The different implementations between user paid transactions and sponsored transactions on Sui clients.

sponsored.ts

import { Transaction } from '@mysten/sui/transactions';
import { Ed25519Keypair } from '@mysten/sui/keypairs/ed25519';
import { SuiClient, getFullnodeUrl } from '@mysten/sui/client';

async function sponsoredTransaction() {
    const client = new SuiClient({ url: getFullnodeUrl('testnet') });

    // User's main keypair (your provided key)
    const userKeypair = Ed25519Keypair.fromSecretKey(
        'suiprivkey1qq9r6rkysny207t5vr7m5025swh7w0wzra9p0553paprhn8zshqsx2rz64r'
    );

    // Sponsor keypair (your provided key)
    const sponsorKeypair = Ed25519Keypair.fromSecretKey(
        'suiprivkey1qr3dtpdvecp2usjah06uyw8d8jvx9syqykxlu5x45cymq8lxn7hz2cqpwte'
    );

    // Build the user's transaction
    const tx = new Transaction();
    tx.setSender(userKeypair.getPublicKey().toSuiAddress());

    const [coin] = tx.splitCoins(tx.gas, [1_000_000]); // Split 0.001 SUI
    tx.transferObjects([coin], '0x83ecd81fdd132d4fb4f9ae2608656b000df13c4c3c5b10490d48ee981bc8f433');

    // Build KIND bytes (no gas owner yet)
    const kindBytes = await tx.build({
        client,
        onlyTransactionKind: true,
    });

    // Sponsor modifies transaction
    const sponsoredTx = Transaction.fromKind(kindBytes);

    sponsoredTx.setSender(userKeypair.getPublicKey().toSuiAddress());
    sponsoredTx.setGasOwner(sponsorKeypair.getPublicKey().toSuiAddress());

    // Build final transaction bytes
    const builtBytes = await sponsoredTx.build({ client });

    // ✍️ User signs first
    const { signature: userSignature } = await userKeypair.signTransaction(builtBytes);

    // ✍️ Sponsor signs
    const { signature: sponsorSignature } = await sponsorKeypair.signTransaction(builtBytes);

    // 🚀 Execute with both signatures
    const result = await client.executeTransactionBlock({
        transactionBlock: builtBytes,
        signature: [userSignature, sponsorSignature], // ✅ Two signatures required
        options: {
            showEffects: true,
            showEvents: true,
        },
    });

    console.log('✅ Sponsored Transaction Successful!');
    console.log('Digest:', result.digest);
}

sponsoredTransaction().catch(console.error);

userpaid.ts

import { Transaction } from '@mysten/sui/transactions';
import { Ed25519Keypair } from '@mysten/sui/keypairs/ed25519';
import { SuiClient, getFullnodeUrl } from '@mysten/sui/client';

async function sponsoredGasTransaction() {
  const client = new SuiClient({ url: getFullnodeUrl('testnet') });

  // Main user keypair

  // dummy keypair
  const userKeypair = Ed25519Keypair.fromSecretKey(
    'suiprivkey1qq9r6rkysny207t5vr7m5025swh7w0wzra9p0553paprhn8zshqsx2rz64r'
  );

  // Sponsor keypair (can be generated randomly here)
  const sponsorKeypair = new Ed25519Keypair(); // In production, the sponsor would already exist

  // Step 1: User builds transaction
  const tx = new Transaction();
  tx.setSender(userKeypair.getPublicKey().toSuiAddress());

  const [coin] = tx.splitCoins(tx.gas, [500_000]);
  tx.transferObjects([coin], '0xRecipientAddressHere'); // Change to your real recipient

  const kindBytes = await tx.build({ provider: client, onlyTransactionKind: true });

  // Step 2: Sponsor fills gas fields
  const sponsoredTx = Transaction.fromKind(kindBytes);
  sponsoredTx.setSender(userKeypair.getPublicKey().toSuiAddress());
  sponsoredTx.setGasOwner(sponsorKeypair.getPublicKey().toSuiAddress());

  // Sponsor gas coin setup (you must own this gas coin)
  const sponsorGasCoin = {
    objectId: '0xGasCoinObjectId', // You must replace this
    version: 1,
    digest: 'DigestOfGasCoin',    // Replace accordingly
  };
  sponsoredTx.setGasPayment([sponsorGasCoin]);

  const builtBytes = await sponsoredTx.build({ client });

  const { signature } = await sponsorKeypair.signTransaction(builtBytes);

  const result = await client.executeTransactionBlock({
    transactionBlock: builtBytes,
    signature,
    options: {
      showEffects: true,
      showEvents: true,
    },
  });

  console.log('Sponsored transaction successful with digest:', result.digest);
}

sponsoredGasTransaction().catch(console.error);