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x402 turns the HTTP 402 status code into a real payment channel. A client requests a paid resource, the server responds with a price and a payment requirement, the client signs and retries with a payment header, the server verifies (locally or through a facilitator) and settles onchain, then returns the resource with a receipt. The whole exchange is two HTTP round-trips and no signup, API key, or contract. Polygon’s mainnet and Amoy facilitators run x402 v2. If you have a V1 integration, see Migration: V1 to V2.

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How it works

x402 payment flow
1Client→Resource ServerGET /endpoint
2Resource Server→Client402 Payment Required + PaymentRequirements (accepts array)
3Client→Resource ServerRetries with PAYMENT-SIGNATURE header
4Resource Server→FacilitatorPOST /verify (signature + requirements)
5Facilitator→PolygonSettles USDC onchain
6Resource Server→Client200 OK + body + PAYMENT-RESPONSE receipt header
Steps 2 and 3 collapse into a single call if the client already knows the route’s price ahead of time (for example, from a manifest). The signed payment header travels with the first request.

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Protocol resources

ResourceRole
ClientRequests a resource and signs the payment (browser, agent, SDK)
Resource ServerProtects endpoints, issues 402, verifies payment payloads
FacilitatorOptional verification + onchain settlement layer (Polygon: x402.polygon.technology mainnet, x402-amoy.polygon.technology Amoy)
Payment SchemeDefines chain, token, network, format (exact or upto)
PaymentRequirementsaccepts array in the 402 body: scheme, network, amount, description
PAYMENT-SIGNATURE headerSigned payment payload on the retry request
PAYMENT-RESPONSE headerBase64-encoded receipt with the settlement transaction hash

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Server side (sellers)

Protect routes with v2 middleware that pairs an x402ResourceServer with a facilitator client. The first unpaid request returns a 402 with an accepts array: 
{
  "x402Version": 2,
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:137",
      "maxAmountRequired": "1000",
      "description": "Access to premium API"
    }
  ],
  "error": null
}
maxAmountRequired is in the smallest unit of the currency. USDC has six decimals, so 1000 = $0.001. On retry, verify the PAYMENT-SIGNATURE against the facilitator and respond 200 OK with a PAYMENT-RESPONSE header carrying the receipt. For setup, see the x402 Quickstart for Sellers.

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Client side (buyers)

The buyer sends the initial request, reads the 402, picks one accepts entry, signs against its wallet, and retries with the payment header.
  • Create an x402Client, register ExactEvmScheme for "eip155:*".
  • Use wrapFetchWithPayment or wrapAxiosWithPayment to handle 402s transparently.
  • The signed payload becomes a base64 string on the retry:
PAYMENT-SIGNATURE: <base64-encoded payload>
  • Read the resulting receipt via x402HTTPClient.getPaymentSettleResponse() after a successful response.
For setup, see the x402 Quickstart for Buyers.

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Facilitator (optional)

The facilitator exposes verification and settlement endpoints: 
POST /verify
{
  "x402Version": 2,
  "paymentHeader": "<payload>",
  "paymentRequirements": { ... }
}
Some facilitators settle each payment instantly; others batch settlements for high-volume sellers. Polygon’s facilitators settle instantly on mainnet and Amoy.
NetworkFacilitator URL
Polygon mainnethttps://x402.polygon.technology
Polygon Amoyhttps://x402-amoy.polygon.technology

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Design goals

  • HTTP-native. Uses standard status codes and headers. No additional protocol layer.
  • Chain and token agnostic. Works across any chain or stablecoin.
  • Minimal integration. Middleware on the server side; a few wrappers on the buyer side.
  • Micropayments. Low friction and low cost, suitable for per-request pricing.
  • Autonomous agents. AI systems can transact without per-transaction human approval.

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Implementation

x402 Quickstart for Sellers

Add x402 v2 middleware to Express, Next.js, or Hono.

x402 Quickstart for Buyers

Pay x402-protected endpoints programmatically.

Migration: V1 to V2

Upgrade an existing V1 integration to v2.

Mainnet Facilitator Addresses

Signer addresses for the Polygon mainnet and Amoy facilitators.