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x402: HTTP 402 payments for AI agents

What x402 (Coinbase) is, how an agent pays via HTTP 402, a minimal example, how to declare support, and how we verify it.

Updated:

What it is

x402 is an open HTTP payment protocol from Coinbase that revives the reserved HTTP 402 Payment Required status. The server replies 402 with a PAYMENT-REQUIRED header, the agent sends a PAYMENT-SIGNATURE, and a facilitator verifies the signature and settles the on-chain transaction — all with no human in the loop. Payments are in stablecoins (USDC) on Base, Polygon, Arbitrum, Solana.

Why it matters for AI agents

Autonomous agents need a way to pay without a human. x402 covers pay-per-use APIs (an agent pays $0.001 per call), monetising agent traffic without subscriptions, and agent-to-agent settlement. By April 2026: ~165M transactions and ~69k connected agents.

Minimal working example

GET /premium-report HTTP/1.1

HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: scheme="exact", amount="0.01", currency="USDC",
  network="eip155:8453", recipient="0x..."

# The agent pays and retries:
GET /premium-report HTTP/1.1
PAYMENT-SIGNATURE: <signed_payment_payload>

HTTP/1.1 200 OK

network is in CAIP-2 form (eip155:8453 = Base).

How to declare support

In ai-agent.json:

{ "payments": { "x402": "https://api.example.com/x402" } }

Or via a Link header:

Link: <https://api.example.com/x402>; rel="x402-endpoint"

Common mistakes

  • Declaring an endpoint that doesn’t implement the 402/PAYMENT-SIGNATURE flow.
  • Wrong network (not CAIP-2) — the agent can’t tell the chain.
  • No declaration in ai-agent.json or a Link header — the agent never learns of support.

How to verify

The x402 check is informational (doesn’t affect the score). The scanner looks for a declaration two ways: a payments.x402 field in /.well-known/ai-agent.json, or a Link header with rel="x402-endpoint". The real payment flow isn’t tested.

curl -s https://example.com/.well-known/ai-agent.json | jq '.payments.x402'
curl -sI https://example.com/ | grep -i 'x402-endpoint'

Sources