languagetool-org/languagetool · error · BadRequestException

'lang' parameter missing

Error message

'lang' parameter missing

What it means

The GET /v2/ruleExamples endpoint needs the language short code (e.g. en-US, de-DE) to build the JLanguageTool instance for which rule examples are requested. If the 'lang' query parameter is absent, handleRuleExamplesRequest throws a BadRequestException before any further validation.

Source

Thrown at languagetool-server/src/main/java/org/languagetool/server/ApiV2.java:267

      limits.getAccount().getDefaultDictionary() != null &&
      !limits.getAccount().getDefaultDictionary().isEmpty()) {
      dict = limits.getAccount().getDefaultDictionary();
    }
    boolean deleted;
    if("batch".equals(parameters.get("mode"))) { //Experimental
      List<String> words = Arrays.asList(parameters.get("words").split("\\s+"));
      deleted = db.deleteWordBatch(words, limits.getPremiumUid(),dict);
      writeResponse("deleted", deleted, httpExchange);
    } else {
      deleted = db.deleteWord(parameters.get("word"), limits.getPremiumUid(), dict);
      writeResponse("deleted", deleted, httpExchange);
    }
  }

  private void handleRuleExamplesRequest(HttpExchange httpExchange, Map<String, String> params) throws Exception {
    ensureGetMethod(httpExchange, "/rule/examples");
    if (params.get("lang") == null) {
      throw new BadRequestException("'lang' parameter missing");
    }
    if (params.get("ruleId") == null) {
      throw new BadRequestException("'ruleId' parameter missing");
    }
    Language lang = Languages.getLanguageForShortCode(params.get("lang"));
    JLanguageTool lt = new JLanguageTool(lang);
    if (textChecker.config.languageModelDir != null) {
      lt.activateLanguageModelRules(textChecker.config.languageModelDir);
    }
    List<Rule> rules = lt.getAllRules();
    List<Rule> foundRules = new ArrayList<>();
    for (Rule rule : rules) {
      if (rule.getId().equals(params.get("ruleId"))) {
        foundRules.add(rule);
      }
    }
    if (foundRules.isEmpty()) {
      throw new PathNotFoundException("Rule '" + params.get("ruleId") + "' not found for language " + lang +

View on GitHub (pinned to 2e990059ce)

Solutions

1. Add the 'lang' query parameter with a valid LanguageTool short code, e.g. lang=en-US.
2. Check for typos: the key must be exactly 'lang', not 'language'.
3. Validate the short code with Languages.getLanguageForShortCode client-side equivalents to ensure the language is supported.

Example fix

// before
GET /v2/ruleExamples?ruleId=TOO_LONG_SENTENCE
// after
GET /v2/ruleExamples?lang=en-US&ruleId=TOO_LONG_SENTENCE

if (!params.lang) throw new Error('ruleExamples requires a "lang" short code, e.g. en-US');

When it happens

Trigger: GET /v2/ruleExamples?ruleId=SOME_RULE without '?lang=xx-YY'; parameter omitted by a template or query builder; calling the endpoint with only ruleId.

Common situations: Clients porting from internal APIs where the language was implicit; generic API explorers that drop empty parameters; typo'd parameter name like 'language' instead of 'lang'.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Missing 'text' or 'data' parameter
• 'ruleId' parameter missing
• Use parameter 'dicts', not 'dict' in GET /words API method.
• Rule '<ruleId>' not found for language <lang> (LanguageTool
• Expected Basic Authentication

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/a19b4b5e1b28dd10. Report an issue: GitHub.