X402 Vercel Gateway

FastAPI Solana Vercel Redis License: MIT

A production-ready reference implementation for protecting FastAPI endpoints with the HTTP 402 Payment Required (x402) protocol on serverless infrastructure.

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💡 The Problem: The Agent Friction Wall

Autonomous AI agents (via AutoGPT, LangChain, MCP, or custom bots) cannot fill out credit card forms or complete 2FA challenges. As agent-to-agent (A2A) economic interactions grow, APIs need a machine-native monetization standard.

The x402 protocol leverages standard HTTP error codes combined with cryptographic micro-transactions (Solana USDC / EVM) to challenge callers for payment before serving protected compute or data.

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🚨 The Fatal Flaw: Ephemeral State in Serverless

Most developers protect their gateway using an in-memory dictionary or local cache to track spent transaction hashes:

# ❌ THE VULNERABILITY (Works in Docker, fails on Serverless)
_burned_hashes = {}
if tx_hash in _burned_hashes:
    raise HTTPException(status_code=402, detail="Replay Attack")
_burned_hashes[tx_hash] = True

Why this breaks: On serverless platforms (Vercel, AWS Lambda), compute is stateless and horizontally ephemeral. If an attacker pays 0.005 USDC once and sends 10,000 concurrent requests with the identical tx_hash, Vercel spins up dozens of cold micro-VMs. Every single instance starts with an empty dictionary. All 10,000 requests pass validation, draining your upstream LLM or database quotas while you only get paid once.

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🛡️ The Solution: Atomic Distributed State + On-Chain Proof

This gateway resolves the serverless state dilemma through a two-phase cryptographic & atomic protocol:

1. On-Chain Delta Verification: We query Solana JSON-RPC (getTransaction with jsonParsed) to mathematically prove that the target Associated Token Account (ATA) received the exact payment by computing postTokenBalances - preTokenBalances.
2. Atomic Distributed Lock (SETNX): We leverage Upstash Redis with the SETNX (Set if Not eXists) command to achieve a globally atomic burn of the transaction hash across all serverless regions.

# ✅ THE FIX: Verify On-Chain, then Burn Globally
is_valid = await verify_solana_transaction(tx_hash, required_memo=invoice_id)
if not is_valid:
    raise HTTPException(status_code=402, detail="Invalid payment proof")

# Atomic lock across all serverless cold starts (24h TTL)
acquired = redis_client.set(f"x402:tx:{tx_hash}", current_time, ex=86400, nx=True)
if not acquired:
    raise HTTPException(status_code=402, detail="Replay Attack Detected")

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📐 Architecture

sequenceDiagram
    autonumber
    participant Agent as Autonomous AI Agent
    participant Gateway as Vercel Edge (FastAPI)
    participant Redis as Upstash Redis (SETNX)
    participant RPC as Solana JSON-RPC Node

    Agent->>Gateway: POST /api/v1/protected-data
    Gateway-->>Agent: HTTP 402 Payment Required<br/>{recipient, amount_usdc, invoice_id}
    Note over Agent: Agent signs & broadcasts SPL token transfer
    Agent->>RPC: Broadcast USDC Transfer + Memo(invoice_id)
    RPC-->>Agent: tx_hash confirmed
    Agent->>Gateway: POST /api/v1/protected-data<br/>Header: X-Payment-Proof: <tx_hash>
    Gateway->>RPC: getTransaction(tx_hash)
    RPC-->>Gateway: Transaction metadata & token balances
    Note over Gateway: Verify postTokenBalance - preTokenBalance == amount
    Gateway->>Redis: SETNX x402:tx:<tx_hash> (24h TTL)
    alt Lock Acquired (nx=True)
        Redis-->>Gateway: OK (1)
        Gateway-->>Agent: HTTP 200 OK (Protected Data Delivered)
    else Replay Attempt (nx=False)
        Redis-->>Gateway: Nil (0)
        Gateway-->>Agent: HTTP 402 Payment Required (Replay Attack)
    end

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🚀 Quickstart

1. Clone & Install

git clone https://github.com/roblambert9/x402-vercel-gateway.git
cd x402-vercel-gateway
python -m venv .venv
source .venv/bin/activate  # Or: .venv\Scripts\activate on Windows
pip install -r requirements.txt

2. Configure Environment Variables

Create a .env file or set in your Vercel Dashboard:

Variable	Description	Example
SOLANA_RPC_URL	Solana JSON-RPC endpoint	https://api.devnet.solana.com
RECIPIENT_WALLET	Your receiving Solana address	YourWalletPublicKey...
USDC_MINT_ADDRESS	Mint address for USDC	Devnet: 4zMMC9srt5Ri5X14GAgXhaHii3GnPAEERYPJgZJDncDU
UPSTASH_REDIS_REST_URL	Upstash Redis REST URL	https://your-db.upstash.io
UPSTASH_REDIS_REST_TOKEN	Upstash Redis REST Token	AXxxxx...

3. Run Locally

uvicorn main:app --reload --port 8000

4. Run Live-Fire Integration Test

To run an automated test that signs and broadcasts a real SPL transfer on Devnet and verifies the gateway's 402 challenge response:

python tests/live_fire_devnet.py

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📦 Deployment to Vercel

vercel --prod

Ensure your Upstash Redis integration is bound to your Vercel project environment variables.

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📄 License

MIT License — free to use and integrate into your own agentic services. Built by Rob Lambert.