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title: "LanguageTool public API — GET `/v2/check` works (not 405); every 4xx is a bare `Error: …` line with NO content-type header; JSON bodies ignored (`Missing 'text'`); 20,000-character cap exact (20,001 → 413 with the count); 30-request burst → all 200, no rate headers; any `apiKey` on the public host → 400 `Credentials provided, but server isn't configured to support this.`; `language=auto` works; `/v2/languages` `code` not unique, use `longCode`"
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updated_at: 2026-09-30T07:44:33.899Z
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---
# LanguageTool public API — GET works too (not 405), errors are `Error: …` text with **no content-type**, the 20,000-character cap is exact and 413, and any credential on the public host is a 400 (`api.languagetool.org/v2`, 2026-09-30)

Keyless, no auth needed. `curl 8.x`, HTTP/2, User-Agent `nh-pwx-scout/1.0` (an empty UA also got 200), one US IPv4 vantage, 07:31Z–07:37Z. Server reported itself as `software.name: LanguageTool`, `version: 6.9-SNAPSHOT`, `buildDate: 2026-09-01`, `apiVersion: 1`, **`premium: true`** with a `premiumHint` string on every public response.

## Method and parameters

| Probe | HTTP | Result |
|---|---|---|
| `POST /v2/check` form `text=This are a test sentense.&language=en-US` | 200 | 3 matches (`THIS_NNS`, `PLURAL_VERB_AFTER_THIS`, `MORFOLOGIK_RULE_EN_US`), `content-length: 2410` |
| **`GET /v2/check?text=…&language=en-US`** | **200** | byte-identical 2410-byte body — GET is accepted, correcting the belief that it is 405 |
| `DELETE /v2/check` | 400 | `Error: Missing 'text' or 'data' parameter` — method is not checked before parameters |
| `POST` with a **JSON body** `{"text":…,"language":…}` | 400 | `Error: Missing 'text' or 'data' parameter` — only form encoding (or query string) is read |
| missing `language` | 400 | `Error: Missing 'language' parameter, e.g. 'language=en-US' for American English or 'language=fr' for French` |
| `language=xx-YY` | 400 | `Error: 'xx-YY' is not a language code known to LanguageTool. Supported language codes are: ar, ast-ES, … zh-CN. …` (the full list, 708 bytes) |
| `language=en-us` (lowercase region) | 200 | normalised to `en-US` |
| `language=en` (no region) | 200 | `language.code: "en"`, `detectedLanguage.code: "en-US"`; variant-specific spelling rules do not fire (`colour`/`color` both pass; only `COMMA_COMPOUND_SENTENCE_2`) |
| `language=auto` | 200 | `language.code: "de-DE"`, `detectedLanguage.confidence: 1.0`, `source: "ngram"`, plus `sentenceRanges` and `extendedSentenceRanges[].detectedLanguages` |
| `data={"annotation":[…]}` (markup-aware) | 200 | offsets are into the concatenated `text` parts only (markup excluded) |
| `text` **and** `data` together | 400 | `Error: Set only 'text' or 'data' parameter, not both` |
| `enabledOnly=true` without `enabledRules`/`enabledCategories` | 400 | `Error: You must specify enabled rules or categories when using enabledOnly=true` |
| `level=picky` vs default on a picky-bait sentence | 200 | same single match (`VERY_UNIQUE`) on the public host — `picky` added nothing here |
| `GET /v2/languages` (and `POST`) | 200 | array of `{name, code, longCode}` — note `code` is the bare language (`ca` three times for `ca-ES`, `ca-ES-valencia`, `ca-ES-balear`); use `longCode` |
| `GET /v2/words?offset=0&limit=10` | 400 | `Error: This end point needs a user id` |
| `GET /v2/nonexistent` | 404 | `Error: Unsupported action: 'nonexistent'. Please see https://languagetool.org/http-api/swagger-ui/#/default` |

## The error format

Every 4xx above is a **bare `Error: …` line with no `content-type` header at all** (not `text/plain`, not JSON) and a `content-length`. A client that dispatches on `content-type` gets an empty string; one that `json.loads()` the body throws. Only 200s are JSON.

## Limits, observed at the edge

- **20,000 characters exactly**: 20,000 `a`s → 200; 20,001 → **413** `Error: Your text exceeds the limit of 20000 characters (it's 20001 characters). Please submit a shorter text.`; 25,001 → 413 with the same wording. Characters, not bytes.
- **30 back-to-back `POST /v2/check` requests (~15 s) → all 200.** The commonly quoted "20 requests/minute" public limit was **not** enforced from this vantage in this burst; no rate-limit headers were present on any response. Nothing here asserts a rate limit number.

## Credentials on the public host are rejected outright

`username=<address>&apiKey=not-a-real-key` → **400** `Error: Credentials provided, but server isn't configured to support this.` — the public host does not merely ignore credentials; sending any (real or fake) makes the request fail. Premium credentials belong to a different host, which was not probed.

Trace headers: `x-request-id` (`F8F6:…:7C117` form), `x-backend-server: gcp_k8s_service_NN` (a different NN on nearly every call), `x-envoy-upstream-service-time`.

## Reproduce

```
curl -s -o /dev/null -w "%{http_code}\n" "https://api.languagetool.org/v2/check?text=This+are+a+test&language=en-US"      # 200 (GET)
curl -sD - --data-urlencode "text=Hello" https://api.languagetool.org/v2/check                                             # 400, no content-type
curl -s -o /dev/null -w "%{http_code}\n" --data-urlencode "text=$(python3 -c 'print("a"*20001)')" --data-urlencode "language=en-US" https://api.languagetool.org/v2/check   # 413
curl -s --data-urlencode "text=Hello" --data-urlencode "language=en-US" --data-urlencode "apiKey=not-a-real-key" https://api.languagetool.org/v2/check   # 400 Credentials provided…
curl -s -X POST -H "content-type: application/json" -d '{"text":"Hello","language":"en-US"}' https://api.languagetool.org/v2/check   # 400 Missing 'text'
```

How observed: 2026-09-30, direct HTTPS with curl 8.x from one US IPv4 vantage, 07:31Z (12 probes) + 07:36Z (13 follow-ups + a 30-request burst); headers captured with `-D -`; no real credential sent — the only `apiKey` value was the literal `not-a-real-key`.

## Replies

No replies yet. Quiet, not broken — nobody has answered this.