Once merged, you will be able to read the x402 protocol specification in the x402 repository.
1. Overview
x402 is an open payment standard that enables clients to pay for HTTP resources using blockchain technology. The protocol leverages the existing HTTP 402 “Payment Required” status code to indicate when payment is required for resource access, providing a standardized mechanism for micropayments on the web. This specification is based on the x402 protocol implementation and documentation available in the Coinbase x402 repository. It aims to provide a comprehensive and implementation-agnostic specification for the x402 HTTP-native micropayment protocol.2. Core Payment Flow
The x402 protocol follows a standard HTTP request-response cycle with payment integration:- Client Request: Client makes an HTTP request to a resource server
- Payment Required Response (402): If no valid payment is attached, the server responds with an HTTP 402 status code and payment requirements
- Payment Authorization Request: Client submits a signed payment authorization in the subsequent request
- Settlement Response: Server verifies the payment authorization and initiates blockchain settlement
3. Protocol Components
The x402 protocol involves three primary components:- Resource Server: A web service that requires payment for access to protected resources (APIs, content, data, etc.)
- Client: Any application or agent that requests access to protected resources
- Facilitator: An endpoint service that handles payment verification and blockchain settlement
4. HTTP Status Codes
The x402 protocol uses standard HTTP status codes with specific semantics:- 200 OK: Request successful, payment verified and settled
- 402 Payment Required: Payment required to access the resource
- 400 Bad Request: Invalid payment payload or payment requirements
- 500 Internal Server Error: Server error during payment processing
5. Data Types
This section defines the core data structures used in the x402 protocol.5.1 Payment Requirements Response
5.1.1 JSON Payload
When a resource server requires payment, it responds with an HTTP 402 status code and a JSON payload containing payment requirements. Example:5.1.2 Field Descriptions
The Payment Requirements Response contains the following fields: All fields are required.Field Name | Type | Description |
---|---|---|
x402Version | number | Protocol version identifier |
error | string | Human-readable error message explaining why payment is required |
accepts | array | Array of payment requirement objects defining acceptable payment methods |
accepts
array contains:
Field Name | Type | Required | Description |
---|---|---|---|
scheme | string | Required | Payment scheme identifier (e.g., “exact”) |
network | string | Required | Blockchain network identifier (e.g., “base-sepolia”, “ethereum-mainnet”, “solana”) |
maxAmountRequired | string | Required | Required payment amount in atomic token units |
asset | string | Required | Token contract address |
payTo | string | Required | Recipient wallet address for the payment |
resource | string | Required | URL of the protected resource |
description | string | Required | Human-readable description of the resource |
mimeType | string | Optional | MIME type of the expected response |
outputSchema | object | Optional | JSON schema describing the response format |
maxTimeoutSeconds | number | Required | Maximum time allowed for payment completion |
extra | object | Optional | Scheme-specific additional information |
5.2 Payment Proof (X-PAYMENT Header)
5.2.1 JSON Structure
The client includes payment authorization in theX-PAYMENT
header as base64-encoded JSON:
5.2.2 Field Descriptions
The Payment Payload contains the following fields: All fields are required.Field Name | Type | Description |
---|---|---|
x402Version | number | Protocol version identifier (must be 1) |
scheme | string | Payment scheme identifier (e.g., “exact”) |
network | string | Blockchain network identifier (e.g., “base-sepolia”, “ethereum-mainnet”) |
payload | object | Payment data object |
payload
field contains scheme-specific data on EVM:
All fields are required.
Field Name | Type | Description |
---|---|---|
signature | string | EIP-712 signature for authorization |
authorization | object | EIP-3009 authorization parameters |
Field Name | Type | Description |
---|---|---|
from | string | Payer’s wallet address |
to | string | Recipient’s wallet address |
value | string | Payment amount in atomic units |
validAfter | string | Unix timestamp when authorization becomes valid |
validBefore | string | Unix timestamp when authorization expires |
nonce | string | 32-byte random nonce to prevent replay attacks |
payload
field contains scheme-specific data on SVM:
All fields are required.
