ProofKey: AI Grammar Checker & Writing Assistant with Your Own API Key

ProofKey is a writing assistant that keeps the interaction model you already know — inline underlines, a suggestion card, quick rewrite actions — and swaps the engine for an endpoint you control. Your text goes from your browser straight to the provider you picked, using the API key you supplied, and nowhere else. There is no ProofKey server to route it through, because there is no ProofKey server. Point it at a model running on your own machine and nothing leaves it at all. In Google Chrome you do not even need a provider: ProofKey starts on the model Chrome keeps on your computer, with no key and no account. In Brave, which switches that model off, ProofKey can run one inside the browser on your GPU instead. WHAT IT DOES • Quick actions on any selected text, from the right-click menu or Ctrl+Shift+K: fix grammar, improve writing, make professional, make friendly, simplify, summarize, expand, convert to bullet points, translate. • Editable prompts. Every built-in action is a prompt you can rewrite, and you can add your own. • A key per action. Any action, including one you wrote, can be given its own keyboard shortcut. ProofKey suggests one that Chrome, your OS and the page are not already using — and refuses the ones that cannot work. • Live checking that is quieter than the quick actions, on purpose. It fires on a pause you did not ask for, so it treats messaging conventions as valid: no full stop added to a line that lacks one, no capitalising a lowercase sentence start, no expanding slang. Ctrl+Shift+K you pressed deliberately, so that one completes the correction, capitals included. • Any language the model knows. The interface is English; the text does not have to be. The prompts detect the language and work inside it — including text that mixes two languages, which rule-based checkers cannot handle at all. • Translate into the language your profile already names. It reuses the language you set for explanations, or your first language, so there is usually nothing to configure — and when none is set it says which setting is missing instead of guessing one. • It does not flatten your voice. Regional variety (en-GB/en-US, pt-BR/pt-PT, zh-Hans/zh-Hant) and politeness register (tú/usted, du/Sie, tu/vous, Japanese registers) are treated as choices to preserve, not errors to normalise. • A writing profile: house style rules, terms never to flag, and your first language so the model watches for the interference errors typical of its speakers. • 36 provider presets over two transports, plus a free-form Custom option for any OpenAI-compatible endpoint. • Fallback chain. Put a local model first and a cloud key second; ProofKey moves down the list when one fails, and tells you which one failed. GETTING STARTED In Google Chrome, there is nothing to sign up for. Settings opens after you install; click Download model once, and ProofKey checks your writing with Chrome's built-in model, on your computer. That model is smaller than the cloud ones, so ProofKey offers it live checking and Fix grammar, the two it measured well on, and keeps the other actions for a provider with a key. In Brave, the same card offers Run a model inside this browser: ProofKey runs Qwen3.5 4B itself, on your GPU through WebGPU. It is one 2.4 GB download from huggingface.co, started only by its own button, and it needs a GPU with about 4 GB of memory. It is offered live checking, Fix grammar, bullet points and Translate. For every action, you need an API key. A Gemini key from Google AI Studio takes about two minutes, and the free allowance covers ordinary use, so you can try all of ProofKey without paying anything. Two things are worth knowing before you paste a key: • On a free key the provider may use what you send to improve their products, which is not how paid keys are handled. ProofKey never sees your text either way, but "no telemetry" is a promise about this extension, not about the provider you point it at. For anything confidential, use a paid key or a local model, where nothing leaves your machine. • If underlines quietly stop appearing, you have reached a daily limit. Live checking sends a request on every pause, so a cap notices it. Switch live checking off and the shortcut and right-click actions carry on working. This is a statement about cost. We publish no quality scores for free tiers on any provider, so it is not a claim about how well any particular free model performs. PRIVACY • No backend. No account. No telemetry, analytics or crash reporting. • Your API keys stay in your browser's extension storage. • Site access is requested per site, by you. Not up front, and not for all sites. • With Chrome's built-in model, ProofKey makes no request at all: the check runs on your computer. • With the model inside the browser, the one request is the download you start. After that, checks make none. • Otherwise, your text is subject to your chosen provider's privacy policy — providers vary enormously on retention and training, and that is worth reading first. HONEST STATUS This is an early release. What is built is larger than what has been confirmed by hand: • Confirmed by hand on nine real sites: seven work as installed, and two more need one extra origin granted because their editor runs inside a frame. Each one is listed with the evidence for it, and what has not been tried is marked untested rather than assumed. • Confirmed against real API keys: Google Gemini, xAI (Grok), OpenRouter, OpenCode Go. • Confirmed self-hosted: a local llama.cpp server, three models measured end to end. • Confirmed on one computer: Chrome's built-in model, for live checking and Fix grammar. • Confirmed on one computer: the model inside the browser, in Brave on Windows with an 8 GB GPU. It scored the same as Chrome's model on the live check. Other browsers and graphics cards are untested. • Everything else — every other site, and the other 31 provider presets — takes the same code path as something that was…