kaeru

Convert anything. Nothing leaves your machine. One tool your AI agents and you both use.

npx @linno-inc/kaeru --doctor     # what can this machine convert?

Verified, not claimed

Every conversion listed below was actually run, one by one, on real machines — and the output was re-opened with a second tool to prove it isn't empty. The test suite ships with the code, so you can re-run the count yourself.

npm test        # re-run every combination on your machine
npm run list    # print the conversion table (this is what the numbers count)

Numbers differ per machine on purpose — see below.

Related MCP server: Text-Toolkit

The table adapts to your machine

kaeru doesn't bundle converters. It finds the ones you already have — and asks each one what its version actually supports. A conversion that can't work is never offered.

$ npx @linno-inc/kaeru --doctor
This machine can do: 4,062 conversions

Tools:
  ✓ magick      C:\Program Files\ImageMagick-7.1.2-Q16-HDRI\magick.EXE
  ✗ ffmpeg      not found
  ...

Install these to unlock more:
  ffmpeg      +468 conversions   winget install Gyan.FFmpeg

Nothing installed? Nothing breaks — you get 0 conversions and a list of one-line installs.

Two doors, one engine

Both call the same code, so an agent and a human can never disagree about what's possible.

For agents (MCP)

{ "mcpServers": { "kaeru": { "command": "npx", "args": ["-y", "@linno-inc/kaeru"] } } }

One tool, convert. Ask for something impossible and it replies with what is possible from that input — or which tool to install.

For humans

npm run ui opens a single page on 127.0.0.1 only. Drop a file, pick a target, save the result. It does not stay running — Ctrl-C and it's gone. No Docker, no daemon, no account.

What it converts

Images · video · audio · documents (md, html, tex, docx, odt, epub, and ~30 more) · Office and spreadsheets · PDF · 3D models · Parquet.

PDFs can be merged, split, compressed and rasterised. Images, audio and PDFs can be shrunk without changing format. Pass a URL to pull from video sites.

CJK encodings are detected automatically (a Shift_JIS CSV from Excel just works).

Every result comes with its receipt

Converted: csv>xlsx
Output:    /path/to/data.xlsx
Tool:      soffice
Command:   soffice --headless --convert-to xlsx --outdir /tmp/... /path/to/data.csv
Took:      3,512 ms

You can always see which program touched your file, and repeat it by hand.

Install the converters

kaeru calls these; install the ones you need (or none, and add them later).

brew install imagemagick ffmpeg pandoc weasyprint qpdf img2pdf yt-dlp assimp duckdb ghostscript
brew install --cask libreoffice

winget install ImageMagick.ImageMagick Gyan.FFmpeg JohnMacFarlane.Pandoc QPDF.QPDF `
  yt-dlp.yt-dlp Assimp.Assimp DuckDB.cli ArtifexSoftware.GhostScript TheDocumentFoundation.LibreOffice
pip install weasyprint img2pdf

sudo apt install imagemagick ffmpeg pandoc qpdf img2pdf yt-dlp assimp-utils ghostscript libreoffice
pip install weasyprint

Privacy and security

• Files are processed by local programs. kaeru makes no network requests (except when you explicitly pass a video-site URL).

• The web page binds to 127.0.0.1, rejects non-localhost Host headers, and hands results back by an unguessable id.

• Existing files are never overwritten unless you ask for it.

• No telemetry. Ever. Not anonymous, not opt-out — none.

Three independent models reviewed the source on 2026-08-18; every exploitable finding was fixed the same day. One known risk is not fixed: converting untrusted HTML or SVG can embed local files in the output. Details, severities and mitigations: SECURITY.md.

Tests

Known gaps are written down, not hidden — see docs/.

Support

Issues are welcome. This is a one-person project: no response time is promised. For a bundled installer or a support contract, see the bottom of this file.

License

MIT. The converters it calls have their own licenses.

Need this installed for a team, or with a support contract? Open an issue titled commercial and I'll get back to you.