A writing tool with layers under the text.
Try other words in place, dim what you might cut, and keep your leftovers close.
Build and run · Shortcuts · AI and the Lab · Issues
All credit for the ideas goes to Jason Fried. In this tweet he showed Write_On, a writing tool he built for himself: alternatives for words, sentences, and paragraphs, dimmed text, a place to stash extra writing, and an AI "lab" that trims without rewriting. Undertext is an open-source rebuild of those ideas, made from watching his video. It is not affiliated with Jason Fried or 37signals.
A local writing tool with layers under the text. Your words stay plain Markdown. The layers hold the other ways you might say them.
- Alternatives. A word or sentence can have several versions. Hover over it and press ↑ or ↓, or press ⌥J and ⌥K, to read each one in place.
- Paragraph versions. Fork a paragraph, rewrite it in place, and switch between the versions.
- Ghost. Dim text you might cut. It stays readable but easy to ignore.
- Overflow. A side panel for leftovers you might use later.
- The Lab. Optional AI help that marks, fixes, and trims, but never rewrites your text on its own.
- Keyboard first. Every action has a shortcut, and ⌘K lists them all.
An experiment that works for daily writing. It is built and tested on macOS. Windows and Linux should build with Tauri, but nobody has tried them yet.
You need Node 22+, pnpm, and Rust. On macOS, install Rust with
brew install rustup && rustup default stable and add /opt/homebrew/opt/rustup/bin to your PATH.
pnpm install
pnpm tauri dev # the desktop app, with live reload
pnpm tauri build # src-tauri/target/release/bundle/macos/Undertext.appThen copy Undertext.app to /Applications.
pnpm dev runs the editor in a plain browser. It has no file access, keeps your document in local
storage, and answers AI requests with fake results so you can try the UI.
| Keys | Action |
|---|---|
| ⌘K | Command palette: every action available right now, with its shortcut |
| ⌘⇧A | Alternatives for the word at the cursor or the selection |
| ⌘⇧G | Ask Claude for alternatives |
| ⌘⇧E | The Lab: typo fixes, checks that mark text, and trims |
| ⌥J ⌥K | Next / previous version at the cursor (word or sentence, else paragraph) |
| ⌥⇧J ⌥⇧K | Next / previous version of the paragraph |
| ⌥L ⌥H | Jump to the next / previous item that has versions (or Lab result) |
| hover + ↑ ↓ | Cycle the hovered word, sentence, or paragraph |
| ⌘⇧D | New version of the current paragraph |
| ⌘/ | Ghost the selection, or revive the ghost at the cursor |
| ⌘⇧X | Stash the selection in Overflow |
| ⌘⇧O | Open Overflow and type in it. Press again to close it. Esc goes back to the page. |
| ⌘⇧L | Show or hide the layers (same as clicking the word count) |
| ⌘O, ⌘S, ⌘⇧S, ⌘N | Open, save, save as, new |
| Right click | Context menu. Shift + right click shows the system menu with spelling fixes. |
- ⌥J, ⌥K, ⌥H, and ⌥L match the physical key, so they work on any keyboard layout. In Undertext they no longer type ∆ ˚ ˙ ¬. Dead keys such as ⌥E and ⌥N still work for accents and ñ.
- Hover cycling only works after you move the mouse. Typing or clicking turns it off until the mouse moves again, so arrow keys never cycle by accident.
- Files that have a path save automatically. Untitled documents survive restarts in local storage.
- ⌘K → "Open the welcome document" brings back the built-in tour at any time.
Plain Markdown with four small additions:
# {{The Obvious, the Easy, and the Possible|Three Buckets}}
Much of the {{tension|pressure|>struggle}} comes from here. ~~Maybe cut this.~~
:::alt
First version of a paragraph.
:::or*
Second version, which is active.
:::
<!-- overflow -->
Leftovers live after the marker.{{a|b|>c}}: alternatives. The first item is the original.>marks the active one.{{calls|~judgments}}:~marks a suggestion that came from AI.~~text~~: ghosted text. Other Markdown tools show it as strikethrough.:::alt…:::or…:::: paragraph versions.*marks the active one.<!-- overflow -->: everything after this line is the Overflow panel.- Use
\to escape~ { } | >. All other Markdown stays as literal text.
The AI features are optional. They never rewrite your text on their own: every result is something you can see and undo.
- Alternatives (⌘⇧G): about 6 suggestions join the word's versions. Your current text stays.
- Fix punctuation and typos: each fix becomes an alternative with the fix active. ⌥K restores your text.
- Checks: weakest sentences, convoluted sentences, off-tone words, and hedges and filler. Marks are temporary highlights with a reason. "Sentences that run long" runs locally, without AI.
- Trim (−10% to −50%): Claude may only delete. Proposed cuts appear faded. Click one to keep it, then make the cuts, keep them as ghosts, or go back to the original.
Privacy. Each AI request sends your whole document to Anthropic's API. Nothing is sent until you use an AI feature.
Setup. Press ⌘K, choose "Set the Anthropic API key…", and paste a key from
platform.claude.com. The key is stored in the macOS Keychain, and
requests go out from the app's Rust side, so the key never reaches the web page. During development,
ANTHROPIC_API_KEY works too. Requests use claude-opus-5-5; a Lab run on a 1,000-word piece costs a
few cents.
Or use your Claude subscription. With no API key set, requests can go through
Claude Code on your Claude plan instead. Install Claude Code and sign in by
running claude in a terminal, then press ⌘K, choose "Use your Claude subscription…", and confirm.
Undertext never signs you in and never uses the subscription until you choose it there; "Stop using
subscription" turns it off without signing Claude Code out. Requests count against your plan's usage
limits. If claude isn't in ~/.local/bin, /opt/homebrew/bin, or /usr/local/bin, set
CLAUDE_PATH. A saved API key always takes priority.
- A ghost that covers only part of an alternative is lost when you save.
- Empty paragraphs collapse when you save.
- When you save,
~and{in your text get a backslash (\~,\{), so they are not read as syntax. - Autosave overwrites the file even if another editor changed it.
- Typing at the end of an alternative adds plain text after it, not to it.
pnpm test # editor and format tests
pnpm icons # rebuild the app icons from brand/ (needs ImageMagick)
cd src-tauri && cargo testThe editor is ProseMirror inside a Tauri app. The icon borrows the mark of Klipstack, with a handwritten u in place of its clip.