Standalone LanguageTool grammar-check server used by the Overleaf integration in services/web/modules/languagetool (Overleaf CE fork repo, see the module's README.md).

Image: erikvl87/languagetool:latest (community server fork of LanguageTool; supports level=default and level=picky — other levels return HTTP 400)
Container name: languagetool (overleaf reaches it by this hostname on the shared network)
Host port: 127.0.0.1:32768 → container 8010 (pinned, stable across restarts)
Compose project name: languagetools_server_compose (directory name)

Layout

tools/languagetools_server_compose/
├── compose.yaml            # top-level: `extends` the service in languagetool/compose.yaml
├── up.sh                   # docker compose up -d
├── down.sh                 # docker compose down
├── up_languagetool.sh      # docker compose up -d languagetool
├── down_languagetool.sh    # docker compose down languagetool
├── logs_languagetool.sh    # docker compose logs -f languagetool
├── autoupdate_languagetool.sh   # pull latest image, recreate container only if changed
├── lt-test.tex             # test document: sentences with verified expected detections
└── languagetool/
    ├── compose.yaml        # actual service definition
    ├── download_ngrams.sh          # main n-gram set  (de en es fr nl)  → ngrams/
    ├── download_ngrams_untested.sh # untested set      (he it ru zh)    → ngrams/untested/
    └── ngrams/             # n-gram data, mounted read-only into the container (gitignored)

Quick start

cd tools/languagetools_server_compose

# 1. Populate n-gram data (one-time, ~45 GB disk; resumable, idempotent)
./languagetool/download_ngrams.sh
./languagetool/download_ngrams_untested.sh   # optional: he/it/ru/zh

# 2. Start
./up.sh

# 3. Verify (container must be healthy)
docker ps --filter name=languagetool
curl -s http://127.0.0.1:32768/v2/languages | head -c 200

N-gram data

LanguageTool loads per-language n-gram models from the directory given in langtool_languageModel (here: /ngrams ← languagetool/ngrams/). Every subdirectory named after a language code (e.g. en/, de/) is picked up — currently present: de en es fr nl (main) and he it ru zh (untested).

Set	Languages	Size (extracted)	Zip sources
main	de en es fr nl	~24 GB	`ngrams-<lang>-YYYYMMDD.zip` (~1.7–9 GB each)
untested	he it ru zh	~14 GB	`ngram-<lang>-YYYYMMDD.zip` (~0.2–2.3 GB each)

Downloads come from https://languagetool.org/download/ngram-data/ (untested set: .../ngram-data/untested/).
Both scripts are idempotent: existing zips are skip-validated (unzip -t), downloads are resumable (curl -C -). Re-run safely after an interrupted transfer.
The downloaded zip archives stay in ngrams/ (~21 GB total). Safe to rm ngrams/*.zip once extraction succeeded, to reclaim that space.
an (Aragonese) is not available on the server; it is kept commented out in download_ngrams_untested.sh in case it appears later.

Pitfall (cost us a debugging session): if ngrams/ does not exist on the host, Docker creates an empty directory for the bind mount and the server runs without any n-gram models — spelling detection (recieve, realy, …) silently degrades while the API still answers. Verify with ls languagetool/ngrams/en and a check request that MORFOLOGIK_RULE_EN_US still fires.

Service configuration (`languagetool/compose.yaml`)

Setting	Value	Notes
`image`	`erikvl87/languagetool:latest`	community fork, HTTP REST API on 8010
`Java_Xms` / `Java_Xmx`	`512m` / `2g`	JVM heap
`langtool_languageModel`	`/ngrams`	n-gram model directory (read-only bind mount)
`ports`	`127.0.0.1:32768:8010`	host port pinned — do not reintroduce the ephemeral `127.0.0.1::8010` form
`healthcheck`	`wget --spider http://localhost:8010/v2/languages`	30s interval, 60s start period
`restart`	`unless-stopped`

Connecting Overleaf

The matching Overleaf side (CE fork v6.3.1) exposes the feature through services/web/modules/languagetool/ — enabled via overleaf service env vars. In this deployment they live in /data_1/docker/compose/overleaf/compose.yaml:

env:
  LANGUAGE_TOOL_ENABLED: "true"
  LANGUAGE_TOOL_HOST: languagetool   # resolves on the shared docker network
  LANGUAGE_TOOL_PORT: "8010"

The overleaf container reaches LT through the shared network (languagetools_server_compose_default); from inside overleaf the host is languagetool:8010, from the host it is 127.0.0.1:32768.
Overleaf checks /v2/check (POST, form-encoded text, language, level, plus disabledRules); client-side defaults: picky level on, spellcheck language = Overleaf default (en-US), and the checks are stored as project settings (shared with collaborators).
If LANGUAGE_TOOL_ENABLED is false or the proxy can't reach LT, Overleaf simply does not offer the feature (no client errors).

Verification

# basic
curl -s -X POST http://127.0.0.1:32768/v2/check \
  -d 'language=en-US' -d 'text=You can recieve a bonus.' -d 'level=picky'

# picky vs default (passive voice is picky-only on this build)
curl -s -X POST http://127.0.0.1:32768/v2/check \
  --data-urlencode 'language=en-US' --data-urlencode 'text=The error was detected by the test suite.' \
  -d 'level=default'   # → no passive-voice match
curl -s -X POST http://127.0.0.1:32768/v2/check \
  --data-urlencode 'language=en-US' --data-urlencode 'text=The error was detected by the test suite.' \
  -d 'level=picky'     # → PASSIVE_VOICE_SIMPLE

Editor test document: lt-test.tex in this directory. Verified expectations (2026-10-26, with the exact disabledRules the proxy sends):

Picky mode	Underlines
on (Overleaf default)	27 — section A (2 passive-voice) + section B (23)
off	25 — section A disappears, section B stays

See the file header for the per-sentence rule annotations.

Update / rollback

./autoupdate_languagetool.sh   # pull erikvl87/languagetool:latest, recreate only if image changed
docker compose down && docker compose up -d        # manual restart
docker compose logs -f languagetool                # tail logs

Troubleshooting:

Overleaf can't reach LT — from the overleaf container: wget -O- http://languagetool:8010/v2/languages. If DNS fails, the overleaf service is not attached to the languagetools_server_compose_default network (see the two networks blocks above; both must carry the same name).
Port changed after a recreate — always use the pinned form "127.0.0.1:32768:8010"; the old "127.0.0.1::8010" form picks a random host port per container creation.
No spelling detections — n-gram mount empty, see the pitfall above.

README.md Unescape Escape

LanguageTool server (Docker Compose)