⚡ x402 Pay-per-Proof

The flow: 402 → pay → proof

1. 1Your agent POSTs a message or hash. With no payment it gets back 402 plus an x402 accepts[] block: pay $0.02 USDC to the treasury on Base.
2. 2The agent sends the USDC transfer on Base (its own wallet / SDK) and captures the txHash.
3. 3It retries with an X-PAYMENT header carrying the txHash. We verify the transfer on-chain, then return a Dilithium-2 signature over your bytes + an X-PAYMENT-RESPONSE receipt. One txHash = one signature (anti-replay).

Try the live 402 challenge (free — no payment sent)

This calls the real endpoint without paying, so it returns the 402 body an x402 client would read.

Integrate (curl)

# 1. Ask for a proof — no payment yet → HTTP 402
curl -i https://fractalai.net.co/api/x402/sign \
  -H 'Content-Type: application/json' \
  -d '{"message":"agent-run #4821 completed: swapped 100 USDC→ETH"}'

# → 402 Payment Required
# {
#   "x402Version": 1,
#   "accepts": [{
#     "scheme": "exact", "network": "base", "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
#     "maxAmountRequired": "20000", "payTo": "0xC13789e82661635d9Cea38a53A0390CF9939ef4f",
#     "maxTimeoutSeconds": 300, "extra": { "settlement": "txHash", ... }
#   }],
#   "digest": "…sha256 of your message…"
# }

# 2. Pay 0.02 USDC to payTo on Base (any wallet / agent SDK), get the txHash.
# 3. Retry with X-PAYMENT = base64(JSON({ txHash })):
PAY=$(printf '{"txHash":"0xYOUR_TX_HASH"}' | base64)
curl -i https://fractalai.net.co/api/x402/sign \
  -H 'Content-Type: application/json' \
  -H "X-PAYMENT: $PAY" \
  -d '{"message":"agent-run #4821 completed: swapped 100 USDC→ETH"}'

# → 200 OK  (+ header X-PAYMENT-RESPONSE with the settlement receipt)
# {
#   "digest": "…", "signature": "…dilithium2…", "algorithm": "dilithium2",
#   "public_key": "…", "payment": { "settled": true, "txHash": "0x…" }
# }

Any x402-aware agent SDK can read accepts[] and automate steps 2–3. In our subset the X-PAYMENT header carries base64(JSON({ txHash })).

What we support (honest x402 subset)

• Scheme: exact — a fixed-amount USDC transfer.
• Network / asset: Base mainnet, USDC (0x833589fC…, 6 decimals).
• Settlement: on-chain txHash verification — you pay first (a normal transfer), we verify it landed to the treasury, then sign.
• 402 body: follows the x402 accepts[] payment-requirements shape (scheme/network/asset/payTo/maxAmountRequired/maxTimeoutSeconds), so x402 clients can auto-read the price.
• Anti-replay: each txHash redeems exactly one signature. Per-IP rate limit on the free 402 discovery.

Also supported (facilitator-native): gasless EIP-3009 transferWithAuthorization. Your agent signs a payment authorization off-chain and puts it in the X-PAYMENT header; we delegate verify + settle to an x402 facilitator (Coinbase CDP by default) which submits the USDC transfer on-chain and pays the gas — we never custody funds. This is a one-shot call (no "pay first, come back") and is what gets this endpoint cataloged in the x402 Bazaar so autonomous agents can discover it. The accepts[] block advertises both the EIP-3009 entry (extra.assetTransferMethod="eip3009") and the txHash fallback, so any x402 client picks the one it supports.

Honest status: the facilitator is third-party payment infrastructure, not part of our product sovereignty (the PQC key, digest and treasury are ours). Facilitator settlement activates once a CDP API credential is configured server-side; until then the endpoint serves the txHash path and cleanly declines to sign an EIP-3009 authorization it cannot yet settle (it never fakes a settlement).