Payment-Identifier (Idempotency)

The payment-identifier extension enables idempotency for x402 payments, allowing clients to safely retry requests without duplicate payment processing.

The payment-identifier extension provides an idempotency mechanism for x402 payments. Clients can include a unique payment ID with their requests, and servers can use this ID to deduplicate payment processing - ensuring that retries with the same payment ID return cached responses without re-processing payments.

Use Cases

• Network failures: Safely retry failed requests without duplicate payments
• Client crashes: Resume requests after restart using persisted payment IDs
• Load balancing: Same request can hit different servers with shared cache
• Testing: Replay requests during development without spending funds

How It Works

1. Server advertises payment-identifier extension support in the PaymentRequired response
2. Client generates a unique payment ID and includes it in the PaymentPayload
3. Server caches responses keyed by payment ID (with configurable TTL)
4. Retry requests with the same payment ID return cached responses without re-processing payment

Quickstart for Buyers (Clients)

TypeScript

Step 1: Generate a Payment ID

Use the generatePaymentId() utility to create a unique identifier:

import { generatePaymentId } from "@x402/extensions/payment-identifier";

const paymentId = generatePaymentId();
// Example: "pay_7d5d747be160e280504c099d984bcfe0"

// Custom prefix
const orderId = generatePaymentId("order_");
// Example: "order_7d5d747be160e280504c099d984bcfe0"

Step 2: Add Payment ID to Extensions

Hook into the payment flow to add the payment ID before payload creation:

import { x402Client, wrapFetchWithPayment } from "@x402/fetch";
import {
  appendPaymentIdentifierToExtensions,
  generatePaymentId,
} from "@x402/extensions/payment-identifier";

const client = new x402Client();
// ... register schemes ...

// Generate a unique payment ID for this logical request
const paymentId = generatePaymentId();

// Hook into payment flow to add the payment ID
client.onBeforePaymentCreation(async ({ paymentRequired }) => {
  if (paymentRequired.extensions) {
    // Only appends if server declared the extension
    appendPaymentIdentifierToExtensions(paymentRequired.extensions, paymentId);
  }
});

const fetchWithPayment = wrapFetchWithPayment(fetch, client);

// First request - payment is processed
const response1 = await fetchWithPayment(url);

// Retry with same payment ID - cached response returned (no payment)
const response2 = await fetchWithPayment(url);

Python

Step 1: Generate a Payment ID

Use the generate_payment_id() utility to create a unique identifier:

from x402.extensions.payment_identifier import generate_payment_id

payment_id = generate_payment_id()
# Example: "pay_7d5d747be160e280504c099d984bcfe0"

# Custom prefix
order_id = generate_payment_id("order_")
# Example: "order_7d5d747be160e280504c099d984bcfe0"

Step 2: Add Payment ID to Extensions

Hook into the payment flow to add the payment ID before payload creation:

from x402 import x402Client
from x402.extensions.payment_identifier import (
    append_payment_identifier_to_extensions,
    generate_payment_id,
)
from x402.http.clients import x402HttpxClient
from x402.schemas import PaymentCreationContext

client = x402Client()
# ... register schemes ...

# Generate a unique payment ID for this logical request
payment_id = generate_payment_id()

# Hook into payment flow to add the payment ID
async def before_payment_creation(context: PaymentCreationContext) -> None:
    extensions = context.payment_required.extensions
    if extensions is not None:
        # Only appends if server declared the extension
        append_payment_identifier_to_extensions(extensions, payment_id)

client.on_before_payment_creation(before_payment_creation)

async with x402HttpxClient(client) as http:
    # First request - payment is processed
    response1 = await http.get(url)

    # Retry with same payment ID - cached response returned (no payment)
    response2 = await http.get(url)

Go

Step 1: Generate a Payment ID

Use the GeneratePaymentID() utility to create a unique identifier:

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

// Generate with default prefix "pay_"
paymentID := paymentidentifier.GeneratePaymentID("")
// Example: "pay_7d5d747be160e280504c099d984bcfe0"

// Generate with custom prefix
paymentID = paymentidentifier.GeneratePaymentID("order_")
// Example: "order_7d5d747be160e280504c099d984bcfe0"

Step 2: Add Payment ID to Extensions

Hook into the payment flow to add the payment ID before payload creation:

import (
    x402 "github.com/x402-foundation/x402/go"
    "github.com/x402-foundation/x402/go/extensions/paymentidentifier"
)

client := x402.Newx402Client()
// ... register schemes ...

