Think with AI beyond the chat box.

PenEcho is a shared canvas where handwriting, equations, diagrams, and spatial context become part of the conversation.

Think on the canvas

Put a question, equation, diagram, or half-formed idea anywhere on the canvas and pause. PenEcho reads your marks and their spatial relationships, then answers beside them. You can work through a problem without translating every step into a chat message or rebuilding it with rigid diagram tools.

• Get answers, hints, explanations, continuations, formulas, plots, and diagrams directly on the canvas.
• Move, resize, accept, or discard every AI draft before it becomes part of your work.
• Draw naturally with a stylus or mouse, then pan and zoom across a sparse 20,000 x 20,000 canvas.
• Choose Arcane, Sci-fi, or Research mode to match the kind of problem you are exploring.
• Save lightweight snapshots locally in your browser. Starting a new canvas can overwrite the current snapshot, save a new copy, or continue without saving; unconfirmed AI drafts are never included.

PenEcho has no npm runtime dependencies. It only allocates 512 x 512 tiles where ink exists, so the huge logical canvas does not become a huge bitmap.

How it works

flowchart LR
  User["Handwriting, equations, and sketches"] --> Canvas["Browser canvas<br/>sparse confirmed tiles"]
  Canvas --> Atlas["Cropped visual atlas<br/>plus geometry"]
  Atlas --> Server["PenEcho server<br/>validation and prompt"]
  Server --> Executor{"Configured executor"}
  Executor --> API["API mode<br/>OpenAI-compatible or Anthropic"]
  Executor --> Codex["Codex CLI mode<br/>local codex exec"]
  API --> Draft["Structured editable draft"]
  Codex --> Draft
  Draft --> Canvas

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The browser sends only the relevant canvas crop and geometry. The server validates the request, uses the selected executor, and returns a movable draft that stays separate from confirmed ink until you accept it.

Quick start

You need Node.js 18+ and either an API key or an installed Codex CLI.

Option 1: API

cp env.api.example .env
# Add your API key, endpoint, and model to .env
npm start

API mode supports OpenAI-compatible Chat Completions and Anthropic Messages endpoints. Credentials stay on the server and are never sent to browser code.

Option 2: Codex on your machine

codex login
cp env.codex.example .env
npm start

This runs codex exec locally for each canvas request. It uses your authenticated Codex CLI directly and does not require an API key. It is a local execution path through Codex, not a local model.

Open http://localhost:3888. Other devices on the same trusted LAN can use http://<this-computer-LAN-IP>:3888.

Token use and cost

The following is an illustrative estimate, not an enforced PenEcho token budget. Assuming a request uses 10,000 input tokens and 1,000 output tokens, the standard short-context API cost would be:

• gpt-5.6-sol: 10,000 x $5.00 / 1M + 1,000 x $30.00 / 1M = $0.080
• gpt-5.6-terra: 10,000 x $2.50 / 1M + 1,000 x $15.00 / 1M = $0.040
• gpt-5.6-luna: 10,000 x $1.00 / 1M + 1,000 x $6.00 / 1M = $0.016

At those example quantities, that is about 1.6 to 8 cents per request. Actual input, reasoning, and output usage varies by canvas content, model, provider, and retry behavior. Prices can change, so check the OpenAI API pricing page for current rates.

If you sign in to Codex with ChatGPT, PenEcho uses the Codex usage included with your plan instead of an API key. Included limits vary by plan, and additional usage may require ChatGPT credits. See Codex pricing for current plans and limits.

Help test more models

PenEcho has been tested most heavily with GPT-5.6 Sol, Terra, and Luna. The Anthropic Messages adapter supports Claude endpoints, but Claude coverage is still limited. If you use Claude, please open an issue with the model name, a reproducible canvas example, expected and actual results, and a screenshot with secrets removed. Test notes and adapter fixes are both welcome.

Safe deployment

PenEcho listens on 0.0.0.0:3888 by default so localhost and trusted-LAN access work immediately. Choose the deployment boundary that matches your executor:

• Codex CLI mode: use it only on the local machine or a trusted, directly connected LAN. A valid request starts a local codex exec process, so do not expose this mode directly to the public internet or an untrusted reverse proxy. PenEcho checks the Host, client network, exact Origin, process-lifetime session cookie, JSON content type, and concurrency before launching Codex, but these checks are not a substitute for an isolated operating-system account, VM, or container in higher-risk environments.
• API mode: local, LAN, proxy, and remote requests are intentionally accepted without PenEcho-level Host or Origin restrictions. If you expose it publicly, place it behind HTTPS, authentication, rate limiting, and request-size controls. Keep .env and provider keys private; credentials remain in the Node.js process and are never sent to browser code.

For either mode, keep debug artifacts disabled in production unless you are actively diagnosing a problem, and never publish .env, logs, screenshots, or saved requests containing private content.

Useful configuration

The example files are ready to run. These settings cover most custom setups:

Setting	Purpose
OPENAI_API_URL	OpenAI-compatible or Anthropic base URL
OPENAI_MODEL	Model used in API mode
CODEX_CLI_MODEL	Optional model override for Codex CLI mode
AUTO_AI_DELAY_SECONDS	Initial delay before automatic recognition; the browser control can override it from 0 to 10 seconds
HOST / PORT	Server address and port, default 0.0.0.0:3888

Run the checks before submitting a change:

npm run check

For implementation details, see the architecture notes.

Build it with us

PenEcho is still young, with real work left in recognition, visual tools, model support, and pen interaction. Open an issue, propose an idea, or send a pull request. If PenEcho clicks for you, star the repo, share the demo, and help us make it better.

Read CONTRIBUTING.md to get started.

License and commercial use

PenEcho is open source under GNU AGPL v3.0 only. Commercial use is allowed under the AGPL. If you modify PenEcho and provide that version to users over a network, you must offer those users the corresponding source code as required by the license.

An alternative commercial license is available for proprietary products and hosted services that cannot meet the AGPL requirements. The PenEcho name and logo are governed separately by the PenEcho trademark policy.

Contributors keep ownership of their work and grant the project the rights needed to offer both AGPL and commercial editions. See the contributor agreement.