Authentication

LimitGuard supports two authentication modes: API key (subscription) and x402 USDC micropayments (pay-per-use). Sandbox mode is available for testing without either.

API Key Authentication

Pass your API key in the X-API-Key header:

curl -X POST https://api.limitguard.ai/v1/entity/check \
  -H "X-API-Key: lg_live_xxxxxxxxxxxxxxxxxxxx" \
  -H "Content-Type: application/json" \
  -d '{"entity_name": "Acme Corp BV", "country": "NL"}'

import httpx

response = httpx.post(
    "https://api.limitguard.ai/v1/entity/check",
    headers={"X-API-Key": "lg_live_xxxxxxxxxxxxxxxxxxxx"},
    json={"entity_name": "Acme Corp BV", "country": "NL"},
)

const response = await fetch("https://api.limitguard.ai/v1/entity/check", {
  method: "POST",
  headers: {
    "X-API-Key": "lg_live_xxxxxxxxxxxxxxxxxxxx",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({ entity_name: "Acme Corp BV", country: "NL" }),
});

API key users bypass x402 payment — you pay via subscription. Rate limits and monthly quotas apply based on tier.

Key Format

Prefix	Type	Use
`lg_live_`	Production key	Live data, real sources
`lg_test_`	Test / sandbox key	Automatically activates sandbox mode

Creating Keys

curl -X POST https://api.limitguard.ai/v1/keys/create \
  -H "Content-Type: application/json" \
  -d '{"email": "you@example.com", "tier": "free"}'

response = httpx.post(
    "https://api.limitguard.ai/v1/keys/create",
    json={"email": "you@example.com", "tier": "free"},
)
print(response.json()["api_key"])

const response = await fetch("https://api.limitguard.ai/v1/keys/create", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ email: "you@example.com", tier: "free" }),
});
const { api_key } = await response.json();

Available tiers: free, starter, pro, enterprise.

The plaintext key is returned once only — store it securely. It is never stored server-side.

x402 USDC Micropayments

For AI agents and pay-per-use access without a subscription. Include the X-PAYMENT header with a base64-encoded JSON payment object.

See the full x402 Protocol guide for step-by-step implementation with code examples.

x402 V2 Flow

Request without payment

Make your API request normally. You’ll receive HTTP 402 with payment requirements.

Build payment signature

Construct an EIP-3009 TransferWithAuthorization signature using the payment details from the 402 response.

Retry with payment

Retry the same request with the X-PAYMENT header containing the base64-encoded payment object.

HTTP 402 Response

When you make a request without payment, the API returns the payment requirements:

{
  "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)"
    }
  ]
}

Supported Networks

Network	CAIP-2 Chain ID	USDC Contract
Base Mainnet	`eip155:8453`	`0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913`
Base Sepolia (testnet)	`eip155:84532`	`0x036CbD53842c5426634e7929541eC2318f3dCF7e`
Solana Mainnet	`solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp`	`EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v`
Solana Devnet	`solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1`	`4zMMC9srt5Ri5X14GAgXhaHii3GnPAEERYPJgZJDncDU`

V1 Backward Compatibility

The legacy PAYMENT-SIGNATURE header is still accepted (V1 format). V2 (X-PAYMENT) is recommended for new integrations.

Response Headers on Success

Header	Value	Description
`X-Payment-Verified`	`true`	Payment accepted and verified
`X-Payment-Amount`	`50000`	Amount in USDC 6-decimal units
`X-Payment-Fallback`	`true`	Circuit breaker triggered (rare)

Free Endpoints

These endpoints never require payment or authentication:

Endpoint	Purpose
`GET /health`	API health status
`POST /v1/keys/create`	Self-service key provisioning
`GET /.well-known/x402.json`	x402 service listing
`GET /.well-known/agent.json`	A2A agent card
`GET /.well-known/mcp.json`	MCP manifest
`GET /.well-known/security.txt`	Security contact (RFC 9116)
`GET /v1/self-verify`	LimitGuard’s own trust score
`GET /v1/methodology`	Scoring methodology
`GET /v1/badge/{entity_id}`	SVG trust badge
`GET /v1/legal/*`	Legal documents
`GET /llms.txt`	LLM-readable summary
`GET /llms-full.txt`	LLM-readable full reference

Get Started

Core Concepts

Guides

API Key Authentication

Key Format

Creating Keys

x402 USDC Micropayments

x402 V2 Flow

HTTP 402 Response

Supported Networks

V1 Backward Compatibility

Response Headers on Success

Free Endpoints

​API Key Authentication

​Key Format

​Creating Keys

​x402 USDC Micropayments

​x402 V2 Flow

​HTTP 402 Response

​Supported Networks

​V1 Backward Compatibility

​Response Headers on Success

​Free Endpoints

API Key Authentication

Key Format

Creating Keys

x402 USDC Micropayments

x402 V2 Flow

HTTP 402 Response

Supported Networks

V1 Backward Compatibility

Response Headers on Success

Free Endpoints