A developer named Maxime Rivest built an open-source app that turns the reMarkable Paper Pro e-ink tablet into Tom Riddle's diary from Harry Potter: you handwrite a question on the page, pause, and a vision AI writes back in animated handwriting while your own ink fades into the paper. It was one of the fastest-rising stories on Hacker News on July 6, 2026 - a small, vivid demonstration that today's models are cheap and portable enough to power a magical piece of consumer hardware.
Key facts
- The app, called 'riddle,' is written in Rust with low-level C for the e-ink display, released MIT-licensed at github.com/maximerivest/riddle.
- It works with any OpenAI-compatible vision API - OpenAI, OpenRouter, Groq, Gemini, or a local model.
- After a 2.8-second pause in writing, it captures the page and streams a reply rendered as animated handwriting.
- It reached #2 on Hacker News as the fastest-rising story, at about two hours old.
The experience is designed to feel like the enchanted diary from the books, which wrote back to whoever wrote in it. You write on the reMarkable with its pen, using the tablet's raw pressure-sensitive input. After you rest the pen for 2.8 seconds, the app commits the page to an image and sends it to a vision language model it calls 'the oracle,' which streams a reply back sentence by sentence. Your original ink 'drinks into the page' - fades away - and the AI's answer appears, rendered as handwriting that animates stroke by stroke as if an invisible hand were writing it. There are gestures, too: flip the marker to erase, draw a large question mark to summon a guide, tap with five fingers to leave the diary, and press the power button to make 'the diary sleep.'
The engineering is what makes it more than a toy. To render the AI's text as convincing handwriting, the app takes the Dancing Script font and applies Zhang-Suen thinning - an algorithm that reduces shapes to single-pixel-wide skeletons - so the letters become pen paths that can be drawn one stroke at a time. It offers two display modes: a windowed mode that runs inside the tablet's normal interface, and a full-takeover 'quill' mode that stops the vendor software and drives the e-ink engine directly for the lowest possible latency, using what the README describes as an interposition shim over the tablet's waveform engine. This is a real app talking to real hardware at a low level, not a mockup.
The detail that ties it to the day's bigger story is model compatibility. Riddle works with any OpenAI-compatible endpoint - the same interchangeability that, at industry scale, lets developers swap open-weight models like GLM-5.2 in for frontier APIs by changing a URL. Here it means a hobbyist can point the diary at whatever vision model they prefer, cloud or local. The commodification of the interface is what makes both the billion-dollar margin-collapse debate and this weekend hack possible from the same underlying shift.
Why it matters: consumer AI moments like this are a useful barometer. When a single developer can wire a frontier-class vision model into a niche e-ink tablet over a weekend and produce something genuinely delightful, it signals that the models have crossed from 'research demo' to 'raw material anyone can build on.' The e-ink medium is a clever fit - the slow, deliberate refresh of electronic paper actually enhances the illusion of a hand writing back, turning a technical limitation into atmosphere.
The honest caveat: it is a novelty, and a literary one - the diary it imitates is, in the source material, a dangerous object that manipulates its writer, which is a wink the project leans into rather than a warning it takes seriously. There is no deeper capability here beyond 'vision model reads a page and replies.' But as a piece of interaction design - fading ink, animated strokes, the patience the medium demands - it is a small, real reminder that the interesting frontier of AI is increasingly about experience, not just benchmarks.
Originally published on Ground Truth, where every claim is checked against the primary source.
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