Smith

When to reach for Smith: you have a set of UTXOs you control and you want to build a payment that is efficient, private, and ready for your signing device, without hand-rolling the coin selection or fee math yourself.

Smith is your transaction builder. Give it a set of UTXOs and payment targets, and it picks the optimal coins, calculates fees, and produces a ready-to-sign Partially Signed Bitcoin Transaction (PSBT).

What it does

Smith handles two operations:

PSBT construction. Given a fixture describing available UTXOs, target addresses, amounts, and fee rate, Smith runs coin selection to pick the best inputs, calculates the exact fee, handles change output creation, and produces a valid PSBT in base64 format.

PSBT inspection. Given an existing PSBT (base64), Smith parses it and reports signing status (unsigned, partially signed, fully signed), missing fields, input/output details, fee analysis, script types involved, and what to do next.

Best for

• Building transactions with smart coin selection
• Understanding how fee estimation and change output creation work
• Comparing different UTXO selection strategies and their tradeoffs
• Inspecting PSBTs before you sign them
• Learning the PSBT format (BIP 174) hands-on

What you see

When building a PSBT, Smith returns:

• Selected inputs showing which UTXOs were chosen and why
• Outputs with payment outputs and change (if any)
• Fee calculation with total fee in sats and effective fee rate in sat/vB
• PSBT base64 ready for a signing device
• Scrollytelling breakdown on the web UI, an animated step-by-step walkthrough of the whole construction process

When inspecting an existing PSBT:

• Signing status (unsigned, partially signed, or fully signed)
• Input details including script types, UTXO data, partial signatures present
• Output details with addresses, amounts, and script types
• Fee analysis showing the extracted fee and rate
• Missing fields listing what's still needed to complete signing
• Next steps with recommendations for what to do next

Fixture format

Smith takes a JSON fixture describing the transaction to build:

{
  "inputs": [
    {
      "txid": "abc123...",
      "vout": 0,
      "value": 100000,
      "script_pubkey": "0014..."
    }
  ],
  "outputs": [
    {
      "address": "bc1q...",
      "value": 50000
    }
  ],
  "fee_rate": 5.0
}

The web UI includes 6 preset fixtures covering common scenarios (simple payment, consolidation, multi-output batch, etc.) plus a custom JSON editor.

CLI usage

# build a PSBT from a fixture
txray build fixtures/simple_payment.json

Selected 1 input / 2 outputs
  fee:         705 sats
  fee_rate:    5.0 sat/vB
  change:      0.00049295 BTC -> bc1q...aa9
  psbt (base64):
    cHNidP8BAH0CAAAAAT...

# inspect an existing PSBT
txray inspect "cHNidP8BAH0CAAAAA..."

Signing status: unsigned
Inputs:
  [0] P2WPKH, 0.00060000 BTC, no partial sigs
Outputs:
  [0] 0.00050000 BTC -> bc1q...kk8
  [1] 0.00009295 BTC -> bc1q...aa9  (likely change)
Next steps:
  - send the PSBT to the signing device
  - return the signed PSBT to `txray inspect` to verify

How Smith connects to the other tools

After building a PSBT with Smith, you can analyze the resulting transaction with Lens to verify the structure, or run it through Sherlock to check the privacy implications of your coin selection. This is the full txray loop: build with Smith, verify with Lens, audit with Sherlock.