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LimitGuard implements the x402 V2 micropayment protocol for AI agent pay-per-use access. No subscription or API key required — agents pay per call in USDC.

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Overview

x402 extends HTTP with a payment layer. The flow is simple:
1

Request without payment

Agent makes a normal API request.
2

Receive HTTP 402

Server returns payment requirements — chain, amount, recipient, contract address.
3

Build payment signature

Agent constructs an EIP-3009 TransferWithAuthorization signature.
4

Retry with X-PAYMENT header

Agent retries the same request with the base64-encoded payment header.
5

Receive response

Server verifies signature, processes the request, and returns the result.

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Supported Networks

NetworkCAIP-2 Chain IDUSDC Contract
Base Mainneteip155:84530x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913
Base Sepoliaeip155:845320x036CbD53842c5426634e7929541eC2318f3dCF7e
Solana Mainnetsolana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdpEPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v
Solana Devnetsolana:EtWTRABZaYq6iMfeYKouRu166VU2xqa14zMMC9srt5Ri5X14GAgXhaHii3GnPAEERYPJgZJDncDU

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Step 1: Discover Payment Requirements

Make a request without payment to get the requirements: 
curl -X POST https://api.limitguard.ai/v1/entity/check \
  -H "Content-Type: application/json" \
  -d '{"entity_name": "Acme Corp BV", "country": "NL"}'
HTTP 402 Response
{
  "x402Version": "2",
  "error": "Payment Required",
  "accepts": [
    {
      "chainId": "eip155:8453",
      "currency": "USDC",
      "contractAddress": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "amount": "50000",
      "decimals": 6,
      "recipient": "0xFacilitatorAddress",
      "description": "LimitGuard API call (0.05 USDC)"
    },
    {
      "chainId": "solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp",
      "currency": "USDC",
      "contractAddress": "EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v",
      "amount": "50000",
      "decimals": 6,
      "recipient": "SolanaFacilitatorAddress",
      "description": "LimitGuard API call (0.05 USDC)"
    }
  ]
}

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Step 2: Build EVM Payment (Base / EIP-3009)

import base64, json, secrets, time
from eth_account import Account
from eth_account.messages import encode_typed_data

def build_x402_payment(
    chain_id: str,          # e.g. "eip155:8453"
    amount: int,            # in USDC 6-decimal units
    sender_key: str,        # private key (hex)
    recipient: str,         # facilitator address
    usdc_address: str,      # USDC contract address
) -> str:
    sender = Account.from_key(sender_key).address
    chain_num = int(chain_id.split(":")[1])  # 8453

    nonce = "0x" + secrets.token_hex(32)
    now = int(time.time())

    # EIP-712 domain
    domain = {
        "name": "USD Coin",
        "version": "2",
        "chainId": chain_num,
        "verifyingContract": usdc_address,
    }

    # EIP-3009 TransferWithAuthorization
    message = {
        "from": sender,
        "to": recipient,
        "value": amount,
        "validAfter": now - 10,
        "validBefore": now + 300,
        "nonce": bytes.fromhex(nonce[2:]),
    }

    types = {
        "EIP712Domain": [
            {"name": "name", "type": "string"},
            {"name": "version", "type": "string"},
            {"name": "chainId", "type": "uint256"},
            {"name": "verifyingContract", "type": "address"},
        ],
        "TransferWithAuthorization": [
            {"name": "from", "type": "address"},
            {"name": "to", "type": "address"},
            {"name": "value", "type": "uint256"},
            {"name": "validAfter", "type": "uint256"},
            {"name": "validBefore", "type": "uint256"},
            {"name": "nonce", "type": "bytes32"},
        ],
    }

    signable = encode_typed_data(domain, types, "TransferWithAuthorization", message)
    signed = Account.sign_message(signable, private_key=sender_key)

    payload = {
        "chainId": chain_id,
        "amount": str(amount),
        "sender": sender,
        "recipient": recipient,
        "nonce": nonce,
        "signature": signed.signature.hex(),
        "validAfter": now - 10,
        "validBefore": now + 300,
    }

    return base64.b64encode(json.dumps(payload).encode()).decode()

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Step 3: Make Paid Request

import httpx

x_payment = build_x402_payment(
    chain_id="eip155:8453",
    amount=50_000,           # $0.05 for /v1/entity/check (fresh)
    sender_key="0xYOUR_PRIVATE_KEY",
    recipient="0xFacilitatorFromThe402Response",
    usdc_address="0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
)

response = httpx.post(
    "https://api.limitguard.ai/v1/entity/check",
    headers={"X-PAYMENT": x_payment, "Content-Type": "application/json"},
    json={"entity_name": "Acme Corp BV", "country": "NL"},
)
print(response.json())

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Response Headers

On successful payment verification, these headers are included:
HeaderValueMeaning
X-Payment-VerifiedtruePayment accepted
X-Payment-Amount50000Amount verified (6-decimal USDC)
X-Payment-FallbacktrueCircuit breaker triggered (rare)

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Replay Prevention

Each nonce can only be used once. Nonces are tracked in Redis with a 5-minute TTL. Reusing a nonce returns: 
HTTP 402
{
  "error": "Nonce already used (replay detected)"
}

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Quality Tiers with x402

Control cost via the X-Response-Quality header. The amount in X-PAYMENT must match or exceed the price for the selected tier:
Tier/v1/entity/check/v1/risk/score/v1/kyb/check
cached$0.01 (10,000)$0.005 (5,000)$0.25 (250,000)
fresh$0.05 (50,000)$0.01 (10,000)$1.25 (1,250,000)
enhanced$0.15 (150,000)$0.05 (50,000)$2.50 (2,500,000)
If the payment amount is insufficient for the requested tier, you’ll receive HTTP 402 with the correct amount.

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Circuit Breaker Fallback

If the payment facilitator is unavailable (circuit breaker open), LimitGuard falls back to a verified wallet cache. Wallets that have previously completed verified payments are cached. The response includes X-Payment-Fallback: true when this path is used.

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Discovery

AI agents can discover LimitGuard’s x402 payment requirements automatically: 
curl https://api.limitguard.ai/.well-known/x402.json
Returns a Bazaar-compatible service listing with all endpoints, pricing, and accepted payment methods.

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V1 Backward Compatibility

The older PAYMENT-SIGNATURE header (V1 format) is still accepted for backward compatibility. V2 (X-PAYMENT) is recommended for all new integrations.