What is x402?

x402 is an HTTP payment protocol built on the 402 Payment Required status code. It lets agents pay for API calls directly with on-chain USDC — no API keys, no OAuth, no rate limit tiers.

The flow:

1. Client calls the API without any auth header
2. API responds with 402 Payment Required and payment details (amount, recipient, network)
3. Client signs an EIP-3009 USDC transfer authorization and retries the request with X-PAYMENT header
4. API verifies the signature, executes the USDC transfer, and fulfills the request

Why x402?

For AI agents, x402 is ideal:

• Agents hold wallets, not API credentials
• Payments are programmable (agents can authorize specific amounts)
• No manual API key rotation or rate limits to manage
• Works with any USDC-holding wallet on Base

For developers, it's simple:

// Wrap fetch once, pay automatically
const fetchPay = wrapFetchWithPayment(fetch, client);
 
// Calls that need payment: handled automatically
await fetchPay("https://api.inkdprotocol.com/v1/projects", {
  method: "POST",
  body: JSON.stringify({ name: "my-project" }),
});

EIP-3009

x402 uses EIP-3009 transferWithAuthorization — a standard for signing USDC transfers off-chain. The signed authorization is sent in the X-PAYMENT header. The API server executes the on-chain transfer on the client's behalf.

This means:

• No ETH gas needed from the agent wallet (gas is paid by the API server)
• Payment happens atomically with the API call
• Signatures are single-use (nonce prevents replay)

Pricing

Revenue flow

When you pay to create a project or push a version, the USDC flows through the protocol:

1. USDC → Treasury — collected via EIP-3009
2. Treasury.settle() — splits the payment:
   • 50% → InkdBuyback (accumulates until $50 threshold, then auto-buys $INKD)
   • 50% → Protocol treasury

Libraries

npm install @x402/fetch @x402/evm

• @x402/fetch — wraps fetch with automatic payment handling
• @x402/evm — EVM signer adapter (viem-compatible)