x402 API reference

Three routes, all under https://api.kryard.com/x402/v1. None require authentication — no X-Stamp, no API key. That's what the x402 spec calls for, and it's why the facilitator carries hard limits instead.

GET /supported

What this deployment settles. No body.

{
  "kinds": [{ "x402Version": 1, "scheme": "exact", "network": "base-sepolia" }],
  "signers": { "eip155:*": ["0x433ff8253Faa9A57b6326f69Abc8Ad0bf208f5c1"] }
}

signers is the relayer that fronts gas. One deployment serves one chain.

POST /verify

Read-only. Recovers the signature and checks balance, nonce, and validity window against the chain. Never broadcasts, never costs anyone gas.

{
  "paymentPayload": {
    "x402Version": 1,
    "scheme": "exact",
    "network": "base-sepolia",
    "payload": {
      "signature": "0x…",
      "authorization": {
        "from": "0x…", "to": "0x…", "value": "10000",
        "validAfter": "1753660000", "validBefore": "1753660120",
        "nonce": "0x…32 bytes…"
      }
    }
  },
  "paymentRequirements": {
    "scheme": "exact", "network": "base-sepolia",
    "maxAmountRequired": "10000",
    "asset": "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
    "payTo": "0x…", "resource": "https://…",
    "maxTimeoutSeconds": 120,
    "extra": { "name": "USDC", "version": "2" }
  }
}

→ { "isValid": true, "payer": "0x…" } or { "isValid": false, "invalidReason": "…", "payer": "0x…" }

POST /settle

Same body. Re-verifies, then broadcasts the payer's transferWithAuthorization with the relayer paying gas.

→ { "success": true, "transaction": "0x…", "network": "base-sepolia", "payer": "0x…" } or { "success": false, "errorReason": "…", "transaction": "", "network": "…", "payer": "0x…" }

Idempotent on (chain, asset, payer, nonce). A repeat returns the original transaction hash rather than transferring twice — retry safely.

Reason codes

Returned as invalidReason (verify) or errorReason (settle).

The payment itself

Code	Meaning
invalid_request	Malformed body, or a missing/non-address authorization.from.
invalid_exact_evm_signature	Signature doesn't recover to from. Usually a wrong EIP-712 domain.
invalid_exact_evm_recipient_mismatch	Signed to ≠ the payTo in paymentRequirements.
invalid_exact_evm_insufficient_value	Signed value < maxAmountRequired.
invalid_exact_evm_insufficient_balance	Payer's token balance won't cover it.
invalid_exact_evm_authorization_expired	Outside validAfter…validBefore.
invalid_exact_evm_authorization_used	That nonce is already spent on chain.

Facilitator policy

Code	Meaning
x402_policy_disabled	No enabled policy for this chain. Fail-closed default.
x402_asset_not_allowed	Asset outside the allowlist.
x402_payto_not_allowed	Destination outside the allowlist, when one is configured.
x402_amount_below_minimum	Under the per-settle floor (0.01 USDC).
x402_amount_exceeds_cap	Over the per-settle ceiling (10 USDC).
x402_daily_gas_cap_exceeded	Facilitator's 24h gas budget is spent.
x402_monthly_gas_cap_exceeded	30-day gas budget is spent.
x402_gas_estimate_unavailable	Gas couldn't be estimated, so the spend couldn't be bounded. Fails closed.
x402_settlement_in_flight	The same authorization is mid-settlement. Poll rather than retry hard.
x402_relayer_not_configured	Operator-side fault: the facilitator's relayer key isn't resolvable. Nothing you can fix as a caller — retry later or report it.

Throughput

Code	HTTP	Meaning
x402_rate_limited	429	A limit below was hit. Nothing was reserved or broadcast.
x402_rate_limit_unavailable	503	The limiter couldn't be consulted, so the request was refused.

Limits

Scope	Limit
/verify per IP	60 / minute
/settle per IP	20 / minute
/settle per payer	5 / minute
/settle facilitator-wide	60 / minute
Per-settle amount	0.01 – 10 USDC
Facilitator gas	0.02 ETH / day · 0.1 ETH / month

Per-IP limits are charged on every request. The payer and facilitator-wide budgets are charged only after a payment verifies — so a flood of junk can't burn another payer's allowance or the shared budget.

Gas caps are the operator's spend, not yours. When one is exhausted, settlement stops until the window rolls.

Deployment facts

Base URL	https://api.kryard.com
Chain	Base Sepolia · 84532
Asset	Testnet USDC 0x036CbD53842c5426634e7929541eC2318f3dCF7e
Scheme	exact (EIP-3009 transferWithAuthorization)
Relayer	0x433ff8253Faa9A57b6326f69Abc8Ad0bf208f5c1
Auth	None

Testnet only

No mainnet deployment, no SLA, no uptime guarantee. Suitable for integration work, not for real value.