> ## Documentation Index
> Fetch the complete documentation index at: https://docs.utexo.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Bitcoin Settlement

> Settle Utexo Bitcoin swaps with Taproot HTLC PSBTs, script-path withdrawals, and cooperative fast refunds.

Bitcoin settlement uses Taproot (P2TR) outputs and BIP 174 PSBTs. No smart contract is deployed on Bitcoin.

## HTLC lock output

Every deposit locks funds to a unique Taproot output with three script leaves:

| Leaf              | Script                                                                    |
| ----------------- | ------------------------------------------------------------------------- |
| Resolver withdraw | `OP_HASH256 <secretHash> OP_EQUALVERIFY <resolverPubKey> OP_CHECKSIG`     |
| Timelock refund   | `<deadline> OP_CLTV OP_DROP <protocolPubKey> OP_CHECKSIG`                 |
| Fast refund       | `<protocolPubKey> OP_CHECKSIG <resolverPubKey> OP_CHECKSIGADD 2 OP_EQUAL` |

* `secretHash` is double SHA-256 of the 32-byte secret.
* The internal key may be a NUMS point or a resolver-selected key, per source.
* The resolver returns the refund and fast-refund control blocks to Utexo and keeps the withdraw control block internally.

## Deposit transaction

### Intent tagging

Every deposit contains an `OP_RETURN` output that tags the intent:

```text theme={null}
OP_RETURN <protocolPrefix> <flag> <intentID>
```

### PSBT inputs

| Field              | Value                     |
| ------------------ | ------------------------- |
| `WITNESS_UTXO`     | Required for every input. |
| Sighash            | `SIGHASH_ALL` (`0x01`).   |
| `TAP_INTERNAL_KEY` | User's x-only public key. |
| Sequence           | `0xFFFFFFFF` (no RBF).    |

### Outputs

| Index | Output                                                                                |
| ----- | ------------------------------------------------------------------------------------- |
| `0`   | `OP_RETURN` with `0` sats.                                                            |
| `1`   | P2TR HTLC lock. The amount is the deposit minus a proportional share of network fees. |

<Warning>
  The source states there are exactly two or three outputs, but does not define the optional third output. Do not invent change behavior; see [Validation Gaps](/product-suite/swap/resolver-integration/validation-gaps).
</Warning>

## Spend paths

### Key-spend path

Used for deposit input signing, fulfillment, and native transfers.

Witness:

```text theme={null}
[schnorr_signature]
```

### Resolver withdraw (script-path)

Witness:

```text theme={null}
[schnorr_signature, secret_preimage, tap_script, control_block]
```

### Fast refund (script-path)

Cooperative script-path spend co-signed by the protocol and the resolver.

### Timelock refund

Script-path refund after `deadline`.

<Note>
  The source does not document the final witness stack for the timelock refund path. Confirm with the current implementation before constructing this witness.
</Note>

## Fee model

The resolver computes fees from a header plus per-input and per-output costs:

```text theme={null}
header         = 11 vBytes
size           = header
               + sum(input_size + witness_size) * count
               + sum(output_size) * count
fee            = size * fee_rate
```

Default fee rate: `3 sat/vB`.

### Size table

| Type           | Input | Witness | Output |
| -------------- | ----- | ------- | ------ |
| P2TR           | 41    | 17      | 43     |
| P2WPKH         | 41    | 27      | 31     |
| P2SH (2-of-3)  | 43    | 254     | 32     |
| P2PKH          | 41    | 107     | 34     |
| P2WSH (2-of-3) | 41    | 64      | 43     |