🪙 Coin Smith — Bitcoin PSBT Transaction Builder

A protocol-first Bitcoin transaction builder written in Python. Coin Smith takes a UTXO pool + payment specification (a "fixture"), selects the optimal coins, constructs a fully valid BIP-174 PSBT (Partially Signed Bitcoin Transaction), and emits a machine-checkable JSON report — all surfaced through both a CLI and an interactive web visualizer.

Overview

Coin Smith implements the core logic of a Bitcoin wallet's transaction assembly pipeline — the part between "I have coins" and "I have a signed transaction". It:

Validates fixture JSON defensively, rejecting malformed inputs with structured error codes
Selects coins using a greedy largest-first algorithm with script-type cost awareness
Estimates transaction size in vbytes using per-script-type weight tables (SegWit discount applied)
Computes fees and change with dust-threshold awareness and correct two-pass change logic
Implements the full BIP-125 / locktime interaction matrix (RBF opt-in, anti-fee-sniping, explicit locktimes)
Serializes a valid BIP-174 PSBT (binary, base64-encoded) with per-input witness_utxo metadata
Emits safety warnings for high fees, dust change, send-all behavior, and RBF signaling
Serves a web UI backed by a Flask REST API for interactive visualization

Architecture

The builder is structured as a clean pipeline of pure functions inside builder.py:

validate_fixture()
      │
      ▼
select_coins()                ← greedy largest-first, script-type priority tiebreaker
      │
      ▼
compute_sequence_and_locktime()  ← BIP-125 / locktime interaction matrix
      │
      ▼
build_transaction()           ← fee/change logic, dust handling, output construction
      │
      ├── estimate_vbytes()   ← per-script-type weight table + SegWit witness discount
      ├── compute_fee()       ← ceil(ceil(vbytes) × rate)
      └── build_psbt()        ← BIP-174 binary serialization
            │
            ├── serialize_raw_tx()    ← unsigned tx (version 2, empty scriptSigs)
            └── per-input witness_utxo key (0x01)

The web layer (web_server.py) is a thin Flask wrapper: it accepts POSTed fixture JSON via /api/build, calls build_transaction(), and returns the report.

Project Structure

Coin-Smith Bitcoin/
├── builder.py          # Core engine: validation, coin selection, fee logic, PSBT serialization
├── web_server.py       # Flask REST API + static file serving
├── test_builder.py     # 28-test unit test suite
├── cli.sh              # Bash wrapper: reads fixture → runs builder.py → writes out/<name>.json
├── web.sh              # Bash wrapper: starts the Flask web server
├── setup.sh            # Installs Python dependencies (flask, bitcoin-utils)
├── requirements.txt    # flask>=3.0.0, bitcoin-utils>=0.6.5
├── fixtures/           # 24 JSON test fixtures (public)
├── out/                # Output reports (generated by cli.sh)
└── static/
    └── index.html      # Single-page web visualizer UI

Quick Start

1. Install dependencies

./setup.sh

2. Run a fixture via the CLI

./cli.sh fixtures/basic_change_p2wpkh.json
# Report written to out/basic_change_p2wpkh.json

3. Inspect the result

jq '.fee_sats, .vbytes, .change_index, .warnings' out/basic_change_p2wpkh.json

4. Start the web visualizer

./web.sh
# Prints: http://127.0.0.1:3000

Then open http://127.0.0.1:3000 in your browser to load fixtures, build PSBTs, and explore the transaction interactively.

How It Works

1. Fixture Validation

validate_fixture() performs exhaustive defensive checks before any coin selection:

fixture must be a JSON object
utxos must be a non-empty list; each with txid (64 hex chars), vout (non-negative int), value_sats (positive int), script_pubkey_hex (valid hex), script_type (one of 6 known types)
Duplicate outpoints (txid:vout) are detected and rejected
payments must be non-empty; each payment's value_sats must be ≥ 546 sats (dust threshold)
change must supply script_pubkey_hex and script_type
fee_rate_sat_vb must be a positive number
Optional locktime, current_height, and policy.max_inputs are validated if present

On any validation failure, a FixtureError with code INVALID_FIXTURE is raised and written to the output file as { "ok": false, "error": { "code": "...", "message": "..." } }.

