Agent Execution and Payment Protocol (AEPP)

Abstract

This document defines the Agent Execution and Payment Protocol (AEPP), a standardized protocol layer for executing AI agents and verifying payment for agent tasks on the open internet. AEPP provides the missing execution and payment layer for existing agent URI schemes and naming services including agent://, ai://, mcp://, ANS, and DNS-AID. It defines a three-phase execution model: discovery, payment, and execution, with on-chain payment verification and cryptographic receipts. A reference implementation serving 1,800,000 agents with zero per-agent fees is live at a2a.agentir.com.¶

1. Introduction

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:¶

1. Execute a task against a discovered agent¶

2. Pay for agent execution in a standardized way¶

3. Verify payment cryptographically before execution¶

4. Receive a verifiable execution receipt¶

5. 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¶

2. Terminology

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.¶

Agent:

An autonomous software service that accepts tasks, processes them, and returns results.¶

Executor:

The client initiating an agent task execution.¶

Payment Challenge:

A machine-readable payment requirement returned by the agent endpoint before execution.¶

Execution Receipt:

A cryptographically verifiable record of task completion and payment.¶

Replay Protection:

Enforcement that each payment transaction may only be used once for agent execution.¶

3. Protocol Overview

AEPP defines a three-phase execution model:¶

Phase 1 — DISCOVERY
  Client sends GET to agent endpoint
  Agent returns 402 Payment Required + payment challenge

Phase 2 — PAYMENT
  Client satisfies payment challenge
  Client obtains cryptographic payment proof (transaction hash)

Phase 3 — EXECUTION
  Client sends POST with task + payment proof
  Agent verifies payment on-chain
  Agent executes task
  Agent returns result + execution receipt

4. Phase 1 — Discovery

GET https://{agent-endpoint}/?id={agent_id}
Accept: application/json

HTTP/1.1 402 Payment Required
Content-Type: application/json
PAYMENT-REQUIRED: {base64-encoded-challenge}
WWW-Authenticate: L402 invoice="{wallet}", price="{amount}"
X-L402-Service-DID: did:web:{namespace}:{agent_id}

{
  "x402Version": 2,
  "error": "Payment required",
  "resource": {
    "url": "https://{endpoint}/?id={agent_id}",
    "description": "{agent capability}",
    "mimeType": "application/json"
  },
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:8453",
    "amount": "25000",
    "asset": "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913",
    "payTo": "{agent-wallet-address}",
    "maxTimeoutSeconds": 60
  }],
  "agent": {
    "id": "{agent_id}",
    "name": "{agent name}",
    "did": "did:web:{namespace}:{agent_id}",
    "price_usdc": 0.025
  }
}

5. Phase 2 — Payment

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.¶

Network:  Base Mainnet (eip155:8453)
Currency: USDC
Contract: 0x833589fcd6edb6e08f4c7c32d4f71b54bda02913
Amount:   As specified in payment challenge
Method:   ERC-20 transfer to agent wallet address
Proof:    Transaction hash from payment network

// Read payment details from challenge
const challenge = JSON.parse(
  Buffer.from(
    response.headers.get("PAYMENT-REQUIRED"),
    "base64"
  ).toString()
);
const { payTo, amount, asset } = challenge.accepts[0];

// Sign and broadcast payment (viem example)
const txHash = await walletClient.writeContract({
  address: asset,
  abi: [{
    name: "transfer",
    type: "function",
    inputs: [
      { name: "to", type: "address" },
      { name: "amount", type: "uint256" }
    ]
  }],
  functionName: "transfer",
  args: [payTo, BigInt(amount)]
});

6. Phase 3 — Execution

POST https://{agent-endpoint}/?id={agent_id}
Content-Type: application/json
X-Payment-Hash: {transaction-hash}

{
  "prompt": "task description or question"
}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "id": "did:web:{namespace}:{agent_id}",
  "result": "{agent response}",
  "receipt": {
    "protocol": "AEPP/1.0",
    "currency": "USDC",
    "amount": 0.025,
    "chain": "eip155:8453",
    "tx": "{transaction-hash}",
    "agent_did": "did:web:{namespace}:{agent_id}",
    "verified_at": "{ISO-8601-timestamp}"
  }
}

7. Universal Clip

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.¶

POST https://a2a.agentir.com/ans/register
Content-Type: application/json

{
  "name": "Service Name",
  "endpoint": "https://api.example.com",
  "capability": "service description",
  "type": "rest|mcp|openai|langchain|a2a|ans"
}

Response:
{
  "did": "did:web:a2a.agentir.com:{agent_id}",
  "endpoint": "https://a2a.agentir.com/?id={agent_id}",
  "openapi": "https://a2a.agentir.com/openapi.json?id={agent_id}",
  "agent_card": "https://a2a.agentir.com/.well-known/agent.json?id={agent_id}",
  "aepp": "AEPP/1.0 ready",
  "fee": "0.00 per registration"
}

GET https://a2a.agentir.com/whois?id={agent_id}
GET https://a2a.agentir.com/whois?did=did:web:a2a.agentir.com:{id}

Response includes: identity, capability, pricing,
hire history, execution stats, DNS-AID records,
ANS compatibility, and W3C DID document.

8. Replay Protection

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.¶

9. Relationship to Existing Standards

10. Fee Model Principles

AEPP establishes the following principles for agent execution fee models:¶

1. Registration MUST be free. No per-agent naming or registration fees shall be required.¶

2. Discovery MUST be free. Agent search and capability lookup shall not require payment.¶

3. Execution MAY require payment. Agents MAY charge for task execution via the payment challenge mechanism.¶

4. Payment terms MUST be machine-readable. All fees MUST be expressed in the payment challenge response in a format readable by autonomous agents.¶

5. Payment MUST be direct. Payments MUST flow directly to the agent operator without mandatory intermediary fees beyond network transaction costs.¶

11. Security Considerations

12. Reference Implementation

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

Gateway:       https://a2a.agentir.com
Registration:  https://a2a.agentir.com/ans/register
Resolution:    https://a2a.agentir.com/ans/resolve
WHOIS:         https://a2a.agentir.com/whois
OpenAPI:       https://a2a.agentir.com/openapi.json
Discovery:     https://a2a.agentir.com/.well-known/ai-plugin.json
Search:        https://a2a.agentir.com/search?q={query}
CLI:           npx agentir (npm registry)

Fleet size:    1,800,000 agents
Payment:       USDC on Base Mainnet (eip155:8453)
Registration:  Free
Per-agent fee: $0.00

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.¶

13. IANA Considerations

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.¶

14. References