LanguageTool

Promptfree, for everyone, and the only version there is

Open the prompt in Claude CodeOpen the prompt in CodexOpen the prompt in Cursor

Copy

Run the server yourself. It is the same engine.

WHAT YOU GET
- The open-source server carries the community rules; the premium subscription adds more
- For most writing the open rules are enough, and nothing leaves your machine

RUNNING IT
- One Docker container exposing the HTTP API on your own network
- Give it enough memory: the language models are large and it will fail quietly without it

USING IT
- The official browser add-on can be pointed at your own server in its settings
- A CI step that posts changed Markdown to the API and fails on errors

YOUR OWN RULES
- Add a custom rule file for the words your project always gets wrong
- Keep it in the repository so the whole team benefits from one correction

PRIVACY, WHICH IS THE REAL REASON
- Nothing is sent to a third party. For anything unpublished, that alone justifies the setup

Include the compose file, the add-on setting and the CI step.