x402 session sapient signer

[Agusx1211]

Draft implementation of X402SessionSapientSigner — a digest-aware sapient signer that lets a Sequence wallet authorize x402 exact Permit2 payments under a committed, session-scoped policy.

• Payments: reconstructs the canonical x402 Permit2 witness-transfer digest, validates it against the policy (token, per-payment cap, payment count via a deterministic Permit2 nonce word/slot, expiry, chain), and requires a wallet-bound session-key signature.
• Setup: also authorizes a single token.approve(PERMIT2, max) transaction (KIND_TRANSACTIONS) so a session can self-provision its Permit2 allowance.

Design rationale and trust assumptions in docs/X402_SMART_SESSIONS.md. 39 tests, including end-to-end execute/isValidSignature paths.

[Agusx1211]

Live Polygon x402 checks:

Otto AI, real third-party Permit2 payment

Bought https://x402.ottoai.services/crypto-news for 1000 Polygon USDC base units (0.001 USDC) from Sequence wallet 0xCD2604e4C01EfCfB133A240cA2fA7caf1D44B468.

This exercised:

Sequence wallet -> sapient signer ERC-1271 -> Permit2 nonce -> x402 exact Permit2 proxy -> Otto merchant

Txs:

• deploy wallet
• fund wallet
• approve Permit2 via sapient signer
• x402 Permit2 settlement

Settlement went through proxy 0x402085c248EeA27D92E8b30b2C58ed07f9E20001 and transferred 1000 USDC to Otto 0x0E84dDEdAaE6A779c462C22a59F301EC31B6b808.

Comparison: real x402 but EIP-3009, not sapient signer

PayAI Polygon endpoint only advertised the default USDC/EIP-3009 route:

• payment
• refund

Earlier self-payment smoke

• wallet deploy
• fund wallet
• Permit2 approval
• proxy settle