# Show HN: Penmentor – Claude marks up your handwritten math like a teacher

> Source: <https://github.com/AbdullahAlhariri/penmentor>
> Published: 2026-10-20 15:10:54+00:00

A maths mentor at the tip of your pen. Press a key while you work by hand: Claude, Gemini or GPT marks your work on the

screen like a teacher with coloured pens, gives you hints instead of answers, and shows the idea as a 3Blue1Brown-style animation.

<sub>Handwriting in xournal++ → one key → the slip is marked on the page → box the integral → an interactive manim animation.
([video](https://github.com/AbdullahAlhariri/penmentor/blob/main/docs/demo.mp4); for a repeatable recording the pen is scripted and the model's answer is a fixed example.)</sub>

I study calculus with a pen tablet in xournal++. I kept screenshotting my work into Claude and reading long answers
back, when what I wanted was a teacher looking over my shoulder: circle the mistake, tick what's right, nudge me toward
the next step, and *show* me why. penmentor is that, one key away, on my own Claude subscription.

- **Hints, not answers.** A ladder from a nudge to naming the technique to a faint ghost of the next line: you do
the thinking, which is what makes it stick.
- **Feedback on your own work.** The first real mistake circled where you made it, correct lines ticked, later lines
that inherit the mistake underlined. The marks stay while you keep writing; Esc clears them.
- **Practice that makes you recall.** Questions on your key steps (answer by typing or by writing), find-the-error
drills, "which technique fits?".
- **See it.** Box what you want to understand and get an interactive animation made with 3Blue1Brown's own[manim](https://github.com/3b1b/manim) : drag the bounds, retype a value, watch Riemann sums close in.
- **Whole page aware.** A small xournal++ plugin shares the page you are on, not just what fits on screen.

penmentor runs on the AI command-line agent you already use, on that subscription. Pick the model in *Settings*; its
prefix says which agent answers:

| Prefix | Agent | For example | 
|---|---|---|
| `claude:` | [Claude Code](https://claude.com/claude-code) | `claude:opus` ,`claude:sonnet` ,`claude:haiku` | 
| `codex:` | [Codex](https://github.com/openai/codex) | `codex:gpt-5.6-terra` | 
| `agy:` | Antigravity CLI | `agy:gemini-4-argon-medium` ,`agy:gemini-3.8-flash-medium` | 

Settings lists the agents you have installed. How each one is run and what it sees: [the guide](https://github.com/AbdullahAlhariri/penmentor/blob/main/docs/guide.md#how-it-works-and-what-leaves-your-computer).

```
git clone https://github.com/AbdullahAlhariri/penmentor ~/src/penmentor
~/src/penmentor/bin/penmentor --selftest
```

Linux on X11 (best with i3), with one of those agents logged in (`claude`, `codex` or `agy`), plus `rofi`, `slop`,
`xdotool` and PyQt6. Bind `bin/penmentor` to a key ([example](https://github.com/AbdullahAlhariri/penmentor/blob/main/contrib/i3.conf)). Setup, keys and how it works: [the guide](https://github.com/AbdullahAlhariri/penmentor/blob/main/docs/guide.md).