Introducing monthlet, an AI note-taking app

Hello. I’m t27d, and I’ve released monthlet, a note-taking app for personal use.

You need an API key from Claude, ChatGPT, or Gemini to use it — bring your own key — and beyond that it’s free. You can download it at monthlet.ai, and I’d be glad if you tried it.

In this journal I’ll record how monthlet gets built: design decisions, feature walkthroughs, and what happens along the way. This first post is about what the app does and why I’m making it.

Notes across devices

Which note app do you use? Most people work across a Mac or PC, a phone, and often a tablet, and wanting the same notes on all of them is not a new need. Note apps have been answering it for a long time — the early ones, the tools the large platforms ship today, and a great many independent apps for personal and professional use. I’ve tried a lot of them myself.

AI and note apps

monthlet is an app where you say what’s on your mind and the AI turns it into a note — the structure and the layout included.

ChatGPT arrived at the end of 2022. In under four years, LLMs have moved extraordinarily fast, and they’re still moving. Many people now think things through with an AI and keep the output somewhere.

AI produces more than text these days — images, video, all sorts of file formats. But the output still has to be reshaped, copied, pasted, and filed by hand.

What monthlet does

Write down what comes to mind, as bullets or fragments, and say “turn this into a note.” The AI chooses banners, columns, and tabs, and shapes it into something readable. If you want something changed, say it in plain language, and only that part is updated.

The AI writes the note as an HTML file

Where the name comes from

“monthlet” is an anagram of HTML + NOTE. That isn’t wordplay — it’s the design itself.

Notes are ordinary HTML files. They’re saved on your computer and open in a browser without any special app. Today, and in ten years.
Local-first. Notes live on your own disk, and with your own API key your data never passes through our servers.
The AI writes the note. Shaping and layout are its job, so you can stay on the thinking.

If this service someday goes away, the HTML files are still on your disk. “Your notes are yours” is something I want the structure to guarantee, not a promise to be trusted.

About the AI

monthlet runs on your own API key (Claude / ChatGPT / Gemini). For people who’d rather not deal with keys, monthlet AI is in preparation.

What this journal will cover

Feature walkthroughs — spreadsheets, live charts, themes, the calendar, and more
Design thinking, architecture, and what goes on behind the releases

Availability

monthlet shipped for macOS (Apple Silicon) on August 1, 2026. A Windows version is in progress.

Updates go out here and on X. A follow is always welcome.