong, or nothing showed up at all. There's no referee. Just two inboxes and a payment sitting in the middle.
ProofReceipt fixes the receipt side of that. When you deliver, the app hashes the file with SHA-256 and gives you a receipt that ties that exact file to that moment. Change one character in the file and the hash changes. That's the whole trick, and it holds up because the maths doesn't care who argues louder.
I've written about the app here before. This post is about the piece I shipped after it: a verify endpoint other software can call.
The x402 verify endpoint
It lives at https://proofreceipt-verify.netlify.app and it does one job. Send it a receipt and an invoice amount, and it answers MATCH or MISMATCH, naming the check that failed if it's a mismatch. One character changed, one cent changed — it catches both, because I tested exactly those two cases until they failed loudly.
The payment part is the interesting bit. It uses x402, so there's no account, no API key dashboard, no monthly plan. An unpaid request gets a plain HTTP 402 back with the price attached: 0.5% of the invoice amount on the receipt, with a one-cent floor. A $25 invoice costs 12.5 cents to verify. A $2 one costs the floor. Pay, resend, get the verdict.
Right now it runs in test mode on Base Sepolia while the mainnet keys are sorted out. That part matters and I'm not dressing it up: test mode means test money. The endpoint answers, the 402 challenge is real, the verdicts are real. Settled mainnet payments aren't live yet.
Who'd actually call this
Two groups, mostly. Marketplaces and escrow tools that want a delivery check inside their own flow instead of sending users to a website. And AI agents. An agent that delivers work for a human has the same problem a freelancer does, except it can't argue in a dispute thread at all. A hash receipt plus a machine-readable verify call is the closest thing it gets to a paper trail.
The app itself is still at https://proofreceipt.netlify.app if you just want to make a receipt by hand. It's free to try, it's pre-revenue, and I'd rather you test the tamper case yourself than take my word for any of this. Change a full stop in a text file. Watch the verdict flip.
That's the test that convinced me, anyway.
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