OpenAPITools/openapi-generator · error · RuntimeException

Empty method/operation name (operationId) not allowed

Error message

Empty method/operation name (operationId) not allowed

What it means

AbstractScalaCodegen.toOperationId throws RuntimeException when the operationId handed to it is empty. The default pipeline normally substitutes a synthesized id (derived from the operation summary, else from method + path) before this point, so an empty id reaching the guard means synthesis was skipped or itself produced nothing usable.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractScalaCodegen.java:591

            return;
        }

        String scalaPostProcessFile = System.getenv("SCALA_POST_PROCESS_FILE");
        if (StringUtils.isEmpty(scalaPostProcessFile)) {
            return; // skip if SCALA_POST_PROCESS_FILE env variable is not defined
        }

        // only process files with scala extension
        if ("scala".equals(FilenameUtils.getExtension(file.toString()))) {
            this.executePostProcessor(new String[]{scalaPostProcessFile, file.toString()});
        }
    }

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

        operationId = camelize(sanitizeName(operationId), LOWERCASE_FIRST_LETTER);

        // method name cannot use reserved keyword, e.g. return
        if (isReservedWord(operationId)) {
            String newOperationId = camelize("call_" + operationId, LOWERCASE_FIRST_LETTER);
            LOGGER.warn("{} (reserved word) cannot be used as method name. Renamed to {}", operationId, newOperationId);
            return newOperationId;
        }

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

        return operationId;

View on GitHub (pinned to fcec517be3)

Solutions

  1. Set a unique non-empty operationId on every operation
  2. Delete empty operationId keys so summary/path-based fallback naming applies
  3. If relying on fallback naming, keep a summary with at least some word characters

Example fix

# before
get:
  operationId: ''
  responses: { '200': { description: ok } }

# after
get:
  operationId: list_users
  responses: { '200': { description: ok } }
Defensive patterns

Strategy: validation

Validate before calling

// JS: no empty operationIds; usable summary when id absent
for (const [p, item] of Object.entries(spec.paths || {})) {
  for (const [m, op] of Object.entries(item)) {
    if (!op || !op.responses) continue;
    if (op.operationId === '') fail(`${m} ${p}: empty operationId`);
  }
}

Try / catch

try { generator.generate(); } catch (RuntimeException e) { if ("Empty method/operation name (operationId) not allowed".equals(e.getMessage())) { /* set operationId or a word-bearing summary on the failing operation */ } throw e; }

Prevention

When it happens

Trigger: operationId: "" explicitly set with no summary, or a summary consisting solely of characters that sanitize away (pure symbols/emoji); also direct programmatic use of the generator API passing an empty operationId. Reserved-word or digit-leading ids do NOT throw — those are renamed via camelize('call_' + ...).

Common situations: Templated spec pipelines rendering operationId from data that can be empty; scraping/conversion tools that blank ids; specs whose only summary is decorative characters.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/c623ada517815152. Report an issue: GitHub.