What is the x402 Exact Payment Scheme?
The exact payment scheme is the first and primary payment scheme in the x402 protocol. It transfers a specific, predetermined amount of tokens from client to resource server — for example, pay exactly $0.01 USDC to access an API endpoint. On EVM chains, it uses EIP-3009 transferWithAuthorization signatures.
WHY IT MATTERS
The x402 protocol is designed to be extensible across different schemes (logical ways of moving money) and networks (blockchains). The exact scheme is the foundational one — it handles the most common case: pay a fixed price for a resource.
On EVM chains (Base, Ethereum, Arbitrum), the exact scheme leverages EIP-3009 — a token standard that allows gasless transfers via signed authorisations. The client signs a message authorising transfer of exactly X tokens from their address to the recipient. The facilitator then submits this authorisation on-chain via transferWithAuthorization.
Key properties of the exact scheme:
- Fixed amount — price is known upfront, no variable billing
- Gasless for clients — the facilitator pays gas to submit the on-chain transaction
- Non-custodial — the signed authorisation can only transfer the specified amount to the specified recipient
- Replay-resistant — each authorisation has a unique nonce, usable only once
The x402 roadmap includes additional schemes: upto (authorise up to a maximum, settle actual usage — useful for LLM inference), and Cloudflare's proposed deferred (cryptographic commitment now, settlement later). The protocol also supports Solana via its SVM implementation, with different signing mechanics but the same logical exact semantics.
HOW POLICYLAYER USES THIS
PolicyLayer validates exact scheme payments by checking the authorised amount against per-request limits, daily budgets, and per-endpoint caps. Amount hallucination — where an agent signs for a wildly incorrect amount — is caught by comparing the payment payload's value against the 402 response's stated price and configured maximums.