OpenAPITools/openapi-generator · error · RuntimeException

Empty method name (operationId) not allowed

Error message

Empty method name (operationId) not allowed

What it means

The scala-lagom-server-deprecated generator overrides toOperationId() and rejects an empty or null operationId with RuntimeException. openapi-generator normally derives an operationId from method+path when the spec omits one, so this is hit when the spec supplies an explicitly empty operationId (or tooling passes one through as empty).

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaLagomServerCodegen.java:174

    public CodegenType getTag() {
        return CodegenType.SERVER;
    }

    @Override
    public String getName() {
        return "scala-lagom-server-deprecated";
    }

    @Override
    public String getHelp() {
        return "Generates a Lagom API server (Beta) in scala. IMPORTANT: this generator has been deprecated";
    }

    @Override
    public String toOperationId(String operationId) {
        // throw exception if method name is empty
        if (StringUtils.isEmpty(operationId)) {
            throw new RuntimeException("Empty method name (operationId) not allowed");
        }

        // method name cannot use reserved keyword, e.g. return
        if (isReservedWord(operationId)) {
            throw new RuntimeException(operationId + " (reserved word) cannot be used as method name");
        }

        return camelize(operationId, LOWERCASE_FIRST_LETTER);
    }

    @Override
    public ModelsMap postProcessModelsEnum(ModelsMap objs) {
        objs = super.postProcessModelsEnum(objs);
        for (ModelMap mo : objs.getModels()) {
            CodegenModel cm = mo.getModel();

            for (CodegenProperty var : cm.vars) {
                if (var.isEnum) {

View on GitHub (pinned to fcec517be3)

Solutions

  1. Give every operation a non-empty operationId in the spec
  2. Remove the empty operationId field entirely so the generator derives one from method+path
  3. Prefer a spec linter (e.g. Spectral rule operation-operationId) in CI to catch empty operationIds before generation
  4. Migrate off the deprecated Lagom generator if possible

Example fix

# before
paths:
  /pets:
    get:
      operationId: ''
# after
paths:
  /pets:
    get:
      operationId: listPets
Defensive patterns

Strategy: validation

Validate before calling

# Lint every operation for a non-empty operationId (Python):
for path, item in spec['paths'].items():
    for method, op in item.items():
        if method in {'get','put','post','delete','options','head','patch','trace'}:
            oid = op.get('operationId')
            assert oid and oid.strip(), f'{method.upper()} {path} has an empty operationId'

Try / catch

try {
    new DefaultGenerator().opts(input).generate();
} catch (RuntimeException e) {
    // 'Empty method name (operationId) not allowed' - fill in or delete the empty operationId key, then rerun
}

Prevention

When it happens

Trigger: -g scala-lagom-server-deprecated against a spec containing operationId: '' (or a template/null that serializes to empty) on an operation. The check runs per operation during model/operation processing, before files are written.

Common situations: Specs post-processed by scripts that blank out operationIds for dedup; YAML where operationId is present but empty; merging specs where the field was dropped. The generator itself is deprecated, so this mostly appears in legacy pipelines.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/fe04de41fb0569bd. Report an issue: GitHub.