# Bitcoin Hardware Coldcard

> Coldcard hardware wallets (Coinkite): Mk4, Q. Bitcoin-only firmware, airgap support via SD card / NFC / QR, BIP85, MuSig2 (experimental), Trick PINs. USE WHEN: integrating with Coldcard, supporting Mk4 / Q, designing airgap signing flows.

- Skill: `claude-dev-suite/bitcoin-hardware-coldcard` (Agent Skill)
- Install (CLI): `npx skillmds@latest add claude-dev-suite/bitcoin-hardware-coldcard`
- Raw SKILL.md: https://api.skillmd.com/api/skills/claude-dev-suite/bitcoin-hardware-coldcard/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: claude-dev-suite (https://skillmd.com/u/claude-dev-suite)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/claude-dev-suite/bitcoin-hardware-coldcard

---


# Coldcard (Coinkite)

Bitcoin-only hardware wallet. Industry favorite for **airgapped**
signing. Models: Mk4, Q.

## Models

| Model | Year | Notes |
|-------|------|-------|
| Mk4 | 2022 | USB-C + microSD + NFC, no battery |
| Q | 2024 | Color touchscreen, USB-C, microSD, NFC, full keyboard, battery |

## Airgap support

Multiple transport options:
- **microSD card**: write PSBT to card, sign on device, write back.
- **NFC**: tap-to-transfer.
- **QR codes** (Q only): scan + display.
- **USB**: direct (less airgap).

## Bitcoin-only firmware

Coldcard intentionally supports ONLY Bitcoin. No altcoins. Smaller
attack surface, simpler mental model.

## Trick PINs

Defense against coercion:
- Set up multiple PINs.
- "Real" PIN unlocks main wallet.
- "Duress" PIN unlocks decoy wallet (real-looking but with different
  seed).
- "Login countdown" — N attempts before wallet wipe.

## BIP85

Native support for deriving sub-seeds via BIP85. UI lets you generate:
- BIP39 mnemonic (any word count).
- HEX bytes.
- WIF private key.
- xprv.

## MuSig2 (experimental)

Mk4 has experimental MuSig2 support for cooperative Taproot signing.
Not yet standard across HW wallets.

## Backup

- Standard 24-word seed.
- "Tarot card" backup grids for resilient recovery.
- BIP85-derived sub-seeds.
- Encrypted backups to microSD with a separate backup password.

## API

`ckcc-protocol` Python:
```python
from ckcc_protocol.client import ColdcardDevice
dev = ColdcardDevice()
pk = dev.get_xpub("m/84'/0'/0'")
dev.sign_psbt(psbt_bytes)
```

## Connection

- USB-C.
- microSD (airgap).
- NFC (Mk4, Q).
- Q only: QR code.

Third-party: HWI, Sparrow, Specter, Electrum.

## Common issues

- microSD format must be FAT32 (max 32 GB).
- USB driver issues on some Linux distros.
- NFC on iPhone limited to read-only — use Android for tap-to-sign.

## See also

- [trezor/SKILL.md](../trezor/SKILL.md)
- [ledger/SKILL.md](../ledger/SKILL.md)
- [psbt-flows/SKILL.md](../psbt-flows/SKILL.md)
- [hwi/SKILL.md](../hwi/SKILL.md)
- [../wallets/entropy/SKILL.md](../../wallets/entropy/SKILL.md)