// Generate a unique payment ID for this logical request
paymentID := paymentidentifier.GeneratePaymentID("")

// Hook into payment flow to add the payment ID
client.OnBeforePaymentCreation(func(ctx x402.PaymentCreationContext) (*x402.BeforePaymentCreationHookResult, error) {
    if ctx.Extensions == nil {
        return nil, nil
    }

    // Only add if server declared the extension
    if ctx.Extensions[paymentidentifier.PAYMENT_IDENTIFIER] == nil {
        return nil, nil
    }

    err := paymentidentifier.AppendPaymentIdentifierToExtensions(ctx.Extensions, paymentID)
    if err != nil {
        return nil, err
    }

    return nil, nil
})

// First request - payment is processed
response1, err := client.MakeRequest(url)

// Retry with same payment ID - cached response returned (no payment)
response2, err := client.MakeRequest(url)

Best Practices

1. Generate payment IDs at the logical request level, not per retry
2. Persist payment IDs for long-running operations so they survive restarts
3. Use descriptive prefixes (e.g., generatePaymentId("order_")) to identify payment types
4. Don’t reuse payment IDs across different logical requests

Quickstart for Sellers (Servers)

TypeScript

Step 1: Advertise Extension Support

Declare the payment-identifier extension in your route configuration:

import {
  paymentMiddlewareFromHTTPServer,
  x402ResourceServer,
  x402HTTPResourceServer,
} from "@x402/express";
import {
  declarePaymentIdentifierExtension,
  PAYMENT_IDENTIFIER,
} from "@x402/extensions/payment-identifier";

const routes = {
  "GET /weather": {
    accepts: [
      {
        scheme: "exact",
        price: "$0.001",
        network: "eip155:84532",
        payTo: address,
      },
    ],
    extensions: {
      [PAYMENT_IDENTIFIER]: declarePaymentIdentifierExtension(false), // optional
    },
  },
};

Optional vs Required:

// Payment ID is optional (clients can omit it)
declarePaymentIdentifierExtension(false)

// Payment ID is required (clients must provide it or receive 400 Bad Request)
declarePaymentIdentifierExtension(true)

Step 2: Cache Responses After Settlement

Store responses after successful payment settlement:

import { extractPaymentIdentifier } from "@x402/extensions/payment-identifier";

// In-memory cache (use Redis in production)
const idempotencyCache = new Map<string, { timestamp: number; response: unknown }>();
const CACHE_TTL_MS = 60 * 60 * 1000; // 1 hour

const resourceServer = new x402ResourceServer(facilitatorClient)
  .register("eip155:84532", new ExactEvmScheme())
  .onAfterSettle(async ({ paymentPayload }) => {
    const paymentId = extractPaymentIdentifier(paymentPayload);
    if (paymentId) {
      idempotencyCache.set(paymentId, {
        timestamp: Date.now(),
        response: { /* your response data */ },
      });
    }
  });

Step 3: Check Cache Before Payment

Use the onProtectedRequest hook to return cached responses and skip payment processing:

const httpServer = new x402HTTPResourceServer(resourceServer, routes)
  .onProtectedRequest(async (context) => {
    if (!context.paymentHeader) return;

    try {
      const paymentPayload = JSON.parse(
        Buffer.from(context.paymentHeader, "base64").toString("utf-8"),
      );
      const paymentId = extractPaymentIdentifier(paymentPayload);

      if (paymentId) {
        const cached = idempotencyCache.get(paymentId);
        if (cached && Date.now() - cached.timestamp < CACHE_TTL_MS) {
          return { grantAccess: true }; // Skip payment, serve from cache
        }
      }
    } catch {
      // Invalid payment header, continue to normal payment flow
    }
  });

Python

Step 1: Advertise Extension Support

Declare the payment-identifier extension in your route configuration:

from x402.server import x402ResourceServer
from x402.http import FacilitatorConfig, HTTPFacilitatorClient, PaymentOption
from x402.http.middleware.fastapi import PaymentMiddlewareASGI
from x402.http.types import RouteConfig
from x402.mechanisms.evm.exact import ExactEvmServerScheme
from x402.extensions.payment_identifier import (
    declare_payment_identifier_extension,
    PAYMENT_IDENTIFIER,
)

routes = {
    "GET /weather": RouteConfig(
        accepts=[
            PaymentOption(
                scheme="exact",
                price="$0.001",
                network="eip155:84532",
                pay_to=address,
            ),
        ],
        extensions={
            PAYMENT_IDENTIFIER: declare_payment_identifier_extension(required=False),  # optional
        },
    ),
}

