API Reference

Complete reference for the x402-go packages and APIs.

Facilitator Service Endpoints

GET /supported

Returns supported configurations, extensions, and signer addresses.

Response:

{
  "kinds": [
    {"x402Version": 2, "scheme": "exact", "network": "eip155:8453"},
    {"x402Version": 2, "scheme": "exact", "network": "eip155:84532"}
  ],
  "extensions": [],
  "signers": {
    "eip155:8453": ["0xYourSignerAddress"],
    "eip155:84532": ["0xYourSignerAddress"]
  }
}

---

POST /verify

Verifies a payment payload against requirements without settling.

Request:

{
  "paymentPayload": {
    "x402Version": 2,
    "accepted": {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "1000000",
      "payTo": "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"
    },
    "payload": {
      "signature": "0x...",
      "authorization": {
        "from": "0x...",
        "to": "0x...",
        "value": "1000000",
        "validAfter": 1700000000,
        "validBefore": 1700003600,
        "nonce": "0x..."
      }
    }
  },
  "paymentRequirements": {
    "scheme": "exact",
    "network": "eip155:8453",
    "amount": "1000000",
    "payTo": "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913"
  }
}

Response:

{
  "isValid": true,
  "payer": "0xPayerAddress"
}

Error Response:

{
  "isValid": false,
  "invalidReason": "insufficient balance"
}

---

POST /settle

Executes the payment on-chain via TransferWithAuthorization.

Request: Same structure as /verify.

Response:

{
  "success": true,
  "transaction": "0xTransactionHash",
  "network": "eip155:8453",
  "payer": "0xPayerAddress"
}

Error Response:

{
  "success": false,
  "errorReason": "gas price exceeds maximum"
}

---

Facilitator Client Package

github.com/vorpalengineering/x402-go/facilitator/client

NewClient

Creates a new facilitator client.

func NewClient(facilitatorURL string) *Client

Parameters:

• facilitatorURL - Base URL of the facilitator service (e.g., http://localhost:4020)

---

Supported

Queries supported payment schemes, networks, and signers.

func (c *Client) Supported() (*types.SupportedResponse, error)

Returns:

• *types.SupportedResponse - Supported configurations
• error - Any error that occurred

---

Verify

Verifies a payment payload against requirements.

func (c *Client) Verify(req *types.VerifyRequest) (*types.VerifyResponse, error)

Parameters:

• req - Verify request containing payment payload and requirements

Returns:

• *types.VerifyResponse - Verification result with IsValid, InvalidReason, and Payer
• error - Any error that occurred

---

Settle

Settles a payment on-chain.

func (c *Client) Settle(req *types.SettleRequest) (*types.SettleResponse, error)

Parameters:

• req - Settle request containing payment payload and requirements

Returns:

• *types.SettleResponse - Settlement result with Success, Transaction, Network, and Payer
• error - Any error that occurred

---

Resource Middleware Package

github.com/vorpalengineering/x402-go/resource/middleware

NewX402Middleware

Creates a new x402 middleware instance.

func NewX402Middleware(cfg *MiddlewareConfig) *X402Middleware

Parameters:

• cfg - Middleware configuration

---

Handler

Returns the Gin handler function.

func (m *X402Middleware) Handler() gin.HandlerFunc

---

MiddlewareConfig

Configuration for the x402 middleware.

type MiddlewareConfig struct {
    // FacilitatorURL is the base URL of the x402 facilitator service
    FacilitatorURL string

    // DefaultRequirements specifies default payment requirements
    DefaultRequirements types.PaymentRequirements

    // ProtectedPaths is a list of path patterns requiring payment
    // Supports glob patterns like "/api/*" or exact paths like "/data"
    ProtectedPaths []string

    // RouteRequirements maps specific routes to custom payment requirements
    RouteRequirements map[string]types.PaymentRequirements

    // RouteResources maps specific routes to ResourceInfo metadata
    RouteResources map[string]*types.ResourceInfo

    // PaymentHeaderName is the HTTP header containing payment
    // Defaults to "PAYMENT-SIGNATURE"
    PaymentHeaderName string

    // MaxBufferSize is the maximum response buffer size in bytes
    // If the handler response exceeds this size, the request is aborted
    // 0 means unlimited
    MaxBufferSize int

    // DiscoveryEnabled enables serving the /.well-known/x402 discovery endpoint
    DiscoveryEnabled bool

    // OwnershipProofs is a list of pre-generated EIP-191 signatures
    // proving ownership of the protected resource URLs
    OwnershipProofs []string

    // Instructions is an optional markdown-formatted string containing
    // instructions or information for users of your resources
    Instructions string

    // BaseURL is the public base URL of the server (e.g., "https://api.example.com")
    // Used to construct full endpoint URLs in the discovery response
    BaseURL string

