Build a Transaction

This guide builds and submits a simple payment transaction with the WASM SDK. It assumes an existing network connection. Use testnet-10 while learning - test KAS is free from the faucets.

Overview

Kaspa is a UTXO network, so building a transaction means: gather spendable UTXOs for an address, define the outputs to create, add a change output, compute mass/fees, sign the inputs, and submit. The SDK handles most of this. See Transaction Structure for what’s inside the result. All amounts are bigint sompi.

Keys and addresses

const { Mnemonic, XPrv, NetworkType } = kaspa;

const mnemonic = Mnemonic.random(); // or new Mnemonic(existingPhrase)
const xprv = new XPrv(mnemonic.toSeed());

// Kaspa's BIP-44 coin type is 111111
const privateKey = xprv.derivePath("m/44'/111111'/0'/0/0").toPrivateKey();
const address = privateKey.toAddress(NetworkType.Testnet);

Build, sign, submit

createTransactions() performs coin selection, change output creation, and mass/fee calculation:

const { createTransactions, kaspaToSompi } = kaspa;

// 1. Gather UTXOs for the sending address
const { entries } = await rpc.getUtxosByAddresses([address]);

// 2. Build
const { transactions, summary } = await createTransactions({
	entries,
	outputs: [{ address: destinationAddress, amount: kaspaToSompi('0.2') }],
	priorityFee: 0n, // extra sompi on top of the base fee; 0n is fine when the network is quiet
	changeAddress: address
});

// 3. Sign and submit
for (const pending of transactions) {
	await pending.sign([privateKey]);
	const txid = await pending.submit(rpc);
	console.log('Submitted', txid);
}

transactions is an array because a large enough payment can exceed the per-transaction mass limit, in which case the SDK automatically chains multiple transactions. A typical payment produces one.

Notes

• Check (await rpc.getServerInfo()).isSynced before submitting.
• The transaction can be watched in an explorer, or via utxos-changed event subscriptions.
• For finer control, the SDK’s Generator class takes the same parameters and yields pending transactions one at a time.
• Fees are implicit (inputs - outputs). Expected fee = fee rate × mass; rpc.getFeeEstimate() returns suggested fee rate buckets.
• The Python SDK has the same flow (create_transactions, sign, submit) - see its docs.
• For multi-party or offline signing flows, see PSKT.