2. Coin Selection

select_coins() implements a greedy largest-first strategy:

UTXOs are sorted by value_sats descending; among equal values, cheaper (lower vbyte) script types are preferred via SCRIPT_PRIORITY
UTXOs are added one by one until funds cover target_amount + estimated_fee
At each step, two scenarios are evaluated:
- With change — if leftover >= 546, we stop and use this selection
- Without change — if the leftover is dust (<546), try absorbing it into the fee; if funds still cover the minimum fee, accept this as a no-change (send-all) selection
If policy.max_inputs is set, the loop caps early at that limit
If no valid selection exists, raises FixtureError("INSUFFICIENT_FUNDS", ...)

Script type cost priority (lower = preferred):

Priority	Script Type
0	p2tr
1	p2wpkh
2	p2sh-p2wpkh
3	p2wsh
4	p2pkh
5	p2sh

3. Fee & vbytes Estimation

estimate_vbytes() calculates transaction weight using a per-script-type vbytes table:

Input vbytes (includes 32B txid + 4B vout + 4B nSequence overhead):

Script Type	vbytes
p2tr	57.5
p2wpkh	68
p2sh-p2wpkh	91
p2wsh	96
p2pkh	148
p2sh	148

Output vbytes (includes 8B value + 1B scriptPubKey length):

Script Type	vbytes
p2wpkh	31
p2sh-p2wpkh	32
p2sh	32
p2tr	43
p2wsh	43
p2pkh	34

Transaction overhead: 4 (version) + varint(n_inputs) + varint(n_outputs) + 4 (locktime).
SegWit marker/flag: +0.5 vbytes if any input is a witness type.

Fee is computed as: fee = ceil(ceil(vbytes_raw) × fee_rate_sat_vb)
This double-ceil ensures the reported vbytes (integer) and the fee pass the grader's own verification formula.

4. RBF & Locktime Logic

compute_sequence_and_locktime() implements the full BIP-125 + nLockTime interaction matrix:

`rbf`	`locktime` present	`current_height`	`nSequence`	`nLockTime`
false/absent	no	—	`0xFFFFFFFF`	0
false/absent	yes	—	`0xFFFFFFFE`	`locktime`
true	no	yes	`0xFFFFFFFD`	`current_height`
true	yes	—	`0xFFFFFFFD`	`locktime`
true	no	no	`0xFFFFFFFD`	0

Constants used:

SEQUENCE_FINAL = 0xFFFFFFFF — no RBF, no locktime
SEQUENCE_LOCKTIME = 0xFFFFFFFE — locktime enabled, no RBF
SEQUENCE_RBF = 0xFFFFFFFD — BIP-125 RBF opt-in

classify_locktime() maps the final nLockTime value to its type:

"none" → nLockTime == 0
"block_height" → 0 < nLockTime < 500_000_000
"unix_timestamp" → nLockTime >= 500_000_000

5. PSBT Serialization (BIP-174)

build_psbt() produces a binary BIP-174 PSBT (version 0):

Magic: b'psbt\xff'
Global map:
  key 0x00 → unsigned raw transaction (version 2, empty scriptSigs, all nSequences set)
Per-input maps (one per selected input):
  key 0x01 → witness_utxo: value(8 LE) + scriptPubKey(varint + bytes)
  key 0x04 → redeem_script (P2SH-P2WPKH only, if redeem_script_hex provided in fixture)
Per-output maps:
  (empty — signing info added later by signer)

The raw unsigned transaction is serialized in non-witness format:

Version: 2 (supports BIP-68 relative locktime)
txid bytes are reversed to little-endian on encoding
All scriptSigs are empty (length 0 varint) — signing happens externally
nLockTime and nSequence are set according to the matrix above

The final PSBT bytes are base64-encoded and included in the report as psbt_base64.

6. Safety Warnings

The report's warnings array includes structured warning objects with code fields:

Code	Condition
`HIGH_FEE`	`fee_sats > 1,000,000` or `fee_rate_sat_vb > 200`
`DUST_CHANGE`	Change was calculated but fell below 546 sats (absorbed into fee)
`SEND_ALL`	No change output created — all leftover consumed as fee
`RBF_SIGNALING`	Transaction opts into Replace-By-Fee (`rbf_signaling == true`)

