LanguageTool module

Grammar and style checking in the Overleaf source editor, powered by an external LanguageTool server (API v2).

Based on the upstream PR: https://github.com/yu-i-i/overleaf-cep/pull/179 (commit 8337e00), adapted for ext-ce-v6.3.1 with:

Picky level as the default and an on/off "Picky mode" toggle in the project settings (the deployed LT fork, erikvl87/languagetool, supports default + picky; other levels return HTTP 400).
Project-wide settings (stored on the project document, shared by all collaborators, synced over the editor socket):
- languageToolEnabled — grammar check on/off (default: on)
- languageToolPicky — picky level on/off (default: on → level=picky)
- languageToolLanguage — LT language code, e.g. en-US, de (default: auto)
Dark/light theme support: tooltip + underlines use the Overleaf themed CSS variables (--bg-primary-themed, --content-primary-themed, ...) with light-mode fallbacks (fixes the dark-theme tooltip bug from the PR).

Server (Overleaf) environment

Variable	Default	Notes
`LANGUAGE_TOOL_ENABLED`	unset	must be exactly `true` to activate
`LANGUAGE_TOOL_HOST`	`languagetool`	host of the LT service
`LANGUAGE_TOOL_PORT`	`8010`	LT API port
`LANGUAGE_TOOL_URL`	derived	overrides host+port when set

The overleaf container needs network access to the LT service. In this deployment the overleaf service joins the external languagetools_server_compose_default network (see /data_1/docker/compose/overleaf/compose.yaml) and talks to http://languagetool:8010.

Endpoints (authenticated)

GET /languagetool/languages — LT language list (used by the settings UI)
POST /languagetool/check — JSON body { language, level?, text? | data? }; proxied to LT /v2/check with a 100 KB cap and LaTeX-noise disabledRules. Exempt from CSRF (webRouter.csrf.disableDefaultCsrfProtection in the router): it is a stateless, login-gated query — and long-lived editor tabs outlive session rotations, where the page's meta token would stale out and 403 every check. The client still sends X-CSRF-Token; it passes when the session is fresh.

Frontend

frontend/js/languagetool-extension.ts — CodeMirror 6 extension (wired via the sourceEditorExtensions slot in services/web/config/settings.defaults.js). Wavy underlines + hover tooltip with replacement buttons; LaTeX-aware annotation via the Lezer syntax tree (regex fallback); debounced 2 s; aborts stale requests; filters TYPOS while the Hunspell spellchecker is active.
frontend/js/languagetool-section.tsx + frontend/js/components/languagetool-settings.tsx — Settings modal, Spelling and Language → LanguageTool (wired via the settingsModalSpellcheckSections slot).

Settings reach the editor through options.languageTool (use-codemirror-scope.ts) and the lt:settings-changed window event.

Files

index.mjs                                  module entry (env-gated router)
app/src/LanguageToolRouter.mjs
app/src/LanguageToolController.mjs
frontend/js/languagetool-extension.ts
frontend/js/languagetool-section.tsx
frontend/js/components/languagetool-settings.tsx
frontend/js/utils/latex-to-annotations.ts

Core Overleaf changes (see git diff):

services/web/config/settings.defaults.js (sourceEditorExtensions, settingsModalSpellcheckSections, moduleImportSequence)
services/web/app/src/infrastructure/ExpressLocals.mjs (exposed setting languageToolAvailable)
services/web/types/exposed-settings.ts
services/web/app/src/Features/Project/ProjectController.mjs (project-settings schema + update)
services/web/app/src/Features/Project/ProjectOptionsHandler.mjs (persist)
services/web/app/src/Features/Editor/EditorController.mjs (socket emit)
services/web/frontend/js/features/ide-settings/... (settings UI context, hooks, socket listener, api.ts)
services/web/frontend/js/features/source-editor/hooks/use-codemirror-scope.ts