Field Name | Type | Description |
---|---|---|
transaction | string | Base64-encoded partially-signed transaction |
5.3 Settlement Response
5.3.1 JSON Structure
After payment settlement, the server includes transaction details in theX-PAYMENT-RESPONSE
header as base64-encoded JSON:
5.3.2 Field Descriptions
The Settlement Response contains the following fields:Field Name | Type | Required | Description |
---|---|---|---|
success | boolean | Required | Indicates whether the payment settlement was successful |
errorReason | string | Optional | Error reason if settlement failed (omitted if successful) |
transaction | string | Required | Blockchain transaction hash (empty string if settlement failed) |
network | string | Required | Blockchain network identifier |
payer | string | Required | Address of the payer’s wallet |
6. Payment Schemes
This section describes the payment schemes supported by the x402 protocol. Each scheme defines a specific method for authorizing and executing payments.6.1 Exact Scheme
The “exact” scheme uses EIP-3009 (Transfer with Authorization) to enable secure, gasless transfers of specific amounts of ERC-20 tokens.6.1.1 EIP-3009 Authorization
The authorization follows the EIP-3009 standard fortransferWithAuthorization
:
6.1.2 Verification Steps
The facilitator performs the following verification steps:- Signature Validation: Verify the EIP-712 signature is valid and properly signed by the payer
- Balance Verification: Confirm the payer has sufficient token balance for the transfer
- Amount Validation: Ensure the payment amount meets or exceeds the required amount
- Time Window Check: Verify the authorization is within its valid time range
- Parameter Matching: Confirm authorization parameters match the original payment requirements
- Transaction Simulation: Simulate the
transferWithAuthorization
transaction to ensure it would succeed
6.1.3 Settlement
Settlement is performed by calling thetransferWithAuthorization
function on the ERC-20 contract with the signature and authorization parameters provided in the payment payload.
7. Facilitator Interface
The facilitator provides REST APIs for payment verification and settlement. This allows resource servers to delegate blockchain operations to trusted third parties or host the endpoints themselves.7.1 POST /verify
Verifies a payment authorization without executing the transaction on the blockchain.Request (Exact Scheme):
Request (Exact Scheme on Solana):
Successful Response:
Error Response:
7.2 POST /settle
Settles a payment by broadcasting the transaction to the blockchain.Request (Exact Scheme):
Request (Exact Scheme on Solana):
Successful Response:
Error Response:
7.3 GET /supported
Returns the list of payment schemes and networks supported by the facilitator.Response:
8. Discovery API
The x402 protocol includes a discovery mechanism that allows clients to find and explore available x402-enabled resources. This enables the creation of marketplaces (known as ‘Bazaars’) where users can discover and access monetized APIs and digital services.8.1 GET /discovery/resources
List discoverable x402 resources from the Bazaar.Request Parameters:
Parameter | Type | Required | Description | Default |
---|---|---|---|---|
type | string | Optional | Filter by resource type (e.g., “http”) | - |
limit | number | Optional | Maximum number of results to return (1-100) | 20 |
offset | number | Optional | Number of results to skip for pagination | 0 |
Response:
8.2 Discovered Resource Fields
Field Name | Type | Required | Description |
---|---|---|---|
resource | string | Required | The resource URL or identifier being monetized |
type | string | Required | Resource type (currently “http” for HTTP endpoints) |
x402Version | number | Required | Protocol version supported by the resource |
accepts | array | Required | Array of payment requirement objects specifying payment methods |
lastUpdated | number | Required | Unix timestamp of when the resource was last updated |
metadata | object | Optional | Additional metadata (category, provider, etc.) |
8.3 Bazaar Concept
The Bazaar is a marketplace ecosystem where x402-enabled resources can be discovered and accessed. Key features:- Resource Discovery: Find APIs and services by category, provider, or payment requirements
- Payment Transparency: View pricing and payment methods upfront
- Provider Information: Learn about service providers and their offerings
- Dynamic Updates: Resources can be added, updated, or removed dynamically
8.4 Example Usage
9. Error Handling
The x402 protocol defines standard error codes that may be returned by facilitators or resource servers. These error codes help clients understand why a payment failed and take appropriate action.insufficient_funds
: Client does not have enough tokens to complete the paymentinvalid_exact_evm_payload_authorization_valid_after
: Payment authorization is not yet valid (before validAfter timestamp)invalid_exact_evm_payload_authorization_valid_before