    // DiscoverableEndpoints is a list of explicit endpoint paths
    // to advertise in the discovery response
    DiscoverableEndpoints []string
}

---

Context Values

Values available in handler context after payment verification:

During Handler Execution:

verified, _ := c.Get("x402_payment_verified")       // bool
paymentHeader, _ := c.Get("x402_payment_header")    // string
requirements, _ := c.Get("x402_payment_requirements") // types.PaymentRequirements

After Settlement:

txHash, _ := c.Get("x402_settlement_tx")        // string
network, _ := c.Get("x402_settlement_network")  // string (CAIP-2)
payer, _ := c.Get("x402_settlement_payer")      // string

---

Payment Flow

The middleware implements the full x402 v2 payment flow:

1. Request without payment — Returns 402 with payment requirements and PAYMENT-REQUIRED header
2. Request with PAYMENT-SIGNATURE header — Verifies payment with facilitator
3. Invalid payment — Returns 402 with error details and PAYMENT-REQUIRED header
4. Valid payment — Handler executes (response is buffered up to MaxBufferSize)
5. Handler succeeds (2xx) — Settles payment on-chain via facilitator
6. Settlement succeeds — Sends buffered response with PAYMENT-RESPONSE header
7. Settlement fails — Returns error (buffered response is discarded)

---

Error Handling

Scenario	Response
No payment header	402 with requirements
Invalid/malformed header	400 Bad Request
Payment verification fails	402 with error
Facilitator unreachable	502 Bad Gateway
Response exceeds MaxBufferSize	500 (payment not settled)
Settlement fails	502 or 402 (response discarded)
Settlement succeeds	Original handler response sent

---

402 Response Format

{
  "x402Version": 2,
  "error": "PAYMENT-SIGNATURE header is required",
  "resource": {
    "url": "/api/data",
    "description": "Protected data endpoint",
    "mimeType": "application/json"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "1000000",
      "payTo": "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "maxTimeoutSeconds": 120
    }
  ]
}

---

Resource Client Package

github.com/vorpalengineering/x402-go/resource/client

NewResourceClient

Creates a new resource client.

func NewResourceClient(privateKey *ecdsa.PrivateKey) *ResourceClient

Parameters:

• privateKey - Ethereum private key for signing payments. Pass nil for read-only usage (checking requirements without paying).

---

Browse

Fetches the /.well-known/x402 discovery endpoint.

func (c *ResourceClient) Browse(baseURL string) (*types.DiscoveryResponse, error)

Parameters:

• baseURL - The base URL of the server (trailing slash is optional)

Returns:

• *types.DiscoveryResponse - The discovery document containing protected endpoints
• error - Any error that occurred

---

Check

Makes an HTTP request and checks if payment is required.

func (c *ResourceClient) Check(method, url, contentType string, body []byte) (*http.Response, *types.PaymentRequired, error)

Parameters:

• method - HTTP method (GET, POST, etc.)
• url - URL of the resource
• contentType - Content-Type header (empty string if not needed)
• body - Request body (nil for GET requests)

Returns:

• *http.Response - The HTTP response (body is closed if 402)
• *types.PaymentRequired - Payment requirements if 402, nil otherwise
• error - Any error that occurred

---

Requirements

Fetches payment requirements from a resource URL.

func (c *ResourceClient) Requirements(method, url, contentType string, body []byte, index int) (*types.PaymentRequirements, error)

Parameters:

• method - HTTP method
• url - URL of the resource
• contentType - Content-Type header
• body - Request body
• index - Index into the accepts array (usually 0)

Returns:

• *types.PaymentRequirements - The selected payment requirements
• error - If resource doesn't require payment or index is out of bounds

---

Payload

Generates a signed payment payload for the given requirements.

func (c *ResourceClient) Payload(requirements *types.PaymentRequirements) (*types.PaymentPayload, error)

What it does:

1. Validates payment scheme (currently only exact)
2. Parses amount, recipient, and token contract addresses
3. Creates EIP-3009 TransferWithAuthorization (random nonce, 1-hour validity window)
4. Signs with EIP-712 typed data
5. Returns the PaymentPayload struct

---

Pay

Generates payment and makes the HTTP request with the PAYMENT-SIGNATURE header.

func (c *ResourceClient) Pay(method, url, contentType string, body []byte, requirements *types.PaymentRequirements) (*http.Response, error)

Parameters:

• method - HTTP method
• url - URL of the resource
• contentType - Content-Type header
• body - Request body
• requirements - Payment requirements to use

Returns:

• *http.Response - The HTTP response
• error - Any error that occurred

---

Types Package

github.com/vorpalengineering/x402-go/types

PaymentPayload

Full signed payment payload sent in PAYMENT-SIGNATURE header.

