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2026-03-02 11:37:34 -08:00

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x402 API (Chain 138–ready)

Minimal Express API that accepts x402 payments using thirdweb’s settlePayment and facilitator. Configured with a custom Chain 138 definition; by default uses Arbitrum Sepolia and USDC so you can test without a Chain 138 token that supports permit.

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 138 and token support

x402 requires the payment token to support ERC-2612 permit or ERC-3009. Currently, cUSDT and cUSDC on Chain 138 do not support these (see CHAIN138_X402_TOKEN_SUPPORT.md).

Default: The API uses Arbitrum Sepolia and default USDC (price: "$0.01") so you can test end-to-end without Chain 138.
Chain 138: Set X402_USE_CHAIN_138=true and optionally RPC_URL_138 in .env once a Chain 138 token has permit/ERC-3009 (e.g. after adding ERC20Permit to compliant tokens and redeploying). The server then uses the custom Chain 138 definition and the configured token for settlement.

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_CHAIN_138`	No	`true` to use Chain 138 (default `false`)
`RPC_URL_138`	No	Chain 138 RPC when using Chain 138 (default public RPC)
`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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