Building a Serverless HTTP 402 Payment Gateway for FastAPI with Solana and Redis

A developer has released an open-source reference implementation for a serverless HTTP 402 payment gateway for FastAPI, using Solana and Redis. The project, x402-vercel-gateway, addresses the replay attack vulnerability in stateless serverless environments by combining on-chain transaction verification with atomic Redis locks. The solution ensures that each transaction hash is globally tracked, preventing attackers from reusing a single payment across multiple concurrent requests.

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. 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. The x402-vercel-gateway resolves the serverless state dilemma through a two-phase cryptographic & atomic protocol: getTransaction with jsonParsed to mathematically prove that the target Associated Token Account ATA received the exact payment by computing postTokenBalances - preTokenBalances . SETNX : ✅ 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" Check out the complete open-source reference implementation on GitHub: 👉 https://github.com/roblambert9/x402-vercel-gateway https://github.com/roblambert9/x402-vercel-gateway