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epub-to-kindle

A Claude Code skill that converts an EPUB (or any Calibre-supported ebook) into a Kindle-readable format — AZW3 (modern) or MOBI (legacy) — using Calibre's ebook-convert. It auto-locates the CLI inside the macOS .app bundle, installs Calibre via Homebrew when missing, and surfaces the exact sudo chown command when /opt/homebrew permissions block the install.

What It Does

  • Disambiguates fuzzy format requests ("aws", "azw", "Kindle") → AZW3, or MOBI for legacy devices
  • Finds ebook-convert on $PATH or inside /Applications/calibre.app/Contents/MacOS/ (it's not on $PATH after a cask install)
  • Installs Calibre via brew install --cask calibre if absent
  • Detects the /opt/homebrew not-writable / to_sym Homebrew failure and prints the one-time sudo chown fix
  • Converts and verifies the output file, leaving the original EPUB untouched

Workflow

flowchart TD
    A(["/epub-to-kindle\n<input.epub> [azw3|mobi]"]) --> B{Format clear?}
    B -->|"'aws'/'azw'/ambiguous"| C[Confirm AZW3 vs MOBI]
    B -->|Yes| D[Check input exists]
    C --> D
    D --> E{ebook-convert\nfound?}
    E -->|"on PATH or in .app"| I[Run ebook-convert]
    E -->|No| F{brew prefix\nwritable?}
    F -->|No| G(["Stop — ask user to run\nsudo chown -R you /opt/homebrew\nin a real terminal"])
    F -->|Yes| H[brew install --cask calibre]
    H --> I
    I --> J{Output file\nexists?}
    J -->|No| K(["Error — conversion failed"])
    J -->|Yes| L(["Done\n<book>.azw3 written · side-load tips"])
Loading

Install

git clone https://github.com/biomystery/claude-skills.git
mkdir -p ~/.claude/skills
ln -s "$(pwd)/claude-skills/epub-to-kindle" ~/.claude/skills/epub-to-kindle

Restart Claude Code — /epub-to-kindle will be available.

Usage

# Convert to AZW3 (modern Kindle) — default
/epub-to-kindle ~/Downloads/book.epub

# Convert to MOBI (older Kindle / max compatibility)
/epub-to-kindle ~/Downloads/book.epub mobi

# Natural language also works — "aws" is read as AZW3
convert ~/Downloads/book.epub to aws format

You can also call the helper script directly:

epub-to-kindle/scripts/convert-ebook.sh ~/Downloads/book.epub azw3

Output

A single Kindle file written next to the input (or to a path you specify). The source EPUB is not modified.

Sample output (illustrative values):

Using: /Applications/calibre.app/Contents/MacOS/ebook-convert
ebook-convert (calibre 9.9.0)
Converting -> ~/Downloads/sample-book.azw3
...
AZW3 output written to ~/Downloads/sample-book.azw3
OK: ~/Downloads/sample-book.azw3 (1100000 bytes)

Side-load it by copying into the Kindle's documents folder over USB, or email it to your Send-to-Kindle address.

Requirements

  • macOS with Homebrew (for the auto-install path), or Calibre already installed
  • Calibre (ebook-convert) — installed automatically if missing
  • Writable /opt/homebrew for auto-install (otherwise a one-time sudo chown -R $(whoami) /opt/homebrew)

Supported Inputs / Edge Cases

  • Inputs: any format Calibre's ebook-convert accepts — .epub, .fb2, .lit, .pdb, .htmlz, etc. (EPUB is the common case)
  • Outputs: azw3 (default) or mobi
  • CLI not on PATH: the macOS cask installs the CLI inside the .app bundle; the script checks the bundle path explicitly
  • Homebrew owned by root: brew dies with undefined method 'to_sym' or not writable; fix is sudo chown -R $(whoami) /opt/homebrew in a real terminal — the agent can't enter the sudo password through the !-prefix shell (no TTY)
  • Stale cask API cache: can independently trigger the to_sym crash; the script clears ~/Library/Caches/Homebrew/api automatically

Key Design Decisions

Check the .app bundle, not just $PATH. Calibre's cask does not symlink ebook-convert onto $PATH. The reliable path is /Applications/calibre.app/Contents/MacOS/ebook-convert.

Permission failures stop with instructions, not retries. A non-writable /opt/homebrew needs interactive sudo that an automated agent cannot supply. The script detects this up front and prints the exact chown command rather than looping on a doomed brew install.

AZW3 is the default. It's the current Kindle (KF8) format; MOBI is offered only for older hardware.

Skill Structure

epub-to-kindle/
├── SKILL.md    (instruction file Claude executes)
├── README.md   (this file)
└── scripts/
    └── convert-ebook.sh   (locate CLI · install Calibre · convert · verify)