Add ERC: x402 Payment Receipt Binding for ERC-8004 Agents

[hilarl]

feat(ERCs): add ERC-XXXX x402 payment receipt binding

Summary

Adds a new ERC standardizing the binding between an x402 HTTP 402 Payment Required receipt and an ERC-8004 agent identity entry.
The on-chain interface maps keccak256(JCS(receipt)) → keccak256(utf8(did))
with deterministic selectors and an optional EIP-712 off-chain envelope.

Motivation

x402 is silent on agent identity — receipts prove a payment happened
but bind to ephemeral wallet addresses. ERC-8004 is silent on
payments — agent records carry no notion of "this agent paid for that
resource." Without a standard binding, every implementer wires the two
together with an ad-hoc off-chain database, defeating ERC-8004's goal
of cross-vendor agent portability for use cases like Sybil-resistant
reputation, paid-interaction validation, and audit trails for
autonomous agentic commerce.

This ERC is intentionally minimal and additive: it does not modify
x402, does not modify ERC-8004, and does not constrain the underlying
payment scheme, ledger, or DID method.

What's in the ERC

• IPaymentReceiptBinding interface — a new sub-registry contract
  with three core methods (bindReceipt, getAgentByReceipt,
  getReceiptsByAgent), one event (ReceiptBound), and selector-
  stability matching ERC-8004's invariant.
• Canonical hash construction — receipt_hash = keccak256(JCS(x402_receipt))
  with RFC 8785 JCS canonicalization REQUIRED.
• Agent identifier — agent_id = keccak256(utf8(payer_did)) per
  ERC-8004, DID-method-agnostic.
• Authorization model — dual: either the payer wallet that signed
  the x402 payment OR the ERC-8004 agentAddress MAY create a binding.
• Off-chain attestation envelope — optional EIP-712 JSON document
  carrying the verbatim receipt for portable audit; JCS-canonicalized
  and signed by the agent's agentAddress.
• Six-step verification procedure chaining trust into ERC-8004's
  IdentityRegistry.
• Storage layout — three mappings sufficient for O(1) lookup in
  both directions, paginated enumeration by agent.

Sections (strict EIP-1 ordering): Abstract, Motivation, Specification,
Rationale, Backwards Compatibility, Reference Implementation, Security
Considerations, Copyright (CC0).

Reference Implementation

Two reference implementations are cited, neither presented as the
canonical or owning implementation:

1. tenzro/tenzro-network — native multi-VM ERC-8004 implementation
   at precompile addresses 0x101a / 0x101b / 0x101c with
   byte-identical calldata to a Solidity deployment. Source:
   https://github.com/tenzro/tenzro-network/tree/main/crates/tenzro-identity/src/erc8004.rs.
2. Companion x402 scheme PR — docs(specs): add exact scheme for Tenzro chain (EVM/SVM/Canton façades) coinbase/x402#135 —
   exact scheme variant emitting receipts in the canonical JSON form
   this ERC hashes.

A canonical Solidity reference will accompany the final draft.

Magicians discussion link

https://ethereum-magicians.org/t/erc-x402-payment-receipt-binding-for-erc-8004-agents/PLACEHOLDER

(Discussion thread will be opened ≥7 days before this PR is opened, per
EIP-1 process. Replace PLACEHOLDER with the assigned thread ID once
the Magicians post is live.)

AI assistance disclosure

Per the project's AI disclosure norms: parts of this ERC text and PR
description were prepared with AI assistance from Claude Code. All
technical decisions — interface shape, hash construction, authorization
model, verification procedure, storage layout — were reviewed and
authored by the human author (Hilal Agil, hilal@tenzro.com). The
reference implementation in tenzro/tenzro-network was developed
independently over preceding months as part of the Tenzro Ledger
codebase.

Checklist

• Single new file under ERCS/ (ERCS/erc-XXXX.md); ERC editors
  will assign the final number.
• Frontmatter follows EIP-1: lowercase keys, exact field order
  (eip, title, description, author, discussions-to,
  status, type, category, created, requires); ISO date
  2026-05-02; status Draft; type Standards Track; category
  ERC; requires: 8004.
• Author entry: Hilal Agil (@hilarl) <hilal@tenzro.com>.
• Sections in strict EIP-1 order: Abstract, Motivation,
  Specification, Rationale, Backwards Compatibility, Reference
  Implementation, Security Considerations, Copyright.
• Copyright section is CC0 with the standard waiver line.
• DCO sign-off on every commit:
  Signed-off-by: Hilal Agil <hilal@tenzro.com>.
• Conventional Commit subject:
  feat(ERCs): add ERC-XXXX x402 payment receipt binding.
• No prescriptive references to any specific ledger, VM, or DID
  method in the normative Specification section.
• All examples use placeholder hex (0x<...>) or generic
  identifiers; no mainnet contract addresses cited as canonical.
• Magicians thread linked above (will be ≥7 days old at PR open
  time).

[eip-review-bot]

File ERCS/erc-draft_x402_erc8004_binding.md

Requires 1 more reviewers from @g11tech, @jochem-brouwer, @samwilsn, @xinbenlv

[github-actions]

The commit 3f32470 (as a parent of f264479) contains errors.
Please inspect the Run Summary for details.