How does an AI agent pay for an API call with x402, step by step, and what exactly is in the 402 response and the retry?

Here's how x402 works on DeskCrew, step by step:

1. The exchange order

1. Your agent sends an HTTP request to a paid tool (e.g. POST /api/mcp/deskcrew calling draft_support_reply).
2. The server replies HTTP 402 Payment Required with a JSON body containing an accepts[] array — one entry per chain, each quoting the exact terms.
3. Your agent signs a USDC payment matching those terms (an EIP-3009 transferWithAuthorization on EVM chains; on Solana the exact-svm scheme).
4. Your agent retries the identical request, this time including the signed payment in the X-PAYMENT header.
5. The server verifies the payment, runs the tool, and returns the result. The X-PAYMENT-RESPONSE header carries the on-chain settlement transaction hash. The authorization is only settled after a non-error result — a tool failure means no charge.

2. Header and payload field names

402 response body (example for draft_support_reply):

{
  "error": "Payment required for tool 'draft_support_reply'",
  "accepts": [
    {
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0xB075aA8206D6De88EDEeD0eE4015a1a33D3659D8",
      "amount": "50000",
      "scheme": "exact",
      "network": "eip155:8453",
      "maxTimeoutSeconds": 300,
      "extra": { "name": "USD Coin", "version": "2" }
    }
  ]
}

Key fields in each accepts[] entry:

• asset — USDC contract address on that chain
• payTo — DeskCrew's receiving wallet
• amount — price in 6-decimal atomic units (USDC has 6 decimals)
• scheme — exact (EVM) or exact-svm (Solana)
• network — CAIP-2 chain identifier (eip155:8453 = Base, eip155:137 = Polygon, etc.)
• maxTimeoutSeconds — window in which the signed authorization stays valid

On the retry, the signed payload goes in a request header named X-PAYMENT. The server responds with the tool result and a response header named X-PAYMENT-RESPONSE containing the settlement tx.

3. Worked example

Suppose your agent calls draft_support_reply with this payload:

POST /api/mcp/deskcrew
Content-Type: application/json

{
  "tool": "draft_support_reply",
  "input": {
    "customer_message": "My order hasn't arrived after 10 days."
  }
}

Step A — 402 returned (one chain shown, Base):

HTTP/1.1 402 Payment Required

{
  "error": "Payment required for tool 'draft_support_reply'",
  "accepts": [
    {
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0xB075aA8206D6De88EDEeD0eE4015a1a33D3659D8",
      "amount": "50000",
      "scheme": "exact",
      "network": "eip155:8453",
      "maxTimeoutSeconds": 300
    }
  ]
}

50000 = 0.050000 USDC (six decimals). This is the $0.05 price for draft_support_reply.

Step B — Agent signs an EIP-3009 authorization for exactly 50000 USDC to 0xB075…659D8 on Base, base64-encodes it.

Step C — Agent retries:

POST /api/mcp/deskcrew
Content-Type: application/json
X-PAYMENT: <base64 encoded authorization>

{
  "tool": "draft_support_reply",
  "input": {
    "customer_message": "My order hasn't arrived after 10 days."
  }
}

Step D — Success:

HTTP/1.1 200 OK
X-PAYMENT-RESPONSE: <base64 settlement tx>

{ "draft": "I'm sorry to hear about the delay...", "disclaimer": "..." }

DeskCrew's relayer broadcasts the transaction and pays gas. Your agent never sends an on-chain transaction itself.

4. What happens when the wallet is unfunded

If the agent's wallet holds no USDC (or insufficient USDC) on the chosen chain:

• The agent can still sign the authorization — signing is a local cryptographic operation that doesn't check balances.
• The server receives the retry, verifies the authorization against the chain, and sees the wallet cannot cover amount. The call is rejected (the tool does not run).
• No on-chain transaction is broadcast, so no gas is spent.
• DeskCrew only settles after a non-error result, so there is no pending charge to clear — the authorization is simply released.

In short: the 402 quote is always returned (discovery is free), the signature is cheap to produce, but the settlement will fail and the tool will refuse to execute until the wallet is funded on at least one of the five supported chains (Base, Polygon, Avalanche, Sei, Solana).