OpenAPITools/openapi-generator · error · RuntimeException

Empty method name (operationId) not allowed

Error message

Empty method name (operationId) not allowed

What it means

AbstractCSharpCodegen.toOperationId throws when the operation name reaching it is null or empty. C# client/interface method names come from operationId; an empty one would produce uncompilable C#, so generation aborts. The comment in code notes this should not occur under normal flow because an auto-generated name is used, so it signals an empty explicit value or a custom code path.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java:1537

        return outputFolder + File.separator + sourceFolder + File.separator + packageName + File.separator + apiPackage();
    }

    @Override
    public String modelFileFolder() {
        return outputFolder + File.separator + sourceFolder + File.separator + packageName + File.separator + modelPackage();
    }

    @Override
    public String toModelFilename(String name) {
        // should be the same as the model name
        return toModelName(name);
    }

    @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;
        }

        // operationId starts with a number
        if (operationId.matches("^\\d.*")) {
            LOGGER.warn("{} (starting with a number) cannot be used as method name. Renamed to {}", operationId, camelize(sanitizeName("call_" + operationId)));
            operationId = "call_" + operationId;
        }

        return camelize(sanitizeName(operationId));
    }

    @Override

View on GitHub (pinned to fcec517be3)

Solutions

  1. Set a unique non-empty operationId on every operation.
  2. Delete the empty operationId key rather than leaving a blank value so the default generator can derive a name from the path.
  3. Audit custom generator subclasses to ensure they never forward empty ids to toOperationId.

Example fix

# before:
paths:
  /users/{id}:
    get:
      operationId: ""
# after:
paths:
  /users/{id}:
    get:
      operationId: getUserById
Defensive patterns

Strategy: validation

Validate before calling

// Pre-flight: every operation must have a non-empty operationId
OpenAPI api = new OpenAPIV3Parser().read("spec.yaml");
api.getPaths().forEach((path, item) -> item.readOperationsMap().forEach((method, op) -> {
    String id = op.getOperationId();
    if (id == null || id.trim().isEmpty())
        throw new IllegalStateException(method + " " + path + " has an empty operationId");
}));

Prevention

When it happens

Trigger: A spec operation carrying 'operationId: ""' combined with a code path that bypasses default operationId derivation, or a custom C# generator override that calls toOperationId with a blank string.

Common situations: Specs exported from tools that emit empty operationIds; copy-paste spec surgery that strips the id; custom generators built on AbstractCSharpCodegen manipulating ids first.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/c7862169061c1160. Report an issue: GitHub.