Optional vs Required:

# Payment ID is optional (clients can omit it)
declare_payment_identifier_extension(required=False)

# Payment ID is required (clients must provide it or receive 400 Bad Request)
declare_payment_identifier_extension(required=True)

Step 2: Cache Responses After Settlement

Store responses after successful payment settlement:

import time
from x402.schemas import SettleContext
from x402.extensions.payment_identifier import extract_payment_identifier

# In-memory cache (use Redis in production)
idempotency_cache: dict = {}
CACHE_TTL_SECONDS = 60 * 60  # 1 hour

async def after_settle(ctx: SettleContext) -> None:
    payment_id = extract_payment_identifier(ctx.payment_payload)
    if payment_id:
        idempotency_cache[payment_id] = {
            "timestamp": time.time(),
            "response": {},  # your response data
        }

server = x402ResourceServer(facilitator)
server.register("eip155:84532", ExactEvmServerScheme())
server.on_after_settle(after_settle)

Step 3: Check Cache Before Payment

Use FastAPI middleware to check the cache before the payment middleware processes the request:

import base64
import json
from fastapi import Request, Response
from x402.schemas import PaymentPayload

@app.middleware("http")
async def idempotency_middleware(request: Request, call_next):
    payment_header = request.headers.get("X-Payment")
    if payment_header:
        try:
            payment_data = json.loads(base64.b64decode(payment_header))
            payment_payload = PaymentPayload.model_validate(payment_data)
            payment_id = extract_payment_identifier(payment_payload)

            if payment_id:
                cached = idempotency_cache.get(payment_id)
                if cached and time.time() - cached["timestamp"] < CACHE_TTL_SECONDS:
                    return Response(
                        content=json.dumps(cached["response"]),
                        media_type="application/json",
                    )
        except Exception:
            pass  # Invalid payment header, continue to normal flow

    return await call_next(request)

# Add payment middleware AFTER idempotency middleware
app.add_middleware(PaymentMiddlewareASGI, routes=routes, server=server)

Go

Step 1: Advertise Extension Support

Declare the payment-identifier extension in your route configuration:

import (
    x402http "github.com/x402-foundation/x402/go/http"
    "github.com/x402-foundation/x402/go/extensions/paymentidentifier"
)

// Optional or required payment identifier (pick one)
paymentIdExtension := paymentidentifier.DeclarePaymentIdentifierExtension(false) // optional
// paymentIdExtension = paymentidentifier.DeclarePaymentIdentifierExtension(true) // required

routes := x402http.RoutesConfig{
    "GET /weather": {
        Accepts: []x402http.PaymentOption{
            {
                Scheme:  "exact",
                Price:   "$0.001",
                Network: "eip155:84532",
                PayTo:   address,
            },
        },
        Extensions: map[string]interface{}{
            paymentidentifier.PAYMENT_IDENTIFIER: paymentIdExtension,
        },
    },
}

Step 2: Extract Payment ID

Use the ExtractPaymentIdentifier() utility to get the payment ID from the payload:

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

// In your handler
payload := c.MustGet("x402_payload").(x402.PaymentPayload)

paymentID, err := paymentidentifier.ExtractPaymentIdentifier(payload, true)
if err != nil {
    // Handle invalid payment ID
    c.JSON(400, gin.H{"error": err.Error()})
    return
}

// Check for duplicate
if existingResponse, found := processedPayments[paymentID]; found {
    // Return cached response
    c.JSON(200, existingResponse)
    return
}

Step 3: Cache Responses

Store responses after successful payment processing:

// In-memory cache (use Redis in production)
var processedPayments = make(map[string]interface{})

// After processing payment
processedPayments[paymentID] = responseData

Idempotency Behavior

Scenario	Server Response
New payment ID	Process payment normally, cache response
Same payment ID (within TTL)	Return cached response, skip payment
Same payment ID (after TTL)	Process payment normally, update cache
No payment ID	Process payment normally (no caching)

Configuration Options

Cache TTL

Adjust CACHE_TTL_MS (TypeScript/Go) or CACHE_TTL_SECONDS (Python) based on your use case:

• Short TTL (5-15 min): For time-sensitive resources
• Long TTL (1-24 hours): For static or infrequently changing resources

Production Considerations

1. Use Redis or similar instead of in-memory cache for distributed systems
2. Handle cache failures gracefully - if cache is unavailable, process payment normally
3. Consider payload hashing - for additional safety, hash the full payload and reject if same ID but different payload (409 Conflict)
4. Monitor cache hit rates to tune TTL and detect abuse

API Reference

TypeScript

Client Functions

generatePaymentId(prefix?)

