ProofKey
ProofKey is an MIT Grammarly-style Chrome extension that underlines problems and runs rewrites against your choice of model. On Google Chrome it uses Chrome's built-in Gemini Nano, which runs on the computer with no key and no account. In other Chromium browsers it needs a provider you pick: Gemini, OpenRouter, xAI, llama.cpp, OpenCode Go, or any OpenAI-compatible endpoint, with the key stored in the browser.
What does ProofKey do?
ProofKey's README calls it "a Grammarly-style writing assistant for Chrome that talks to your LLM, using your API key, or, in Google Chrome, to the model Chrome already keeps on your computer, with no key at all." It underlines problems as you type and offers an inline card with suggestions and quick actions. Live checking is quiet, keeping messaging conventions, while a deliberate action from the right-click menu or Ctrl+Shift+K completes the correction.
Quick actions include fix grammar, improve writing, make professional, make friendly, simplify, summarize, expand, convert to bullet points and translate. Each is a prompt you can rewrite, and you can add your own actions and set a shortcut for any of them. The README documents verified sites and providers in detail, with a cutoff date on each row, so you can see what has actually been run against a real key and what has not.
Key facts
- Licence: MIT, per the README's own badge.
- First-class browser: Google Chrome. Chromium builds like Brave and Edge can run ProofKey, with the limits the README names.
- No backend. The README says text goes from the browser to the provider you picked and nowhere else.
- Chrome's built-in Gemini Nano is selected on a fresh install in Google Chrome; the model is one 4 GB download, measured in the README on a test machine.
- Providers verified against real keys: Google Gemini, xAI (Grok), OpenRouter, llama.cpp (self-hosted), OpenCode Go, and Chrome's built-in model.
- Twenty-nine other providers are prefilled but not confirmed; the README calls these "prefilled, not confirmed."
- WebLLM option: ProofKey can run Qwen3.5 4B inside the browser on your GPU through WebGPU; one 2.4 GB model download, started by its own button.
- A "Find a model on this computer" button scans the usual ports (1234, 11434, 8080) for LM Studio, Ollama or llama.cpp, only when clicked.
- Compatibility measurements, with dates: Gmail, Telegram Web, Outlook, Tuta, X and Reddit verified between 2026-08-02 and 2026-09-02; iCloud Mail and Infomaniak Mail verified 2026-09-14.
What does it replace, and where does it fall short?
ProofKey replaces Grammarly for someone who writes in a browser, is willing to pick a provider or use Chrome's own model, and does not want their text going to Grammarly's servers. It is a candidate on the open-source alternatives to Grammarly page.
Where it falls short: Google Docs is listed as "not supported", because its editor paints text to a canvas and ProofKey has nothing to underline. Chrome's built-in model is "slower than a cloud model" in the README's own numbers (an 8-sentence live check in 8.6 to 8.8 seconds; the eval's larger request in about one second with a cloud model). The built-in model does five languages officially (English, Spanish, German, French and Japanese); others are untested. For a desktop-wide checker not confined to a browser, Nogrammerly or Usami.
How does ProofKey run?
In Google Chrome, install from the Chrome Web Store, click Download model on the Chrome built-in AI card, and reload tabs. The 4 GB Gemini Nano model installs once from Google, then every check runs on the computer. In other browsers, open Settings and set up a provider: paste a key for Gemini, OpenRouter or xAI, run llama.cpp on your own machine, or wire in any OpenAI-compatible endpoint through the Custom option.
Installing from source is in the README: clone the repository, run npm install and npm run build, then load the dist/ folder in chrome://extensions with developer mode on.
Who is ProofKey for?
A browser user who wants quick fixes and tone changes while keeping full control of where the writing goes: on-device in Chrome, in-browser through WebGPU, on a local LM Studio or Ollama, or on a cloud API with their own key. A reader on another browser or a desktop writer should look elsewhere.
What limits does the README state?
- "Google Docs: Not supported, text is painted to a canvas, so there is nothing to underline."
- Chrome's built-in model is "slower than a cloud model" on the README's own measurement.
- Chrome's built-in model is on Google Chrome 153 on a capable desktop; Brave switches the feature off, Edge on Linux does not have it, and Microsoft is still in a developer preview on Windows and macOS Canary.
- Live checking and Fix grammar are the only two actions enabled on Chrome's built-in model.
- Thirty-one other provider presets are "prefilled, not confirmed."
Questions people ask
Is ProofKey open source?
Yes. The README's badge is MIT, verified from the LICENSE file. MIT is an OSI-approved licence with no added conditions, so you can read, change and ship your own extension.
Does ProofKey send my writing to the cloud?
Only if you pick a cloud provider. On Google Chrome with the built-in Gemini Nano, the check runs on the computer. The WebLLM path runs Qwen3.5 4B in your browser on your GPU. Local llama.cpp, LM Studio and Ollama keep the writing on the machine. Any cloud provider you pick sees only the request you make.
Does ProofKey work in Firefox or Safari?
The README does not say. The project is Chrome-first and tested in Google Chrome 153, with notes about Brave and Edge. A Firefox or Safari build is not promised.
How does ProofKey compare with Grammarly?
Grammarly is a hosted writing service with its own model, sold on a subscription. ProofKey is a free MIT extension that keeps the same interaction, but routes to the provider you pick (including on-device Chrome Nano or a local LM Studio). The open-source alternatives to Grammarly page compares ProofKey with other candidates.
Which sites has ProofKey been tested on?
Per the README's compatibility table, verified between 2026-08-02 and 2026-09-02: Gmail, Telegram Web, Outlook and Hotmail, Tuta, X / Twitter and Reddit. Verified 2026-09-10: WhatsApp Web. Verified 2026-09-14: Infomaniak Mail and iCloud Mail. Google Docs is "not supported". Slack, Notion, Discord, LinkedIn and GitHub are listed as untested on the same code path as a verified site.
Sources
- ProofKey README and MIT LICENSE: github.com/jiru-labs/proofkey, read 2026-10-19.
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Added October 19, 2026. Every claim above comes from the project's README, LICENSE or model card, read on October 19, 2026, or from the GitHub API on the date shown in the panel. Found an error? Write to reveneau@licheo.com and it is fixed in the next weekly pass. Repository: github.com/jiru-labs/proofkey.
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