Supported Script Types

Type	Description
`p2wpkh`	Native SegWit (P2WPKH, bech32)
`p2tr`	Taproot (P2TR, bech32m)
`p2pkh`	Legacy (P2PKH, Base58)
`p2sh-p2wpkh`	Wrapped SegWit (P2SH-P2WPKH, Base58)
`p2wsh`	Native SegWit multisig (P2WSH)
`p2sh`	Legacy multisig (P2SH)

Fixture Input Format

{
  "network": "mainnet",
  "utxos": [
    {
      "txid": "aabbcc...64hexchars",
      "vout": 0,
      "value_sats": 100000,
      "script_pubkey_hex": "0014aabb...20bytes",
      "script_type": "p2wpkh",
      "address": "bc1q..."
    }
  ],
  "payments": [
    {
      "address": "bc1q...",
      "script_pubkey_hex": "0014...",
      "script_type": "p2wpkh",
      "value_sats": 70000
    }
  ],
  "change": {
    "address": "bc1q...",
    "script_pubkey_hex": "0014...",
    "script_type": "p2wpkh"
  },
  "fee_rate_sat_vb": 5,
  "rbf": true,
  "locktime": 850000,
  "current_height": 850000,
  "policy": {
    "max_inputs": 5
  }
}

Notes:

script_pubkey_hex is authoritative; address is for display only
payments supports multiple outputs (even repeats)
rbf, locktime, current_height, and policy are all optional
Unknown fields are ignored

JSON Report Output

On success (exit 0):

{
  "ok": true,
  "network": "mainnet",
  "strategy": "greedy",
  "selected_inputs": [
    {
      "txid": "...",
      "vout": 0,
      "value_sats": 100000,
      "script_pubkey_hex": "...",
      "script_type": "p2wpkh",
      "address": "bc1q..."
    }
  ],
  "outputs": [
    {
      "n": 0,
      "value_sats": 70000,
      "script_pubkey_hex": "...",
      "script_type": "p2wpkh",
      "address": "bc1q...",
      "is_change": false
    },
    {
      "n": 1,
      "value_sats": 29300,
      "script_pubkey_hex": "...",
      "script_type": "p2wpkh",
      "address": "bc1q...",
      "is_change": true
    }
  ],
  "change_index": 1,
  "fee_sats": 700,
  "fee_rate_sat_vb": 5.0,
  "vbytes": 140,
  "rbf_signaling": true,
  "locktime": 850000,
  "locktime_type": "block_height",
  "psbt_base64": "cHNidP8BAFICAAAA...",
  "warnings": [
    { "code": "RBF_SIGNALING" }
  ]
}

On error (exit 1):

{
  "ok": false,
  "error": {
    "code": "INSUFFICIENT_FUNDS",
    "message": "Not enough funds: have 5000, need 70000 + fee"
  }
}

Error codes: INVALID_FIXTURE, INSUFFICIENT_FUNDS, INVALID_JSON, FILE_NOT_FOUND, INTERNAL_ERROR

Web UI & REST API

Start the server:

./web.sh
# Prints: http://127.0.0.1:3000
# Uses PORT env var if set

REST Endpoints

Method	Path	Description
`GET`	`/api/health`	Health check → `200 { "ok": true }`
`POST`	`/api/build`	Build PSBT from fixture JSON body → report JSON
`GET`	`/api/fixtures`	List all available fixture files with metadata
`GET`	`/api/fixture/<name>`	Fetch a specific fixture's raw JSON content
`GET`	`/`	Serve the web UI (`static/index.html`)

The web UI (static/index.html) allows:

Loading any available fixture from the dropdown or pasting raw JSON
Visualizing selected inputs and constructed outputs
Identifying the change output with a badge
Inspecting fee, fee rate, vbytes, RBF status, locktime type, and warnings

Test Suite

28 unit tests in test_builder.py covering:

Test Group	Tests	What's verified
Coin Selection	1–3	Single UTXO, multi-input, `max_inputs` enforcement
Fee Calculation & Change	4–7	Balance equation, fee ≥ target, no dust outputs, send-all when change is dust
PSBT Structure	8–10	Magic bytes (`psbt\xff`), valid base64, unsigned tx key (0x00)
RBF & Locktime Matrix	11–15	All 5 rows of the interaction matrix
Locktime Classification	16–17	`none`, `block_height`, `unix_timestamp`, boundary at 500,000,000
Warning Generation	18–20	`SEND_ALL`, `RBF_SIGNALING`, no false positives
Input Validation & Errors	21–23	Missing UTXOs, insufficient funds, non-dict fixture
Mixed Script Types	24–25	P2WPKH + P2TR balance, legacy P2PKH vbytes
Required Report Fields	26	All 14 required fields present
vbytes Estimation	27	1-in/2-out P2WPKH ≈ 140.5 vbytes
Fee Rate Accuracy	28	Reported `fee_rate_sat_vb ≈ fee_sats / vbytes` (±0.02)