Generates a cryptographically secure unique payment identifier.

import { generatePaymentId } from "@x402/extensions/payment-identifier";

const paymentId = generatePaymentId();
// Returns: "pay_<32-character-hex-string>"

const orderId = generatePaymentId("order_");
// Returns: "order_<32-character-hex-string>"

appendPaymentIdentifierToExtensions(extensions, id?)

Adds a payment identifier to the extensions object. Only modifies extensions if the server declared support for the extension. If no payment ID is provided, one is generated automatically.

import { appendPaymentIdentifierToExtensions } from "@x402/extensions/payment-identifier";

const extensions = paymentRequired.extensions ?? {};
appendPaymentIdentifierToExtensions(extensions, "pay_custom_id_1234567890abcdef");
// extensions now contains the payment-identifier extension (only if server declared it)

isValidPaymentId(id)

Validates a payment identifier format.

import { isValidPaymentId } from "@x402/extensions/payment-identifier";

isValidPaymentId("pay_7d5d747be160e280504c099d984bcfe0"); // true
isValidPaymentId("invalid"); // false (too short)

Server Functions

declarePaymentIdentifierExtension(required?)

Creates a payment-identifier extension declaration for resource servers.

import { declarePaymentIdentifierExtension } from "@x402/extensions/payment-identifier";

// Optional payment ID (default)
const extension = declarePaymentIdentifierExtension();

// Required payment ID
const extensionRequired = declarePaymentIdentifierExtension(true);

extractPaymentIdentifier(paymentPayload)

Extracts the payment identifier from a payment payload.

import { extractPaymentIdentifier } from "@x402/extensions/payment-identifier";

const paymentId = extractPaymentIdentifier(paymentPayload);
if (paymentId) {
  // Check cache, implement idempotency logic
}

validatePaymentIdentifier(extension)

Validates the payment identifier extension object structure and ID format.

import { validatePaymentIdentifier } from "@x402/extensions/payment-identifier";

const extension = paymentPayload.extensions?.["payment-identifier"];
const result = validatePaymentIdentifier(extension);
if (!result.valid) {
  console.error(result.errors);
}

Constants

import {
  PAYMENT_IDENTIFIER,      // "payment-identifier"
  PAYMENT_ID_MIN_LENGTH,   // 16
  PAYMENT_ID_MAX_LENGTH,   // 128
  PAYMENT_ID_PATTERN,      // /^[a-zA-Z0-9_-]+$/
} from "@x402/extensions/payment-identifier";

Python

Client Functions

generate_payment_id(prefix="pay_")

Generates a cryptographically secure unique payment identifier.

from x402.extensions.payment_identifier import generate_payment_id

payment_id = generate_payment_id()
# Returns: "pay_<32-character-hex-string>"

order_id = generate_payment_id("order_")
# Returns: "order_<32-character-hex-string>"

append_payment_identifier_to_extensions(extensions, id=None)

Adds a payment identifier to the extensions object. Only modifies extensions if the server declared support for the extension. If no payment ID is provided, one is generated automatically.

from x402.extensions.payment_identifier import append_payment_identifier_to_extensions

extensions = payment_required.extensions or {}
append_payment_identifier_to_extensions(extensions, "pay_custom_id_1234567890abcdef")
# extensions now contains the payment-identifier extension (only if server declared it)

is_valid_payment_id(id)

Validates a payment identifier format.

from x402.extensions.payment_identifier import is_valid_payment_id

is_valid_payment_id("pay_7d5d747be160e280504c099d984bcfe0")  # True
is_valid_payment_id("invalid")  # False (too short)

Server Functions

declare_payment_identifier_extension(required=False)

Creates a payment-identifier extension declaration for resource servers.

from x402.extensions.payment_identifier import declare_payment_identifier_extension

# Optional payment ID (default)
extension = declare_payment_identifier_extension()

# Required payment ID
extension_required = declare_payment_identifier_extension(required=True)

extract_payment_identifier(payment_payload)

