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x402 API (Chain 138–ready)

Minimal Express API that accepts x402 payments using thirdweb’s settlePayment and facilitator. Production target: Chain 138 with cUSDT V2 / cUSDC V2 (ERC-2612 / ERC-3009). Default demo: Arbitrum Sepolia + USDC so you can run the server without a permit-capable Chain 138 token in .env.

Prerequisites

Node.js 18+
thirdweb account and secret key (Dashboard → Settings → API Keys)
A server wallet address (EOA) that will receive payments

Setup

cd x402-api
cp .env.example .env
# Edit .env: set THIRDWEB_SECRET_KEY and SERVER_WALLET_ADDRESS
npm install

Run

npm start
# or with auto-reload
npm run dev

Health (no payment): GET http://localhost:4020/health
Paid routes (402 until payment): GET http://localhost:4020/api/premium, GET http://localhost:4020/api/paid

Clients must send payment authorization in the PAYMENT-SIGNATURE or X-PAYMENT header (e.g. using thirdweb’s useFetchWithPayment or equivalent).

Chain and token support

x402 requires the payment token to support ERC-2612 permit or ERC-3009 when using thirdweb facilitator. For Alltra (651940) we use local verification (no facilitator): server returns 402 + PAYMENT-REQUIRED, client pays USDC on 651940 and retries with PAYMENT-SIGNATURE + txHash; server verifies settlement on-chain. See X402_ALLTRA_ENDPOINT_SPEC.md.

Default: The API uses Arbitrum Sepolia and default USDC so you can test without custom chains.
Alltra (651940) + USDC: Set X402_USE_ALLTRA=true and SERVER_WALLET_ADDRESS in .env. Optional: CHAIN_651940_RPC_URL. Local verification is used; THIRDWEB_SECRET_KEY is not required for the Alltra path.
Chain 138: Set X402_USE_CHAIN_138=true and optionally RPC_URL_138. Use V2 compliant USD token addresses from CHAIN138_X402_TOKEN_SUPPORT.md in the API’s price config (V1 cUSDT/cUSDC are not x402-capable).

Verification script for token support:

./scripts/verify/check-chain138-token-permit-support.sh [RPC_URL]

Env reference

Variable	Required	Description
`THIRDWEB_SECRET_KEY`	Yes	thirdweb API secret key
`SERVER_WALLET_ADDRESS`	Yes	Address that receives x402 payments
`X402_USE_ALLTRA`	No	`true` for Alltra (651940) USDC + local verification (default `false`)
`X402_USE_CHAIN_138`	No	`true` to use Chain 138 (default `false`)
`RPC_URL_138`	No	Chain 138 RPC when using Chain 138 (default public RPC)
`CHAIN_651940_RPC_URL`	No	Alltra RPC when `X402_USE_ALLTRA=true` (default mainnet-rpc.alltra.global)
`PORT`	No	Server port (default `4020`)

References

thirdweb x402 – Server
CHAIN138_X402_TOKEN_SUPPORT.md – Which Chain 138 tokens support permit/ERC-3009
CHAIN138_TOKEN_ADDRESSES.md – Token addresses on Chain 138

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