OpenAPITools/openapi-generator · error · RuntimeException

Empty method name (operationId) not allowed

Error message

Empty method name (operationId) not allowed

What it means

The Protobuf schema generator's toOperationId() rejects empty method names with a RuntimeException (ProtobufSchemaCodegen.java:325). As its comment states, an auto-generated name is normally supplied when the spec omits operationId, so the throw points to an explicitly empty operationId value (or direct programmatic invocation). Reserved-word operationIds are handled by warning and renaming to call_<name>, not by throwing.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ProtobufSchemaCodegen.java:325

            this.supportMultipleResponses = convertPropertyToBooleanAndWriteBack(SUPPORT_MULTIPLE_RESPONSES);
        } else {
            additionalProperties.put(this.SUPPORT_MULTIPLE_RESPONSES, this.supportMultipleResponses);
        }

        if (additionalProperties.containsKey(EXTRACT_ENUMS_TO_SEPARATE_FILES)) {
            this.extractEnumsToSeparateFiles = convertPropertyToBooleanAndWriteBack(EXTRACT_ENUMS_TO_SEPARATE_FILES);
        } else {
            additionalProperties.put(EXTRACT_ENUMS_TO_SEPARATE_FILES, this.extractEnumsToSeparateFiles);
        }

        supportingFiles.add(new SupportingFile("README.mustache", "", "README.md"));
    }

    @Override
    public String toOperationId(String operationId) {
        // throw exception if method name is empty (should not occur as an auto-generated method name will be used)
        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)) {
            LOGGER.warn("{} (reserved word) cannot be used as method name. Renamed to {}", operationId, camelize(sanitizeName("call_" + operationId)));
            operationId = "call_" + operationId;
        }

        return camelize(sanitizeName(operationId));
    }

    /**
     * Creates an array schema from the provided object schema.
     *
     *  @param objectSchema the schema of the object to be wrapped in an array schema
     *  @return the created array schema
     */
    private Schema createArraySchema(Schema objectSchema) {

View on GitHub (pinned to fcec517be3)

Solutions

1. Fill in a unique non-empty operationId for each operation.
2. Remove the empty `operationId:` field so the generator's automatic path+method naming applies.
3. Add a CI spec-lint step that fails on empty operationId values.

Example fix

# before (api.yaml)
paths:
  /items:
    post:
      operationId: ""
      responses: { '201': { description: created } }
# after
paths:
  /items:
    post:
      operationId: createItem
      responses: { '201': { description: created } }

// protobuf-schema: same guard — empty-string operationIds throw, absent ones are synthesized
openAPI.getPaths().forEach((path, item) -> {
  for (Operation op : item.readOperations().values()) {
    if (op.getOperationId() != null && op.getOperationId().trim().isEmpty()) {
      throw new IllegalArgumentException("Empty operationId on " + path
          + " — set a value or remove the key");
    }
  }
});

try {
    new DefaultGenerator().opts(clientOptInput).generate();
} catch (RuntimeException e) {
    throw new BuildException("Protobuf schema generation failed: " + e.getMessage(), e);
}

When it happens

Trigger: Generating protobuf schema (`-g protobuf-schema`) from a spec containing `operationId: ""` (or whitespace-only) on an operation; or calling ProtobufSchemaCodegen.toOperationId("") directly.

Common situations: Specs produced by converters that emit empty operationId keys; hand-edited files where the field was blanked; template-based spec generation with unfilled operationId placeholders.

Related errors

• Empty method name (operationId) not allowed
• Empty method name (operationId) not allowed
• The BLOB and JSON data types cannot be assigned a default va
• Empty database/table/column name for property '{name}' not a
• The BLOB, TEXT, GEOMETRY, and JSON data types cannot be assi

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/db0508049d00df65. Report an issue: GitHub.