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 billing.

NEVER

1. Do not expose API key in server-side only apps to client — for frontend, use NEXT_PUBLIC_ or VITE_ prefixed env vars.
2. Do not skip transactionSubmitter — without it, users pay gas themselves; the provider falls back to normal submission.
3. Do not use Script Composer for arbitrary transactions — Script Composer is for stablecoin transfers (USDC, USDT, etc.) only; use Wallet Adapter for everything else.

Method 1: Wallet Adapter (Recommended — Any Transaction)

Use for swaps, NFT mints, contract calls — any transaction type.

Installation

npm install @smoothsend/sdk @aptos-labs/wallet-adapter-react

Provider Setup (3 lines)

import { SmoothSendTransactionSubmitter } from "@smoothsend/sdk";
import { AptosWalletAdapterProvider } from "@aptos-labs/wallet-adapter-react";
import { Network } from "@aptos-labs/ts-sdk";

const smoothSend = new SmoothSendTransactionSubmitter({
  apiKey: process.env.NEXT_PUBLIC_SMOOTHSEND_API_KEY!,
  network: "mainnet" // or 'testnet' (always free)
});

export function Providers({ children }: { children: React.ReactNode }) {
  return (
    <AptosWalletAdapterProvider
      autoConnect={true}
      dappConfig={{
        network: Network.MAINNET,
        transactionSubmitter: smoothSend
      }}
      onError={(error) => console.error("Wallet error:", error)}
    >
      {children}
    </AptosWalletAdapterProvider>
  );
}

After this, every signAndSubmitTransaction call is gasless. No other code changes needed.

Method 2: Script Composer (Fee-in-Token — Stablecoin Only)

Use for USDC, USDT, WBTC, USDe, USD1 transfers. Fee (~$0.01) is deducted from the token being sent — no APT or SmoothSend credits required.

import { ScriptComposerClient } from "@smoothsend/sdk";

const client = new ScriptComposerClient({
  apiKey: process.env.NEXT_PUBLIC_SMOOTHSEND_API_KEY!,
  network: "mainnet"
});

// USDC Mainnet asset address
const USDC_ASSET = "0xbae207659db88bea0cbead6da0ed00aac12edcdda169e591cd41c94180b46f3b";

const build = await client.buildTransfer({
  sender: walletAddress,
  recipient: "0xRecipient...",
  amount: "1000000", // 1 USDC (6 decimals)
  assetType: USDC_ASSET,
  decimals: 6,
  symbol: "USDC"
});

// Sign with wallet, then submit build.signedTransaction

Error Handling

try {
  const response = await signAndSubmitTransaction(payload);
  await aptos.waitForTransaction({ transactionHash: response.hash });
} catch (error: any) {
  if (error?.status === 402 || error?.message?.includes("Insufficient credits")) {
    // Credits exhausted — show upgrade CTA
    toast.error("Service temporarily unavailable. Please try again later.");
    window.open("https://dashboard.smoothsend.xyz/billing", "_blank");
  } else {
    throw error;
  }
}

Pricing

See SmoothSend Pricing for current rates. Testnet is free; mainnet uses credit packs.

Common Mistakes

References

• SmoothSend Docs: https://docs.smoothsend.xyz
• Pricing: https://docs.smoothsend.xyz/pricing
• Dashboard: https://dashboard.smoothsend.xyz
• npm: https://www.npmjs.com/package/@smoothsend/sdk
• MCP (AI context): npx @smoothsend/mcp — tools for get_docs, estimate_credits, get_code_snippet
• Related: ts-sdk-wallet-adapter, ts-sdk-transactions