spec: propose split scheme for multi-party payment distribution

[0xAxiom]

Summary

Proposes a new split scheme for x402 that enables atomic multi-party payment distribution via on-chain splitter contracts.

Related: #937

Problem

The current exact scheme transfers to a single payTo address. Many real-world use cases require splitting payments between multiple parties:

• Content platforms (90% author / 10% platform)
• Referral systems (70% provider / 20% referrer / 10% protocol)
• Agent marketplaces (80% tool creator / 15% marketplace / 5% protocol)

Today, platforms handle this either off-chain (accounting only) or via custom contracts (like Postera's hardcoded 90/10 splitter). A standardized scheme would let any x402 client automatically pay into multi-party splits.

What's Included

specs/schemes/split/scheme_split.md

• Scheme overview and use cases
• PaymentRequirements format with extra.recipients array
• Basis point (BPS) allocation system
• Splitter contract interface (IPaymentSplitter)
• Client flow, verification, and settlement
• Security considerations
• Comparison with exact scheme

specs/schemes/split/scheme_split_evm.md

• EVM-specific implementation details
• Payment header payload construction
• Verification steps (event-based, smart wallet compatible)
• Gas cost estimates per recipient count
• Splitter contract ABI

Design Decisions

• Basis points over percentages: 0.01% precision, no floating point
• 2-5 recipient limit: Balances flexibility with gas costs
• Calldata-based splits: No on-chain storage needed per split (gas efficient)
• Event-based verification: Works with smart wallets (ERC-4337) where tx.to may be EntryPoint
• Permissionless contract: No admin keys, no upgrade path, fully trustless
• Remainder to first recipient: Deterministic rounding behavior

Next Steps

Per CONTRIBUTING.md, submitting the spec first for review. Once the spec is approved, I'll follow up with:

1. Reference PaymentSplitter.sol contract
2. TypeScript scheme implementation (@x402/evm split mechanism)
3. Facilitator verification logic
4. Example integration

Happy to iterate on the spec based on feedback from the x402 Foundation team.

[cb-heimdall]

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