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​
Create your project

Initialize a new fastapi project.

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Server-side

Follow the steps below to setup your server.
1

Create your api project

shell
mkdir my-fastapi-project
cd my-fastapi-project
python -m venv .venv
source .venv/bin/activate
pip install fastapi uvicorn httpx
2

Setup Express

main.py
from typing import Optional, Dict, Any
import httpx
from fastapi import FastAPI, Depends, HTTPException, Body

app = FastAPI()

# MIDDLEWARES

# ENDPOINTS

if __name__ == "__main__":
    import uvicorn
    uvicorn.run("main:app", host="0.0.0.0", port=8000, reload=True)
3

Add middleware

main.py
# MIDDLEWARES
async def settle_with_flow402(
    *,
    nonce: str,
    tx_hash: str,
    fee_tx_hash: Optional[str],
    intent_id: str,
) -> Dict[str, Any]:
    """
    Call Flow402 simplified /settle endpoint.
    """
    url = "https://api.flow402.net/settle"

    payload = {
        "nonce": nonce,
        "txHash": tx_hash,
        "feeTxHash": fee_tx_hash,
        "intentId": intent_id,
    }

    async with httpx.AsyncClient() as client:
        res = await client.post(url, json=payload)
        # If 4xx/5xx: raise for debug, you can also handle gracefully
        res.raise_for_status()
        return res.json()

async def require_payment(body: Dict[str, Any] = Body(...)):
    """
    FastAPI dependency that checks payment against Flow402.
    It mimics the Express middleware `requirePayment`.
    """

    nonce = body.get("nonce")
    intent_id = body.get("intentId")
    tx_hash = body.get("txHash")
    fee_tx_hash = body.get("feeTxHash")

    # Return 402 "Payment required" if missing fields
    if not nonce or not intent_id or not tx_hash:
        raise HTTPException(
            status_code=402,
            detail={
                "error": "payment_required",
                "message": "Missing Flow402 payment body.",
                "expectedBody": ["nonce", "intentId", "txHash"],
                "optionalBody": "feeTxHash",
            },
        )

    # Call Flow402 facilitator
    result = await settle_with_flow402(
        nonce=nonce,
        tx_hash=tx_hash,
        fee_tx_hash=fee_tx_hash,
        intent_id=intent_id,
    )

    # Return 402 if payment not settled
    if not result.get("settled"):
        raise HTTPException(
            status_code=402,
            detail={
                "error": "payment_not_settled",
                "message": "Payment not settled on-chain.",
                "facilitatorResponse": result,
            },
        )

    # Attach receipt-like info for the endpoint handler
    return {
        "txHash": result.get("txHash"),
        "receipt": result.get("receipt"),
    }
4

Create your endpoints

main.py
# ENDPOINTS

# Free/public endpoint
@app.get("/api/free")
async def free_endpoint():
    return {
        "status": "ok",
        "type": "free",
        "message": "This is a free endpoint. No payment required.",
    }

# Premium endpoint protected with Flow402
@app.post("/api/premium")
async def premium_endpoint(payment = Depends(require_payment)):
    return {
        "status": "ok",
        "type": "premium",
        "message": "Welcome to the premium API",
        "payment": payment,
    }

​
Client-side

Follow the steps below to request Flow402 protocol from your shell.
1

Call /verify endpoint

shell
curl -s -X POST \
-H "Content-Type: application/json" \
-d '{
    "from": "0xPAYER",
    "to": "0xMERCHANT",
    "amount": "0.10",
    "network": "base",
    "scheme": "exact"
}' \
https://api.flow402/verify
2

Send two transactions using USDC and Base network

Use any wallet to send a transaction from PAYER address to your MERCHANT address.Also send a transaction from PAYER address to our FACILITATOR address 0xc4a35c792a020d55218b83c5d77172b86b934dce. This transaction’s amount needs to be greater than 0.001 USDC and greater than AMOUNT*0.1 (1% tax fee).
3

Call your paid api endpoint

shell
curl -s -X POST \
-H "Content-Type: application/json" \
-d '{
    "nonce": "nonce-given-from-first-request",
    "intentId": "intentId-given-from-first-request",
    "txHash": "transaction-hash-from-payer-to-merchant",
    "feeTxHash": "transaction-hash-from-payer-to-facilitator",
}' \
http://localhost:8000/api/premium
Response:
json
{
    "status": "ok",
    "type": "premium",
    "message": "Welcome to the premium API 🚀",
    "payment": {
        "txHash": "0xMERCHANT_TX_HASH",
        "receipt": {
            "intentId": "pi_...",
            "amount": "0.10",
            "currency": "USDC",
            "network": "base",
            "merchant": "0xYOUR_MERCHANT_ADDRESS",
            "facilitatorRecipient": "0xc4a35c792a020d55218b83c5d77172b86b934dce",
            "fee": "0.001",
            "feeTxHash": "0xFEE_TX_HASH"
        }
    }
}