P21 / INTEROPERABILITY
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One protocol.
Across systems.
Bitcoin-native evidence across agent frameworks, payment systems, and execution chains.
Compatibility specifications; released adapters are listed in implementation status.
[Implementation status](https://proof21.xyz/docs/start/status/)
Payment + discovery
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Binance B402 Bazaar
Capability descriptions, payment requirements, and service evidence mapped to B402 discovery.
Payment + discovery
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Coinbase CDP Bazaar
x402 service discovery with payment evidence kept separate from task verification.
Tool distribution
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MCP Registry
Versioned tool contracts with explicit inputs, permissions, findings, and failure states.
Native evidence
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TAP / DMT / Ordinals
Bitcoin-derived rules and asset evidence interpreted under native metaprotocol semantics.
Agent commerce
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Virtuals ACP
Agent commerce records connected to independently evaluated service outcomes.
Evidence + coordination
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Trac / OpenMayhem
Original signed receipts and source context carried across coordinated agent workflows.
Alternative payment adapters
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PayAI / thirdweb
Provider-specific payment schemes mapped to a common evidence contract.
Financial agent tooling
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OKX Onchain OS
Financial-action evidence bound to network, asset, recipient, and execution policy.
Communication + identity
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A2A / ERC-8004
Agent communication and identity references alongside independent evidence evaluation.
Bitcoin payments
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Lightning L402
Bitcoin-native service payments with authorization and verification kept separate.
External evidence + beacons
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Chainlink / drand
External proofs and beacon sources with distinct verification and trust profiles.
Agent clients
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Hermes / local agents
One evidence model across agent clients, local tooling, and service interfaces.
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Independent systems. Shared evidence.
Each interface preserves its own payment, identity, and execution semantics. Proof21 provides the evidence contract between them.
[Interface contracts ↗](https://proof21.xyz/docs/integrations/catalogs/)
The payment for an API service and the requested financial payout must be checked as separate events.