chore: add builder_code.md spec#2034
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…ments Adds a new builder-code extension that enables ERC-8021 Schema 2 attribution tracking for x402 payments. Three parties can attach their builder code to settlement transactions: - Agent (client): sets the "a" field via BuilderCodeClientExtension - Service (server): declares in 402 response via declareBuilderCodeExtension() - Facilitator: adds to "s" array at settlement via BuilderCodeFacilitatorExtension At settlement, the facilitator encodes all builder codes as a Schema 2 CBOR suffix and appends it to the transferWithAuthorization calldata. The EVM ignores trailing calldata bytes, so the transfer executes normally while indexers (Coindexer, Beacon, Dune) can parse the suffix. Changes: - New extension package: @x402/extensions/builder-code - EVM mechanism: executeTransferWithAuthorization accepts optional calldataSuffix - settleEIP3009 reads builder code extension from FacilitatorContext - scheme.ts passes context through to EIP-3009 settlement
…t extension - Service declares its builder code as "a" (app) — it's the application exposing the x402 endpoint - Facilitator adds its code as "w" (wallet) — it's the entity that signs and broadcasts the settlement transaction - Service can optionally include related on-chain services in "s" array (e.g., Morpho, Aerodrome) - Remove BuilderCodeClientExtension — agent doesn't attach builder codes, the service's extension data passes through untouched - CBOR encoder now handles "a", "w", and "s" fields
feat: ERC-8021 builder code attribution for x402 payments
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phdargen
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Apr 15, 2026
CarsonRoscoe
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Apr 16, 2026
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Description
Adds the
builder-codeextension spec for on-chain attribution tracking in x402 payments. This extension appends ERC-8021 Schema 2 (CBOR-encoded) builder codes to settlement transaction calldata, enabling attribution of which application exposed a paid endpoint and which facilitator settled the payment.The spec covers:
Tests
Checklist