Penmentor is a maths mentor that sits at the tip of your pen. While you work by hand in xournal++, one key press sends your page to Claude, Gemini or GPT, which marks your work on the screen like a teacher with coloured pens. Instead of handing you answers, it gives hints, and it can show the underlying idea as a 3Blue1Brown-style animation.
The project was built by a calculus student who was tired of screenshotting work into Claude and reading long replies. What they wanted was a teacher looking over their shoulder: circling mistakes, ticking correct lines, and nudging them toward the next step. Penmentor delivers this one key away, using the user's own Claude subscription.
The learning design centres on hints rather than answers. A ladder moves from a gentle nudge, to naming the technique, to a faint ghost of the next line, so the student does the thinking. Feedback targets the first real mistake, underlines later lines that inherit the error, and keeps the marks on the page until Esc clears them. Practice modes include recall questions, find-the-error drills, and "which technique fits?" prompts. Users can also box any expression to generate an interactive manim animation, where they can drag bounds or retype values and watch Riemann sums converge.
Penmentor runs on existing AI command-line agents, including Claude Code, Codex and Antigravity, and the model is selected in Settings. It currently targets Linux on X11, with setup requiring rofi, slop, xdotool and PyQt6. The project is released under the MIT license.
Source: Hacker News · Summarized by HeadlinesBriefing