type PaymentPayload struct {
    X402Version int                 `json:"x402Version"`
    Resource    *ResourceInfo       `json:"resource,omitempty"`
    Accepted    PaymentRequirements `json:"accepted"`
    Payload     map[string]any      `json:"payload"`
    Extensions  map[string]any      `json:"extensions,omitempty"`
}

---

PaymentRequirements

Single payment option requirements.

type PaymentRequirements struct {
    Scheme            string         `json:"scheme"`
    Network           string         `json:"network"`
    Amount            string         `json:"amount"`
    Asset             string         `json:"asset"`
    PayTo             string         `json:"payTo"`
    MaxTimeoutSeconds int            `json:"maxTimeoutSeconds"`
    Extra             map[string]any `json:"extra,omitempty"`
}

---

PaymentRequired

402 response with payment options.

type PaymentRequired struct {
    X402Version int                   `json:"x402Version"`
    Error       string                `json:"error,omitempty"`
    Resource    *ResourceInfo         `json:"resource,omitempty"`
    Accepts     []PaymentRequirements `json:"accepts"`
    Extensions  map[string]Extension  `json:"extensions,omitempty"`
}

---

ResourceInfo

Endpoint metadata.

type ResourceInfo struct {
    URL         string `json:"url"`
    Description string `json:"description,omitempty"`
    MimeType    string `json:"mimeType,omitempty"`
}

---

VerifyRequest

Request to verify payment.

type VerifyRequest struct {
    PaymentPayload      PaymentPayload      `json:"paymentPayload"`
    PaymentRequirements PaymentRequirements `json:"paymentRequirements"`
}

---

VerifyResponse

Response from payment verification.

type VerifyResponse struct {
    IsValid       bool   `json:"isValid"`
    InvalidReason string `json:"invalidReason,omitempty"`
    Payer         string `json:"payer,omitempty"`
}

---

SettleRequest

Request to settle payment on-chain.

type SettleRequest struct {
    PaymentPayload      PaymentPayload      `json:"paymentPayload"`
    PaymentRequirements PaymentRequirements `json:"paymentRequirements"`
}

---

SettleResponse

Response from payment settlement.

type SettleResponse struct {
    Success     bool   `json:"success"`
    ErrorReason string `json:"errorReason,omitempty"`
    Payer       string `json:"payer,omitempty"`
    Transaction string `json:"transaction"`
    Network     string `json:"network"`
}

---

SupportedKind

Single supported scheme-network combination.

type SupportedKind struct {
    X402Version int            `json:"x402Version"`
    Scheme      string         `json:"scheme"`
    Network     string         `json:"network"`
    Extra       map[string]any `json:"extra,omitempty"`
}

---

SupportedResponse

Response from /supported endpoint.

type SupportedResponse struct {
    Kinds      []SupportedKind     `json:"kinds"`
    Extensions []string            `json:"extensions"`
    Signers    map[string][]string `json:"signers"`
}

---

DiscoveryResponse

Response from /.well-known/x402 discovery endpoint.

type DiscoveryResponse struct {
    Version         int      `json:"version"`
    Resources       []string `json:"resources"`
    OwnershipProofs []string `json:"ownershipProofs,omitempty"`
    Instructions    string   `json:"instructions,omitempty"`
}

---

ExactEVMSchemePayload

EIP-3009 payment payload structure.

type ExactEVMSchemePayload struct {
    Signature     string                      `json:"signature"`
    Authorization ExactEVMSchemeAuthorization `json:"authorization"`
}

---

ExactEVMSchemeAuthorization

EIP-3009 authorization parameters.

type ExactEVMSchemeAuthorization struct {
    From        string `json:"from"`
    To          string `json:"to"`
    Value       string `json:"value"`
    ValidAfter  int64  `json:"validAfter"`
    ValidBefore int64  `json:"validBefore"`
    Nonce       string `json:"nonce"`
}

---

Utilities Package

github.com/vorpalengineering/x402-go/utils

GetChainID

Parse CAIP-2 network identifier to chain ID.

func GetChainID(network string) (*big.Int, error)

Example:

chainID, err := utils.GetChainID("eip155:8453") // Returns 8453

---

EncodePaymentHeader

Encode PaymentPayload to base64 for the PAYMENT-SIGNATURE header.

func EncodePaymentHeader(payload *types.PaymentPayload) (string, error)

---

DecodePaymentHeader

Decode base64 header to PaymentPayload.

func DecodePaymentHeader(header string) (*types.PaymentPayload, error)

---

Transport Headers

Header	Direction	Format	Purpose
PAYMENT-SIGNATURE	Client -> Server	Base64-encoded PaymentPayload	Client sends signed payment
PAYMENT-REQUIRED	Server -> Client	Base64-encoded PaymentRequired	Server requires payment (402 response)
PAYMENT-RESPONSE	Server -> Client	Base64-encoded SettleResponse	Server confirms payment settled (success response)