How can an AI agent pay per API call in USDC?

x402 puts payment inside HTTP itself. There is no signup, no API key and no invoice: the payment header is the credential, and the server proves settlement in its response. That is what lets an agent find a tool and pay for it on its own.

How the flow works, step by step

1. Discover for free. On Legibility's MCP server, initialize, tools/list and ping cost nothing, so an agent can read the tools and their input schemas before deciding to pay.
2. Call without a key. A tools/call with no Authorization header and no X-PAYMENT header returns HTTP 402 with a JSON body: x402Version, an error string, and an accepts array of payment requirements.
3. Read the requirements. Each entry names the scheme (exact), the network (base-sepolia), maxAmountRequired in atomic USDC units (USDC has 6 decimals), the asset contract, the payTo address and maxTimeoutSeconds.
4. Sign and resend. The agent signs an ERC-3009 TransferWithAuthorization for that amount, which is gasless for the buyer, base64-encodes the payment payload and resends the same request with it in the X-PAYMENT header.
5. Settlement and result. Legibility verifies and then settles the payment through an x402 facilitator (by default https://x402.org/facilitator), runs the tool, and returns the result with an X-PAYMENT-RESPONSE header carrying the settlement proof.

What the 402 looks like

The first, unpaid call:

curl -i -X POST https://legibility.io/api/mcp \
  -H "content-type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"read_product","arguments":{"gtin":"8076800195057"}}}'

The response is HTTP 402 with a body shaped like this:

{
  "x402Version": 1,
  "error": "Payment required: provide a lgk_ API key (Bearer) or a settled X-PAYMENT header.",
  "accepts": [
    {
      "scheme": "exact",
      "network": "base-sepolia",
      "maxAmountRequired": "10000",
      "resource": "https://legibility.io/api/mcp",
      "description": "Legibility read_product",
      "mimeType": "application/json",
      "payTo": "0x...",
      "maxTimeoutSeconds": 120,
      "asset": "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
      "extra": { "name": "USDC", "version": "2" }
    }
  ]
}

Treat the quote you receive as authoritative rather than a number from documentation. The amount is server configuration; the default in Legibility's code is 10000 atomic units, which is 0.01 USDC. The asset shown is USDC on Base Sepolia.

What to know before relying on it

• Testnet only today. Settlement runs on Base Sepolia. The on-chain path is wired and testable, but no mainnet settlement is live yet.
• Payment settles before the read runs. On the x402 path the payment is verified and settled first, so a paid call that returns product: null has still been paid for. The rule that misses are free applies to API key billing, where only trusted reads count.
• Input validation is free. Malformed arguments, such as passing both url and gtin, come back as a tool error before any payment is requested.
• MCP only. x402 is accepted on the MCP endpoint at https://legibility.io/api/mcp. The REST endpoints under /api/v1 take an API key.
• No account metering. x402 calls are paid on-chain, so they skip account rate limiting and monthly quota. A settlement failure returns 402 again with a settle_error field.

The API key alternative

If a human sets the agent up, an API key may be simpler. Create an lgk_ key in the dashboard and send it as Authorization: Bearer lgk_.... The free plan includes 1,000 trusted reads a month with a hard stop, and a call that returns nothing trusted is not billed. Plans are listed on the home page, and the payment protocol is documented at MCP and x402.