PenEcho: An Infinite AI Canvas for Handwriting, Diagrams, and Spatial Thinking

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PenEcho

What it isAn infinite canvas where AI reads your handwriting, diagrams, and spatial context
PlatformmacOS, Linux, Windows (Node.js 18.17+)
PriceFree, open source (AGPL-3.0)
Linkgithub.com/penecho/penecho

Chat interfaces are bad at spatial problems. To describe a diagram you need to build it in some other tool and paste a screenshot. To work through a math problem you type step by step in a linear thread. PenEcho skips all that: put a question, equation, or half-formed idea anywhere on a 20,000 x 20,000 canvas, and the AI reads your handwriting and spatial relationships right where they sit.

  1. Draw naturally with a stylus or mouse. Everything stays ink: you pan and zoom across a sparse canvas that only allocates pixels where you’ve drawn. No page breaks, no fixed aspect ratio. Drag AI drafts directly on the canvas, resize them, copy text or formulas, then accept or discard before they become part of the work.

  2. It connects to whatever you already use: Codex CLI, Claude CLI, or any OpenAI-compatible API. Configuration is a single command (penecho configure) that walks you through model selection and effort level. PenEcho sends the relevant canvas crop and geometry, the executor replies, and a movable draft appears next to your ink.

  3. The plugin system adds live data and interactive widgets. Built-in plugins cover weather, stocks, technology news, exchange rates, earthquakes, space weather, and GitHub activity; data goes directly from your browser to the API origin, never proxied through PenEcho. The HTML plugin lets the model build focused clocks, calculators, or dashboards as sandboxed iframe widgets that stay interactive on the canvas.

  4. Recommended models include Opus 4.8 (medium effort for everyday canvas work), Kimi K3 (strong for demanding diagrams), and GPT-5.6-terra (surprisingly fast and responsive). At typical usage, a request costs roughly 2-8 cents on API mode. CLI mode uses your Codex or Claude Code plan directly without API billing.

  5. The honest caveat: PenEcho is young (v0.7.0). Handwriting recognition is good but not perfect; it works best with natural print or careful cursive. The CLI modes should only be exposed on a local machine or trusted LAN. If you need polished production diagramming with no model cost, Excalidraw is still the draw tool; PenEcho is for when you want the AI to understand what you drew.

Worth your time if: you think in diagrams, equations, or spatial sketches and want an AI that meets you on the canvas instead of forcing everything through a chat box.

Install & first run 

npm install -g penecho
penecho configure
penecho

Open http://localhost:3888. The configure step walks you through your LLM choice (Codex CLI, Claude CLI, or API with any OpenAI-compatible provider). Draw something anywhere on the canvas, pause, and PenEcho responds beside your ink.

# Override model or effort per session
penecho --codex --model gpt-5.6-sol --effort xhigh
penecho --claude --model opus --effort medium

The default config lives at ~/.penecho/config.env. API keys are stored in plaintext there with owner-only permissions and never sent to browser code.

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