When your whiteboard scribbles become the AI prompt

What it does

PenEcho is a browser-based shared canvas roughly the size of a small city—20,000 by 20,000 pixels. You scribble equations, diagrams, or half-formed ideas with a stylus or mouse, lasso ink to move or resize it, and pause. The app captures the visual layout, ships it to a configured LLM via Claude Code, OpenAI Codex CLI, or a standard API endpoint, and drops the model’s reply back onto the board as draggable objects you can accept or discard.

The interesting bit

Instead of extracting text and throwing away the spatial relationships, PenEcho sends the actual canvas image to the model, letting the LLM infer meaning from where things sit. It is essentially a very patient interpreter between your chicken-scratch and local CLI agents, complete with per-request reasoning-effort dials and background process cleanup.

Key highlights

• Handwriting, freehand lasso selection, and pan-and-zoom across an infinite canvas.
• Direct integration with Claude CLI, OpenAI Codex CLI, and generic OpenAI-compatible or Anthropic APIs; Kimi K3 is explicitly recommended for demanding canvas work.
• Per-request reasoning levels (none through max/xhigh) exposed in a fixed-width toolbar menu, overriding the saved config without rewriting files.
• AGPL v3 license; backed by Moonshot AI’s Kimi Open Source Friends program.
• API credentials stay in a local plaintext config.env with owner-only POSIX permissions; the browser never touches the key.

Caveats

• The README notes that less capable models can return weaker canvas results, and some provider endpoints may reject effort levels the selected model does not support.
• It relies on external CLIs being separately installed and authenticated, and lingering CLI processes are terminated in the background if they fail to exit cleanly.
• API keys live in a local plaintext file, so credential protection is entirely your responsibility.

Verdict

Grab it if you reason with ink—mathematicians, architects, or anyone who sketches before they type. Pass if you want a self-contained whiteboard with its own brain; this is a frontend that leans heavily on whatever CLI or API you wire up.