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" }),
});

A paid-tier API key skips the per-call x402 payment: each call is debited at list price from the key’s prepaid balance instead. A free key still pays per call. The tier only selects the key’s rate limits (see Rate Limits).

Key Format

Prefix	Type	Use
`lg_live_`	Production key	Live data, real sources
`lg_sandbox_`	Sandbox key (`"tier": "sandbox"` on `POST /v1/keys/create`)	Mock data, never charged, 10 requests per minute per IP

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();

POST /v1/keys/create can only provision free or sandbox tiers directly. To move up the ladder, create a free key first, then pay via POST /v1/keys/upgrade/{tier} (x402 required even if you already hold a key — see Pricing):

Tier	Top-up (credited to the prepaid balance)	Rate limits
`free` / `sandbox`	500 calls (a ceiling, not an allowance: REST calls are still paid per call over x402)	10 / min, 500 / 24 h
`indie`	$29	30 / min, 1,000 / 24 h
`starter`	$99	60 / min, 10,000 / 24 h
`growth`	$299	120 / min, 50,000 / 24 h
`pro`	$999	300 / min, 250,000 / 24 h

A top-up buys balance, not a number of calls: each call is debited at its list price. Check the balance with GET /v1/keys/usage.

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": "850000",
      "decimals": 6,
      "recipient": "0xFacilitatorAddress",
      "description": "Limitguard API call (0.85 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`	`850000`	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

Post-mortems

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