Open-Source Whiteboard Lets Claude Answer Right Next to Handwritten Notes

Civil-Direction-6981 · reddit · 2026-07-17

The author developed an open-source canvas called PenEcho, allowing Claude to provide responses directly alongside handwritten notes and sketches.

Ideal for physics, math, and other scenarios requiring whiteboard brainstorming: users handwrite formulas, draw diagrams, and jot down annotations on the canvas. After a pause, the system sends only the relevant regions to the model, and the reply appears next to the original spot as an editable draft.
This workflow avoids the disconnect of awkwardly copy-pasting half-finished derivations into a chat box. The author emphasizes that it can explain steps, continue lines of thought, and point out errors without switching you to a separate chat window.
Technically, the canvas logically scales up to 20,000×20,000, but only allocates 512×512 tiles for areas with handwriting. Requests send cropped visual atlases and geometric info, meaning daily usage typically consumes only a few thousand input tokens and less than 1,000 output tokens, often costing just a few cents.
It currently supports the Anthropic API, Claude Code CLI login, as well as OpenAI-compatible APIs and Codex CLI. The author claims that in this workflow, Opus 4.8 (max effort) is one of the best they've personally tested at understanding handwriting, unfinished equations, and diagrams.

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