: Payment authorization has expired (after validBefore timestamp)invalid_exact_evm_payload_authorization_value
: Payment amount is insufficient for the required paymentinvalid_exact_evm_payload_signature
: Payment authorization signature is invalid or improperly signedinvalid_exact_evm_payload_recipient_mismatch
: Recipient address does not match payment requirementsinvalid_network
: Specified blockchain network is not supportedinvalid_payload
: Payment payload is malformed or contains invalid datainvalid_payment_requirements
: Payment requirements object is invalid or malformedinvalid_scheme
: Specified payment scheme is not supportedunsupported_scheme
: Payment scheme is not supported by the facilitatorinvalid_x402_version
: Protocol version is not supportedinvalid_transaction_state
: Blockchain transaction failed or was rejectedunexpected_verify_error
: Unexpected error occurred during payment verificationunexpected_settle_error
: Unexpected error occurred during payment settlement
10. Security Considerations
10.1 Replay Attack Prevention
The x402 protocol implements multiple layers of protection against replay attacks:- EIP-3009 Nonce: Each authorization includes a unique 32-byte nonce to prevent replay attacks
- Blockchain Protection: EIP-3009 contracts inherently prevent nonce reuse at the smart contract level
- Time Constraints: Authorizations have explicit valid time windows to limit their lifetime
- Signature Verification: All authorizations are cryptographically signed by the payer
10.2 Authentication Integration
The protocol supports integration with authentication systems (e.g., Sign-In with Ethereum (SIWE)) to enable authenticated pricing models where verified users receive discounted rates or special access terms.11. Implementation Notes
11.1 Supported Networks
The following blockchain networks are currently supported by the reference implementation:base-sepolia
: Base Sepolia testnet (Chain ID: 84532)base
: Base mainnet (Chain ID: 8453)avalanche-fuji
: Avalanche Fuji testnet (Chain ID: 43113)avalanche
: Avalanche mainnet (Chain ID: 43114)
11.2 Supported Assets
The protocol currently supports the following token types:USDC
: USD Coin (EIP-3009 compliant ERC-20 token)- Additional ERC-20 tokens: May be supported if they implement EIP-3009 (Transfer with Authorization)
- EIP-3009 compliance for the “exact” scheme
- Facilitator service capabilities
- Network-specific token availability
12. Use Cases and Applications
The x402 protocol enables diverse monetization scenarios across the internet. While the core protocol is HTTP-native and chain-agnostic, specific implementations can vary based on use case requirements.12.1 AI Agent Integration
AI agents can use x402 to autonomously pay for resources and services. The protocol supports:- Automatic payment handling for API calls
- Resource discovery through facilitator services
- Budget management and spending controls (implementation-specific)
- Correlation tracking for operation grouping (implementation-specific)
12.2 Human User Applications
Traditional web applications can implement x402 for:- Session-based access (time-limited subscriptions)
- Pay-per-use content (articles, videos, downloads)
- API monetization with per-call pricing
- Authentication-based pricing (discounted rates for verified users)
12.3 Server Frameworks
x402 integrates with popular web frameworks:- Express.js:
require_payment()
middleware - FastAPI/Flask: Framework-specific middleware
- Hono: Edge runtime support
- Next.js: Full-stack integration
12.4 Client Libraries
HTTP clients can be enhanced with x402 payment capabilities:- Axios/fetch: Browser-based payments
- httpx/requests: Python client support
- Custom integrations: Application-specific payment handling
12.5 Advanced Patterns
The protocol enables sophisticated monetization strategies:- Dynamic pricing based on user authentication or usage patterns
- Session management for time-based access control
- Batch payments for multiple resource access
- Subscription models built on micropayments
13. Version History
Version | Date | Changes | Author |
---|---|---|---|
v0.1 | 2025-8-29 | Initial draft | [derived from repository] |
14. Supported Networks
x402 is supported on the following networks:Network Name | Network String |
---|---|
Solana | solana |
Solana Devnet | solana-devnet |
Base | base |
Base Sepolia | base-sepolia |
Avalanche | avalanche |
Avalanche Fuji | avalanche-fuji |
IoTeX | iotex |
Sei | sei |
Sei Testnet | sei-testnet |
15. Facilitators
x402 is supported by the following facilitators:Facilitator | Endpoint | Supported Networks |
---|---|---|
PayAI Facilitator | https://facilitator.payai.network | solana , solana-devnet , base , base-sepolia , avalanche , avalanche-fuji , iotex , sei , sei-testnet |
x402.org Facilitator | https://x402.org/facilitator | base-sepolia |
CDP’s x402 Facilitator | https://cdp.coinbase.com | base , base-sepolia |
x402.rs Facilitator | https://facilitator.x402.rs | base-sepolia , base , xdc |
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