x402 Endpoint Guide — $0 Paid API

How we shipped a live, paid x402 endpoint (FastAPI, $0.50 USDC/call on Base) for $0 total: free hosting, free listing, deterministic engine with zero per-call inference cost.

The protocol (30-second version)

1. Client POSTs without payment → you return HTTP 402 with x402Version, accepts[] (scheme, network, asset, amount, payTo), and an error field.
2. Client signs a USDC transfer (EIP-3009/EIP-712) and retries with an X-PAYMENT header (base64-encoded payment payload JSON).
3. You verify via a facilitator (POST https://x402.org/facilitator/verify) → 200 with your payload, or 402 again.

Minimum viable server (FastAPI)

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
import base64, json, httpx

app = FastAPI()
REQUIREMENTS = {
    "scheme": "exact",
    "network": "eip155:8453",  # Base
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",  # USDC on Base
    "amount": "500000",  # $0.50 (6 decimals)
    "payTo": "YOUR_BASE_WALLET",
    "maxTimeoutSeconds": 300,
}

@app.post("/api/your-tool")
async def tool(request: Request):
    x_payment = request.headers.get("x-payment")
    if not x_payment:
        return JSONResponse(status_code=402, content={
            "x402Version": 1,
            "accepts": [REQUIREMENTS],
            "error": "payment required: $0.50 USDC on Base",
        })
    payload = json.loads(base64.b64decode(x_payment).decode())
    r = httpx.post("https://x402.org/facilitator/verify", json={
        "x402Version": 1,
        "paymentPayload": payload,
        "paymentRequirements": REQUIREMENTS,
    }, timeout=15)
    if not r.json().get("isValid"):
        return JSONResponse(status_code=402, content={"error": "payment invalid"})
    return JSONResponse(content=do_the_work(await request.json()))

$0 hosting

• Render free tier (no credit card): 750 hrs/month, public HTTPS, sleeps after 15 min idle — add a 10-minute keepalive ping and it stays awake inside the free budget.
• Bind 0.0.0.0 and read $PORT from env. requirements.txt must list every import (we learned this the hard way: missing python-multipart killed three deploys).

Discovery (so agents find you)

1. Serve /.well-known/x402 describing endpoints, inputs, outputs.
2. Serve a Google A2A agent card at /.well-known/agent.json (name, description, skills, provider).
3. Serve /skill.md + /llms.txt for agent readability.
4. List on 402index.io (largest crawler directory; requires domain-ownership verification via a challenge file).
5. Bazaar/agentic.market auto-indexes after your first settled payment — self-settle one $0.50 call to bootstrap, and include the extensions.bazaar metadata block (discoverable: true + input/output JSON schemas) in your 402 response.

Unit economics

Price above your marginal cost. Our engine is fully deterministic (no LLM per call) → $0.00 marginal cost → 100% margin at $0.50. If your endpoint burns LLM inference, price at 3–5× your per-call cost or keep it free-but-rate-limited as a lead magnet (Model B: free tool → paid offer).

Live reference

https://citrus-coast-ai-teardown.onrender.com — 402 endpoint, agent card, skill.md, all live. Steal the pattern.