Transfer BTC

A complete Bitcoin send flow: check balance, select UTXOs and estimate fee, MPC sign each input, finalize, and broadcast.

Components used:

• GET_BITCOIN_ACCOUNT_BALANCE — verify sufficient funds before building
• BUILD_BITCOIN_TRANSACTION — UTXO selection, fee estimation, and sighash generation
• SIGN_WITH_KEY_SHARE — MPC sign each input's sighash
• SIGN_BITCOIN_TRANSACTION — apply signatures and finalize the PSBT
• BROADCAST_BITCOIN_TRANSACTION — submit to the network

---

Code

import { WorkspaceClient, ComponentModule } from 'caller-sdk';

const workspace = new WorkspaceClient({ apiKey: process.env.WR_API_KEY! });

const KEY_ID          = 'your-key-id';
const DERIV           = [2147532436, 2147483648, 2147483648, 0, 0]; // m/84'/0'/0'/0/0
const SENDER_PUBKEY   = '02abc123...'; // compressed 33-byte public key
const SENDER_ADDR     = 'bc1q...';    // sender's native SegWit address
const RECIPIENT_ADDR  = 'bc1q...';    // recipient address

async function transferBtc(amountSatoshis: number) {
  // 1. Check balance
  const { balance } = await workspace
    .call(ComponentModule.GET_BITCOIN_ACCOUNT_BALANCE, {
      address: SENDER_ADDR,
    })
    .promise();

  console.log(`Balance: ${balance} satoshis`);
  if (balance < amountSatoshis) throw new Error('Insufficient BTC balance');

  // 2. Build unsigned PSBT — selects UTXOs, estimates fee, produces one sighash per input
  const { unsignedTransaction, messageHashes, estimatedFee } = await workspace
    .call(ComponentModule.BUILD_BITCOIN_TRANSACTION, {
      recipientAddress: RECIPIENT_ADDR,
      amount: amountSatoshis,
      publicKey: SENDER_PUBKEY,
      locktime: null,
      opReturnData: null,
      changeAddress: null, // defaults to sender
    })
    .promise();

  console.log(`Estimated fee: ${estimatedFee} satoshis`);
  console.log(`Signing ${messageHashes.length} input(s)`);

  // 3. MPC sign each input sighash
  //    Bitcoin transactions can have multiple inputs — sign each one
  const signatures = await Promise.all(
    messageHashes.map((messageHash) =>
      workspace
        .call(ComponentModule.SIGN_WITH_KEY_SHARE, {
          keyId: KEY_ID,
          derivationPath: DERIV,
          messageHash,
        })
        .promise()
        .then((r) => r.signature),
    ),
  );

  // 4. Apply signatures and finalize the PSBT
  const { signedTransaction } = await workspace
    .call(ComponentModule.SIGN_BITCOIN_TRANSACTION, {
      unsignedTransaction,
      signatures,
      publicKey: SENDER_PUBKEY,
    })
    .promise();

  // 5. Broadcast
  const { txid } = await workspace
    .call(ComponentModule.BROADCAST_BITCOIN_TRANSACTION, {
      signedTransaction,
    })
    .promise();

  console.log('Broadcast txid:', txid);
  return txid;
}

// Send 50,000 satoshis (0.0005 BTC)
const txid = await transferBtc(50_000);

---

Workflow canvas layout

GET_BITCOIN_ACCOUNT_BALANCE
         │ balance (guard check)
         ▼
BUILD_BITCOIN_TRANSACTION
         │ messageHashes[]         │ unsignedTransaction
         ▼                         │
SIGN_WITH_KEY_SHARE (×N inputs)    │
         │ signatures[]            │
         └──────────┬──────────────┘
                    ▼
         SIGN_BITCOIN_TRANSACTION
                    │ signedTransaction
                    ▼
         BROADCAST_BITCOIN_TRANSACTION
                    │ txid
                    ▼
              [Confirmed]

Fee deduction

Set the deductFee config option on BUILD_BITCOIN_TRANSACTION to true to send the exact amount minus fees — useful for draining a wallet to a new address.