Feat: 8021 Schema v2 support for x402

[0xClouds]

Description

Adds a new builder-code extension that enables ERC-8021 Schema 2 attribution tracking for x402 payments. Two parties attach their builder code to settlement transactions:

• Service (resource server): declares its code as the "a" (app) field in the 402 response via declareBuilderCodeExtension()
• Facilitator: adds its code as the "w" (wallet) field at settlement via BuilderCodeFacilitatorExtension

The service can optionally include related on-chain services (e.g., Morpho, Aerodrome) in the "s" array to attribute protocols it depends on.

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.

Schema 2 Suffix

{
  a: "bc_my_service",                            // app that exposed the x402 endpoint
  w: "bc_my_facilitator",                        // wallet/facilitator that settled on-chain
  s: ["bc_morpho", "bc_aerodrome"]               // related on-chain services (optional)
}

Encoded as: [CBOR data][length 2B][schema_id 0x02][8021 marker 16B]

Flow

Client                      Service                      Facilitator
  |                            |                             |
  |-- GET /resource ---------->|                             |
  |<-- 402 + builder-code ext -|                             |
  |   a: service's code        |                             |
  |   s: [related services]    |                             |
  |                            |                             |
  |-- GET /resource + payment >|                             |
  |                            |-- /verify (/settle) ------->|
  |                            |   (passthrough, untouched)  |
  |                            |                             |
  |                            |   Reads a + s from payload  |
  |                            |   Adds own code as w        |
  |                            |   Encodes Schema 2 CBOR     |
  |                            |   Appends to calldata       |
  |                            |                             |
  |<-- 200 + resource ---------|           Settlement TX with ERC-8021 suffix

Why

x402 payments currently have no attribution mechanism. Builder codes (ERC-8021) are the standard for on-chain attribution on Base, but they work by appending a suffix to transaction calldata — which requires the transaction submitter to cooperate. In x402, the client never submits a transaction; the facilitator does. This extension bridges that gap by flowing builder codes through the x402 payment protocol and having the facilitator append them at settlement time.

Out of Scope

• Payment splitting / revenue share
• Platform attribution (agent platform SDK concern)
• Non-EVM chains (SVM/Stellar/Aptos support later)
• Custom registries (all codes use canonical BuilderCodes contract)
• Permit2 settlement path (EIP-3009 only for now)

Tests

Verified On-Chain

Tested on Base Sepolia with a full service → facilitator flow:

• TX 1
• TX 2

Decoded CBOR from settlement calldata:

a: "bc_demo_agent"
w: "bc_demo_facilitator"
s: ["bc_demo_service"]

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)

Changes

New: @x402/extensions/builder-code

• types.ts — extension key, interfaces, builder code validation; fields: a (app), w (wallet), s (services)
• cbor.ts — hand-rolled Schema 2 CBOR encoder (no dependencies)
• resourceServer.ts — declareBuilderCodeExtension(appCode, serviceCodes?) for services to declare their code as "a" with optional "s" array
• facilitator.ts — BuilderCodeFacilitatorExtension with buildCalldataSuffix() that adds its code as "w"

Modified: @x402/evm (exact facilitator)

• eip3009-utils.ts — executeTransferWithAuthorization() accepts optional calldataSuffix; when present, uses encodeFunctionData() + sendTransaction() instead of writeContract()
• eip3009.ts — settleEIP3009() accepts FacilitatorContext, reads builder code extension, passes suffix
• scheme.ts — passes context to settleEIP3009()

[vercel]

@pk-coinbase is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[CarsonRoscoe]

Thank you for the contribution @0xClouds!

Please first PR up a /specs/extensions/builder-code.md proposal. Once that lands, happy to merge this

[phdargen]

Superseded by #2329