Can a prompt replace LanguageTool?

Do not write a grammar checker — run the real, open-source LanguageTool engine locally (it ships as a self-contained server started with one command). Build a small local page: paste text, see issues underlined with an explanation on hover, click a suggestion to apply it. Add a command-line mode that checks a file and exits non-zero on any issue, so it can run as a git pre-commit hook on documentation. Support a personal dictionary file of names and jargon that should never be flagged. Add an optional LLM rewrite pane (key from the environment) that takes a paragraph plus LanguageTool's own findings and returns a rewritten version shown as a diff against the original — skip this feature entirely, cleanly, when no key is configured; the core grammar checking works with zero API keys and no network connection.

Do not build a browser extension or attempt to reproduce LanguageTool Premium's larger proprietary rule set — the open rule set is what's actually available to self-host, and that's an honest, real gap versus the paid tier, not something to paper over.