TL;DR. BuildProof is an open-source registry on Base mainnet where several parties co-sign the SHA-256 of a file. One of the file types is an AI-output manifest: the hashes of the prompt, the input documents and the model's answer, bundled into one canonical JSON. The AI operator signs it, a human reviewer signs or rejects it, and anyone can later check a file against the chain for free, or let an agent pay $0.01 in USDC per lookup over x402. Code, contract and endpoints are live. It has no real users yet; everything on mainnet is demo data.

The problem: AI output has no paper trail

I direct a construction and real-estate development company. Every month the same paperwork moves between the client, the general contractor, subcontractors and technical supervision: acceptance acts, invoices, supplementary agreements that change a quantity or a unit rate. It travels as PDFs over e-mail and messengers.…