stanfordnlp/CoreNLP · error · IllegalArgumentException

Cannot infer requirements for annotator

Error message

Cannot infer requirements for annotator: ${annotator}

What it means

After confirming an annotator exists, ensurePrerequisiteAnnotators looks it up in Annotator.DEFAULT_REQUIREMENTS to compute its prerequisite chain. If the annotator has no entry there, its requirements cannot be inferred and an IllegalArgumentException is thrown.

Solutions

1. Replace the custom annotator with a built-in one that has requirement metadata.
2. Implement the Annotator.Requirements interface / provide a Requirements implementation so requirements can be inferred.
3. Upgrade/downgrade CoreNLP so the annotator has a DEFAULT_REQUIREMENTS entry.
4. Order the annotators manually in the 'annotators' property and avoid the path that infers requirements, if API allows.

Example fix

// before
props.setProperty("annotators", "tokenize,ssplit,mycustom"); // mycustom has no requirements
// after
props.setProperty("customAnnotatorClass.mycustom", "com.example.MyAnnotator");
props.setProperty("annotators", "tokenize,ssplit"); // handle custom annotator separately

for (String a : annotators) {
  if (!Annotator.DEFAULT_REQUIREMENTS.containsKey(a.toLowerCase()) &&
      !isBuiltinAnnotator(a)) {
    throw new IllegalArgumentException("Annotator lacks requirement metadata: " + a);
  }
}

try { StanfordCoreNLP.ensurePrerequisiteAnnotators(props, annotators, true); } catch (IllegalArgumentException e) { /* fall back to manually ordered annotator list */ }

When it happens

Trigger: Requesting a registered annotator (often a custom one registered via customAnnotatorClass, or one lacking a DEFAULT_REQUIREMENTS entry) in ensurePrerequisiteAnnotators, e.g. via ensurePrerequisiteAnnotators call on a pipeline requiring dependency ordering.

Common situations: Custom annotators plugged in through customAnnotatorClass that were never added to DEFAULT_REQUIREMENTS; CoreNLP version skew where an annotator exists but requirements metadata was added later.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Unknown annotator
• Unsatisfiable annotator list
• Unknown LogPriorType:
• unsupported language
• no sieve type specified

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/a073882356cb1413. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/pipeline/StanfordCoreNLP.java:498

   */
  public static String ensurePrerequisiteAnnotators(String[] annotators, Properties props) {
    int posIndex = ArrayUtils.indexOf(annotators, Annotator.STANFORD_POS);
    int parseIndex = ArrayUtils.indexOf(annotators, Annotator.STANFORD_PARSE);
    boolean useParseForPos = ((parseIndex >= 0) && (posIndex < 0)); // Already doing the parsing, use the parsing for the pos tag

    // Get an unordered set of annotators
    Set<String> unorderedAnnotators = new LinkedHashSet<>();  // linked to preserve order
    Collections.addAll(unorderedAnnotators, annotators);
    for (String annotator : annotators) {
      // Add the annotator
      if (!getNamedAnnotators().containsKey(annotator.toLowerCase())) {
        throw new IllegalArgumentException("Unknown annotator: " + annotator);
      }

      // Add its transitive dependencies
      unorderedAnnotators.add(annotator.toLowerCase());
      if (!Annotator.DEFAULT_REQUIREMENTS.containsKey(annotator.toLowerCase())) {
        throw new IllegalArgumentException("Cannot infer requirements for annotator: " + annotator);
      }
      Queue<String> fringe = new LinkedList<>(Annotator.DEFAULT_REQUIREMENTS.get(annotator.toLowerCase()));
      int ticks = 0;
      while (!fringe.isEmpty()) {
        ticks += 1;
        if (ticks == 1000000) {
          throw new IllegalStateException("[INTERNAL ERROR] Annotators have a circular dependency.");
        }
        String prereq = fringe.poll();
        unorderedAnnotators.add(prereq);
        fringe.addAll(Annotator.DEFAULT_REQUIREMENTS.get(prereq.toLowerCase()));
      }
    }

    if (useParseForPos) {
      unorderedAnnotators.remove(Annotator.STANFORD_POS);
    }

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