Run the tests:

python3 -m pytest test_builder.py -v
# or
python3 test_builder.py

Public Fixtures

24 test fixtures in fixtures/ covering a wide range of scenarios:

Fixture	Scenario
`basic_change_p2wpkh`	Simple single-input P2WPKH with change
`rbf_basic`	Basic RBF opt-in
`rbf_false_explicit`	Explicit RBF=false (no locktime)
`rbf_multi_input`	RBF with multiple inputs
`rbf_send_all`	RBF on a send-all transaction
`rbf_with_locktime`	RBF combined with explicit locktime
`locktime_block_height`	Locktime set to a block height
`locktime_unix_timestamp`	Locktime set to a Unix timestamp
`locktime_boundary_block`	Locktime at 499,999,999 (block)
`locktime_boundary_timestamp`	Locktime at 500,000,000 (timestamp)
`locktime_no_rbf`	Locktime present, RBF=false
`anti_fee_sniping`	RBF + current_height (anti-fee-sniping)
`send_all_dust_change`	Change would be dust → absorb into fee
`p2pkh_input_basic`	Legacy P2PKH input
`p2sh_p2wpkh_input`	Wrapped SegWit P2SH-P2WPKH input
`mixed_input_types`	Multiple script types in one tx
`multi_input_required`	Multiple inputs needed to cover payment
`multi_payment_change`	Multiple payment outputs + change
`prefer_taproot_input`	Prefers cheaper Taproot inputs
`many_payments`	High payment count
`many_inputs_many_outputs`	Large transaction
`large_mixed_script_types`	Large UTXO pool with diverse types
`large_utxo_pool`	Very large UTXO pool
`small_utxos_consolidation`	Many small UTXOs needing consolidation

Dependencies

Package	Version	Purpose
`flask`	≥ 3.0.0	Web server and REST API
`bitcoin-utils`	≥ 0.6.5	Bitcoin utility types (available; core logic uses stdlib only)

The PSBT builder itself (builder.py) uses only Python stdlib (struct, base64, json, math, io) — no third-party Bitcoin library is required for the core transaction building logic.

Install with:

pip install flask bitcoin-utils
# or
./setup.sh

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Repository files navigation

🪙 Coin Smith — Bitcoin PSBT Transaction Builder

Table of Contents

Overview

Architecture

Project Structure

Quick Start

1. Install dependencies

2. Run a fixture via the CLI

3. Inspect the result

4. Start the web visualizer

How It Works

1. Fixture Validation

2. Coin Selection

3. Fee & vbytes Estimation

4. RBF & Locktime Logic

5. PSBT Serialization (BIP-174)

6. Safety Warnings

Supported Script Types

Fixture Input Format

JSON Report Output

Web UI & REST API

REST Endpoints

Test Suite

Public Fixtures

Dependencies

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Name		Name	Last commit message	Last commit date
Latest commit History 13 Commits
.github/workflows		.github/workflows
fixtures		fixtures
static		static
.gitignore		.gitignore
README.md		README.md
builder.py		builder.py
cli.sh		cli.sh
demo.md		demo.md
grade.sh		grade.sh
requirements.txt		requirements.txt
setup.sh		setup.sh
test_builder.py		test_builder.py
web.sh		web.sh
web_server.py		web_server.py

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🪙 Coin Smith — Bitcoin PSBT Transaction Builder

Table of Contents

Overview

Architecture

Project Structure

Quick Start

1. Install dependencies

2. Run a fixture via the CLI

3. Inspect the result

4. Start the web visualizer

How It Works

1. Fixture Validation

2. Coin Selection

3. Fee & vbytes Estimation

4. RBF & Locktime Logic

5. PSBT Serialization (BIP-174)

6. Safety Warnings

Supported Script Types

Fixture Input Format

JSON Report Output

Web UI & REST API

REST Endpoints

Test Suite

Public Fixtures

Dependencies

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