Agent Execution and Payment Protocol (AEPP)

The emergence of AI agent naming and discovery standards including ANS, DNS-AID, agent://, ai://, and mcp:// has established mechanisms for agent identity and discovery. However, none of these standards define how to: Execute a task against a discovered agent Pay for agent execution in a standardized way Verify payment cryptographically before execution Receive a verifiable execution receipt Prevent replay attacks on payment transactions AEPP fills this gap. It is protocol-agnostic and works on top of any agent URI scheme or naming service. An agent discovered via ANS, DNS-AID, agent://, or any other mechanism can be executed via AEPP. AEPP is designed to be: Fee-free per agent — no per-registration or per-naming fees Open — any agent network may implement AEPP Federated — no central execution authority Verifiable — all payments cryptographically verified on-chain Interoperable — works with all existing agent standards

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. An autonomous software service that accepts tasks, processes them, and returns results. The client initiating an agent task execution. A machine-readable payment requirement returned by the agent endpoint before execution. A cryptographically verifiable record of task completion and payment. Enforcement that each payment transaction may only be used once for agent execution.

AEPP defines a three-phase execution model:

The agent MUST return HTTP 402 Payment Required with a payment challenge in both the response body and the PAYMENT-REQUIRED header (base64 encoded).

The executor satisfies the payment challenge using any compatible payment method. The reference implementation uses USDC on Base mainnet (eip155:8453). AEPP does not mandate a specific payment network or currency. Implementations MAY support alternative payment schemes registered as extensions to this protocol. The payment proof MUST be a cryptographically verifiable transaction identifier that the agent can independently verify against the payment network.

Upon receiving the execution request the agent MUST: Extract the payment proof from X-Payment-Hash header Verify the transaction exists on the payment network Verify the transaction recipient matches the agent wallet Verify the transaction amount meets the required amount Verify the transaction hash has not been used previously (replay protection) Record the transaction hash as used before execution If any verification step fails the agent MUST return HTTP 403 Forbidden.

AEPP defines a Universal Clip registration mechanism that converts any existing API or agent protocol to AEPP-compatible endpoints. This eliminates per-agent registration fees by generating all required metadata automatically.

REST APIs — any JSON HTTP endpoint Model Context Protocol (MCP) OpenAI plugin manifest LangChain tools Google A2A protocol ANS registered agents DNS-AID discovered agents

Any service registers with a single POST request:

Registration is free. AEPP implementations MUST NOT charge per-agent registration fees. Revenue models MUST be based on execution, not naming or registration.

Any registered agent is queryable via WHOIS:

AEPP implementations MUST enforce replay protection. Each payment transaction hash MUST be accepted for execution exactly once. Subsequent requests using the same transaction hash MUST be rejected with HTTP 403 and the reason "TX_ALREADY_REDEEMED". Implementations MUST maintain a persistent record of redeemed transaction hashes. This record MUST survive service restarts.

ANS provides agent identity and naming via DNS and PKI. AEPP provides the execution and payment layer that ANS does not define. An ANS-registered agent MAY expose an AEPP-compatible endpoint. AEPP complements ANS without replacing it.

DNS-AID provides agent discovery via DNS records. AEPP provides execution once an agent is discovered via DNS-AID. AEPP-registered agents automatically generate DNS-AID compatible SVCB and TXT records.

Every AEPP agent endpoint is associated with a W3C Decentralized Identifier. The DID document is resolvable at the agent's .well-known endpoint. AEPP execution receipts include the agent DID for cryptographic attribution.

AEPP builds on the HTTP 402 payment challenge pattern established by x402 and L402. The PAYMENT-REQUIRED header and WWW-Authenticate header formats follow established conventions. AEPP extends these with agent-specific fields and on-chain verification.

The Google Agent-to-Agent (A2A) protocol defines agent communication patterns. AEPP adds a payment and execution verification layer compatible with A2A agent discovery and communication flows.

Every AEPP-registered agent automatically generates an OpenAPI 3.1.0 specification from capability metadata. The spec includes x402 as a security scheme definition.

AEPP establishes the following principles for agent execution fee models: Registration MUST be free. No per-agent naming or registration fees shall be required. Discovery MUST be free. Agent search and capability lookup shall not require payment. Execution MAY require payment. Agents MAY charge for task execution via the payment challenge mechanism. Payment terms MUST be machine-readable. All fees MUST be expressed in the payment challenge response in a format readable by autonomous agents. Payment MUST be direct. Payments MUST flow directly to the agent operator without mandatory intermediary fees beyond network transaction costs.

All payments MUST be verified on-chain before execution. Agents MUST NOT execute tasks based on unverified payment claims. Transaction verification MUST check recipient address, amount, and transaction finality.

Transaction hashes are single-use. Replay attacks are prevented by maintaining a persistent ledger of redeemed hashes. See Section 7 for requirements.

Agent identity is anchored to W3C DIDs with blockchain account verification. Agent wallet addresses are cryptographically bound to agent identity documents.

All AEPP endpoints MUST use TLS. Payment challenges MUST include the exact recipient wallet address. Executors MUST verify the wallet address matches the agent's DID document before payment.

A production reference implementation of AEPP is live serving 1,800,000 agents at zero per-agent registration fee:

The implementation demonstrates all AEPP phases including Universal Clip translation, on-chain payment verification, replay protection, W3C DID per agent, ANS and DNS-AID compatibility, semantic vector search, and persistent cross-device agent memory.

This document requests registration of the following HTTP header fields: PAYMENT-REQUIRED — base64 encoded payment challenge X-Payment-Hash — payment transaction proof X-L402-Service-DID — agent DID for L402 flows This document requests registration of the AEPP protocol identifier in the appropriate IANA registry.