MegaETH Payment Demo

Next.js 16 + wagmi v2 + RainbowKit scaffold. Targets MegaETH testnet (chain id 6343, returned by carrot.megaeth.com/rpc). Will host x402 and mpp payment demos.

Stack

• Next.js 16 (App Router, TypeScript, Tailwind v4)
• wagmi v2 + viem 2 + RainbowKit
• Server-side viem wallet for on-chain writes (relayer/sponsor)

Setup

pnpm install
cp .env.example .env.local
# fill NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID and (optional) SERVER_PRIVATE_KEY
pnpm dev

Open http://localhost:3000.

Env

Var	Where	Purpose
NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID	client	RainbowKit / WalletConnect
NEXT_PUBLIC_MEGAETH_RPC_URL	client	viem transport for wagmi (optional)
MEGAETH_RPC_URL	server	viem transport for backend (optional)
SERVER_PRIVATE_KEY	server	hex key for backend signer; required for /api/relay, /api/faucet, x402 gas sponsorship, MPP gasless charge settlement, EVM session gasless relays, and EVM session close
NEXT_PUBLIC_USDM_ADDRESS	client + server	USDm token address (shared by x402 / mpp / mpp-session)
PAY_TO	server	Recipient of all USDm payments (falls back to server signer address)
MPP_SECRET_KEY	server	HMAC secret for mppx 402 challenges (required for /api/mpp/charge and /api/mpp/gasless-charge)
MPP_CHARGE_AMOUNT	server	Human-readable USDm amount (default 1)
MPP_GASLESS_CHARGE_AMOUNT	server	Human-readable USDm amount for the gasless MPP charge (defaults to MPP_CHARGE_AMOUNT, then 1)
NEXT_PUBLIC_MPP_GASLESS_TOKEN_VERSION	client + server	Optional EIP-2612 token domain version override for permit20 signing; by default the route reads eip712Domain() from the token, then falls back to NEXT_PUBLIC_X402_TOKEN_VERSION / 1
NEXT_PUBLIC_MPP_SESSION_ESCROW	client + server	Deployed TempoStreamChannelEvm escrow used by /api/mpp/session and /api/mpp/session-gasless
MPP_SESSION_STATE_REDIS_REST_URL	server	Upstash/Vercel Redis REST URL for durable MPP session channel state; use UPSTASH_REDIS_REST_URL or KV_REST_API_URL as alternatives
MPP_SESSION_STATE_REDIS_REST_TOKEN	server	Upstash/Vercel Redis REST token for durable MPP session channel state; use UPSTASH_REDIS_REST_TOKEN or KV_REST_API_TOKEN as alternatives
MPP_SESSION_STATE_KEY_PREFIX	server	Optional Redis key prefix for MPP session channels (default mpp-session:channel:)
MPP_SESSION_ALLOW_MEMORY_STORE	server	Set to 1 only for throwaway Vercel demos that accept losing channel state across instances

Routes

• GET /api/health — chain id, latest block, server signer address
• POST /api/relay — broadcast a pre-signed raw transaction ({ rawTx: "0x..." })
• GET /api/x402/exact — x402 exact protected endpoint; Permit2 signature, gas sponsored by server facilitator
• GET /api/mpp/charge — MPP tempo.charge protected endpoint; plain ERC20 transfer, client pays gas
• GET /api/mpp/gasless-charge — custom MPP permit20.charge protected endpoint; client signs EIP-2612 permit, server pays gas for permit + transferFrom
• POST /api/mpp/session — EVM session pay-as-you-go endpoint; client wallet pays gas for open and topUp, server verifies hash credentials and vouchers, and calls close
• POST /api/mpp/session-gasless — EVM session Permit2 gasless endpoint; server pays gas for openWithPermit2, topUpWithPermit2, and close

Production Hardening Caveats

This repository is a reference demo. Some server-side deployment controls are intentionally omitted to keep the payment flows easy to inspect:

• The local x402 facilitator HTTP route is not authenticated. The default app path can use the in-process facilitator, but any production deployment that exposes /api/x402/facilitator/verify or /api/x402/facilitator/settle must protect those endpoints with server-to-server authentication, platform access controls, an allowlist, or a private network boundary.
• SERVER_PRIVATE_KEY is reused by the server-sponsored flows and is created without signer-scoped nonce management. Production relayers should add a transaction queue or nonce manager per signer, or split duties across separate signers so concurrent sponsored writes cannot race the same nonce.
• Server-sponsored demo endpoints such as /api/faucet, /api/mpp/gasless-charge, and /api/mpp/session-gasless pay gas from the server signer. Production deployments must add authentication, rate limiting, per-wallet/IP quotas, spend caps, monitoring, and a circuit breaker. The faucet is a development convenience and should be disabled or protected outside a throwaway demo.

Docs

• x402 current scheme — x402 exact payment flow, Permit2 transfer method, and EIP-2612 gas sponsorship.
• MPP charge current scheme — one-time charge flow, push mode, and gasless pull mode.
• MPP session current scheme — official-style pay-as-you-go lifecycle, gasless EVM relayer flow, and escrow contract method diff.

MPP session production state

/api/mpp/session and /api/mpp/session-gasless keep the highest accepted voucher for each channel so close settles the correct cumulative amount. Local development can use the in-memory fallback, but serverless production cannot rely on module memory because a close request may run on a different instance from the open/voucher requests.

On Vercel, configure an Upstash/Vercel Redis REST database by setting either:

• MPP_SESSION_STATE_REDIS_REST_URL and MPP_SESSION_STATE_REDIS_REST_TOKEN
• or the marketplace-provided UPSTASH_REDIS_REST_URL / UPSTASH_REDIS_REST_TOKEN
• or older KV_REST_API_URL / KV_REST_API_TOKEN

Without one of those pairs, the route reports missing env in readiness on Vercel. For non-Vercel serverless hosts, set MPP_SESSION_REQUIRE_DURABLE_STORE=1 to fail fast when the durable store is missing.

Chain

MegaETH testnet — chain id 6343 (live RPC), RPC https://carrot.megaeth.com/rpc, explorer [https://testnet-mega.etherscan.io](https://testnet-mega.etherscan.io/). Faucet via testnet.megaeth.com.