> ## Documentation Index > Fetch the complete documentation index at: https://docs.polygon.technology/llms.txt > Use this file to discover all available pages before exploring further. # x402 How It Works > How the x402 v2 protocol turns an HTTP 402 response into a verified onchain payment, with one round-trip and no signup. 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](./migration-v1-to-v2). *** ## How it works

x402 payment flow

1 Client → Resource Server GET /endpoint

2 Resource Server → Client 402 Payment Required + PaymentRequirements (accepts array)

3 Client → Resource Server Retries with `PAYMENT-SIGNATURE` header

4 Resource Server → Facilitator POST /verify (signature + requirements)

5 Facilitator → Polygon Settles USDC onchain

6 Resource Server → Client 200 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. *** ## Protocol resources | Resource | Role | | -------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- | | Client | Requests a resource and signs the payment (browser, agent, SDK) | | Resource Server | Protects endpoints, issues 402, verifies payment payloads | | Facilitator | Optional verification + onchain settlement layer (Polygon: `x402.polygon.technology` mainnet, `x402-amoy.polygon.technology` Amoy) | | Payment Scheme | Defines chain, token, network, format (`exact` or `upto`) | | `PaymentRequirements` | `accepts` array in the 402 body: scheme, network, amount, description | | `PAYMENT-SIGNATURE` header | Signed payment payload on the retry request | | `PAYMENT-RESPONSE` header | Base64-encoded receipt with the settlement transaction hash | *** ## 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: ```json theme={null} { "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](./quickstart-sellers). *** ## 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: ```http theme={null} PAYMENT-SIGNATURE: ``` * Read the resulting receipt via `x402HTTPClient.getPaymentSettleResponse()` after a successful response. For setup, see the [x402 Quickstart for Buyers](./quickstart-buyers). *** ## Facilitator (optional) The facilitator exposes verification and settlement endpoints: ```http theme={null} POST /verify { "x402Version": 2, "paymentHeader": "", "paymentRequirements": { ... } } ``` Some facilitators settle each payment instantly; others batch settlements for high-volume sellers. Polygon's facilitators settle instantly on mainnet and Amoy. | Network | Facilitator URL | | --------------- | -------------------------------------- | | Polygon mainnet | `https://x402.polygon.technology` | | Polygon Amoy | `https://x402-amoy.polygon.technology` | *** ## 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. *** ## Implementation Add x402 v2 middleware to Express, Next.js, or Hono. Pay x402-protected endpoints programmatically. Upgrade an existing V1 integration to v2. Signer addresses for the Polygon mainnet and Amoy facilitators.