Extracts the payment identifier from a payment payload.

from x402.extensions.payment_identifier import extract_payment_identifier

payment_id = extract_payment_identifier(payment_payload)
if payment_id:
    # Check cache, implement idempotency logic
    pass

validate_payment_identifier(extension)

Validates the payment identifier extension object structure and ID format.

from x402.extensions.payment_identifier import validate_payment_identifier

extension = payment_payload.extensions.get("payment-identifier")
result = validate_payment_identifier(extension)
if not result.valid:
    print(result.errors)

Constants

from x402.extensions.payment_identifier import (
    PAYMENT_IDENTIFIER,      # "payment-identifier"
    PAYMENT_ID_MIN_LENGTH,   # 16
    PAYMENT_ID_MAX_LENGTH,   # 128
    PAYMENT_ID_PATTERN,      # re.compile(r"^[a-zA-Z0-9_-]+$")
)

Go

Client Functions

GeneratePaymentID(prefix string)

Generates a cryptographically secure unique payment identifier.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

// Generate with default prefix "pay_"
paymentID := paymentidentifier.GeneratePaymentID("")
// Returns: "pay_<32-character-hex-string>"

// Generate with custom prefix
paymentID = paymentidentifier.GeneratePaymentID("order_")
// Returns: "order_<32-character-hex-string>"

AppendPaymentIdentifierToExtensions(extensions map[string]interface{}, id string) error

Adds a payment identifier to the extensions object. Only modifies extensions if the server declared support for the extension. Pass an empty string to auto-generate an ID.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

extensions := make(map[string]interface{})
err := paymentidentifier.AppendPaymentIdentifierToExtensions(extensions, "pay_custom_id_1234567890abcdef")
// extensions now contains the payment-identifier extension (only if server declared it)

IsValidPaymentID(id string) bool

Validates a payment identifier format.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

valid := paymentidentifier.IsValidPaymentID("pay_7d5d747be160e280504c099d984bcfe0") // true
valid = paymentidentifier.IsValidPaymentID("invalid") // false (too short)

Server Functions

DeclarePaymentIdentifierExtension(required bool) PaymentIdentifierExtension

Creates a payment-identifier extension declaration for resource servers.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

// Optional payment ID
extension := paymentidentifier.DeclarePaymentIdentifierExtension(false)

// Required payment ID
extensionRequired := paymentidentifier.DeclarePaymentIdentifierExtension(true)

ExtractPaymentIdentifier(payload PaymentPayload, validate bool) (string, error)

Extracts the payment identifier from a payment payload.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

paymentID, err := paymentidentifier.ExtractPaymentIdentifier(payload, true)
if err != nil {
    // Handle error
}
if paymentID != "" {
    // Check cache, implement idempotency logic
}

ValidatePaymentIdentifier(extension interface{}) ValidationResult

Validates the payment identifier extension object structure and ID format.

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

extension := payload.Extensions[paymentidentifier.PAYMENT_IDENTIFIER]
result := paymentidentifier.ValidatePaymentIdentifier(extension)
if !result.Valid {
    // Handle validation errors
    fmt.Println(result.Errors)
}

Constants

import "github.com/x402-foundation/x402/go/extensions/paymentidentifier"

const (
    PAYMENT_IDENTIFIER    = "payment-identifier"
    PAYMENT_ID_MIN_LENGTH = 16
    PAYMENT_ID_MAX_LENGTH = 128
)

var PAYMENT_ID_PATTERN = regexp.MustCompile(`^[a-zA-Z0-9_-]+$`)

Examples

Full working examples are available in the x402 repository:

TypeScript:

• TypeScript Client Example
• TypeScript Server Example

Python:

• Python Client Example
• Python Server Example

Go:

• Go Client Example
• Go Server Example

FAQ

Q: What happens if I reuse a payment ID for a different request? A: The server will return the cached response from the first request. Don’t reuse payment IDs across different logical requests.

Q: How long are payment IDs cached? A: This is configurable by the server. Typical TTLs range from 5 minutes to 24 hours depending on the use case.

Q: Can I use custom payment ID formats? A: Payment IDs must be 16-128 characters, alphanumeric with hyphens and underscores allowed. Use isValidPaymentId() to validate custom IDs.

Q: What if the server doesn’t support payment-identifier? A: The extension is optional. If the server doesn’t advertise support, clients can still make payments normally without idempotency.