Accept crypto payments in FastAPI

Step by step

1. 1.Get an API key and a webhook secret

   In the Moonvex dashboard go to “Integration” → “Create key” → type “Secret” (sk_live_…). The key is shown only once: store it on your server as MOONVEX_SECRET_KEY. On the same page add a webhook pointing to an https URL on your server and store its signing secret as MOONVEX_WEBHOOK_SECRET.

   pip install fastapi uvicorn httpx asyncpg
   
   export MOONVEX_SECRET_KEY=sk_live_...
   export MOONVEX_WEBHOOK_SECRET=whsec_...
   export DATABASE_URL=postgresql://user:pass@localhost:5432/shop

2. 2.Create a payment

   The httpx client is created once in lifespan with the base URL and the Authorization header. Each payment gets its own Idempotency-Key, and the amount is str(Decimal) from a numeric column.

   # main.py
   import os
   from contextlib import asynccontextmanager
   
   import asyncpg
   import httpx
   from fastapi import FastAPI, HTTPException, Request
   from fastapi.responses import RedirectResponse
   
   
   @asynccontextmanager
   async def lifespan(app: FastAPI):
       app.state.db = await asyncpg.create_pool(os.environ["DATABASE_URL"])
       app.state.moonvex = httpx.AsyncClient(
           base_url="https://api.moonvex.org",
           headers={"Authorization": "Bearer " + os.environ["MOONVEX_SECRET_KEY"]},
           timeout=15,
       )
       yield
       await app.state.moonvex.aclose()
       await app.state.db.close()
   
   
   app = FastAPI(lifespan=lifespan)
   
   
   @app.post("/orders/{order_id}/pay")
   async def pay(order_id: int, request: Request):
       order = await request.app.state.db.fetchrow("SELECT id, total FROM orders WHERE id = $1", order_id)
       if order is None:
           raise HTTPException(404)
       r = await request.app.state.moonvex.post(
           "/v1/payments",
           headers={"Idempotency-Key": f"order-{order_id}-1"},
           # numeric arrives as Decimal → str(): "25.00"
           json={"orderId": str(order_id), "amount": str(order["total"]), "currency": "USDT", "network": "TRON"},
       )
       payment = r.json()
       if r.status_code >= 400:
           raise HTTPException(502, payment["error"]["code"])
       return RedirectResponse(payment["paymentUrl"], status_code=303)

3. 3.Send the customer to paymentUrl

   paymentUrl is a hosted payment page: the exact amount including the fee, the address, a QR code, the official token contract and a timer, with a status that updates by itself. Open it with a redirect or in a new tab; it cannot be embedded in an iframe. A customer returning to your site is not proof of payment: mark the order paid only from a webhook or the status from GET /v1/payments/:id. RedirectResponse(..., status_code=303) after a POST opens the payment page.

4. 4.Receive the webhook and verify the signature

   Moonvex sends a POST with a Moonvex-Signature header: t=<unix time>,v1=<signature>. The signature is a hex HMAC-SHA256 of the string “t.raw_body” keyed with the webhook secret. Compute it over the body bytes before parsing JSON, compare in constant time and reject events older than 5 minutes. For 24 hours after a secret rotation the header carries two v1 values; either one is valid. In FastAPI do not declare a Pydantic model for the webhook body: take the Request and await request.body(), which gives you the original bytes.

   # main.py (continued)
   import hashlib
   import hmac
   import json
   import time
   
   
   def verify_moonvex(raw: bytes, header: str, secret: str, tolerance: int = 300) -> bool:
       parts = [p.split("=", 1) for p in (header or "").split(",") if "=" in p]
       t = next((v for k, v in parts if k == "t"), None)
       if t is None or not t.isdigit() or abs(time.time() - int(t)) > tolerance:
           return False
       expected = hmac.new(secret.encode(), t.encode() + b"." + raw, hashlib.sha256).hexdigest()
       return any(hmac.compare_digest(expected, v) for k, v in parts if k == "v1")
   
   
   @app.post("/moonvex/webhook")
   async def moonvex_webhook(request: Request):
       # Request.body(), not a Pydantic model: you need the original bytes
       raw = await request.body()
       if not verify_moonvex(raw, request.headers.get("moonvex-signature", ""), os.environ["MOONVEX_WEBHOOK_SECRET"]):
           raise HTTPException(400, "bad signature")
       event = json.loads(raw)
       if event["type"] in ("payment.confirmed", "payment.overpaid"):
           await mark_paid_once(request.app.state.db, event["id"], event["data"]["orderId"])
       return {"received": True}

5. 5.Mark the order paid exactly once

   A redelivered event keeps the same id, so store it in a table with a unique key in the same transaction as the order update: a duplicate then changes nothing. Fulfil on payment.confirmed and payment.overpaid, never on payment.detected. With asyncpg: INSERT … ON CONFLICT DO NOTHING RETURNING inside conn.transaction(); None means a redelivery.

   # main.py (continued)
   # CREATE TABLE processed_events (event_id text PRIMARY KEY, created_at timestamptz NOT NULL DEFAULT now());
   
   
   async def mark_paid_once(db: asyncpg.Pool, event_id: str, order_id: str) -> None:
       async with db.acquire() as conn, conn.transaction():
           fresh = await conn.fetchval(
               "INSERT INTO processed_events (event_id) VALUES ($1) ON CONFLICT DO NOTHING RETURNING event_id",
               event_id,
           )
           if fresh is None:
               return  # event already processed
           await conn.execute("UPDATE orders SET status = 'paid' WHERE id = $1 AND status <> 'paid'", int(order_id))

6. 6.Test the integration

   Moonvex has no test network: everything runs on mainnets. The “Send test” button under “Integration” sends a webhook.test event, which is how you check the signature. Then pay a small invoice, for example 1 USDT on TRON: it is a regular payment and the money lands on your balance. The delivery log with response codes and a “Retry” button is on the same page.

Pitfalls

• A Pydantic model in the endpoint signature parses and validates the body before you see it, so you cannot verify the signature against it. You need the raw Request.
• Create httpx.AsyncClient once (in lifespan) rather than per request, so connections are reused.
• Do not call synchronous requests inside async endpoints: it blocks the event loop.
• For long jobs after a webhook use BackgroundTasks or a queue; the response must go out within 10 seconds.