TypeScript SDK: SmoothSend Gasless Transactions

Purpose

Guide gasless transaction sponsorship on Aptos using SmoothSend. Users sign transactions via their wallet but never pay gas — you pay per transaction from pre-loaded credits. Works as a drop-in transactionSubmitter for AptosWalletAdapterProvider.

Paid commercial service: Free on testnet; mainnet uses credit-based billing. See Pricing for current rates.

ALWAYS

1. Use @smoothsend/sdk — official npm package for SmoothSend integration.
2. Pass SmoothSendTransactionSubmitter as transactionSubmitter in AptosWalletAdapterProvider — this enables gasless for all signAndSubmitTransaction calls.
3. Store API key in env — use NEXT_PUBLIC_SMOOTHSEND_API_KEY or VITE_SMOOTHSEND_API_KEY (never hardcode).
4. Use testnet for development — testnet is always free; no credits required.
5. Handle 402 (Insufficient credits) — API returns 402 when credits run out; show user-friendly message and link to