OpenAPITools/openapi-generator · error · RuntimeException

" + operationId + " (reserved word) cannot be used as method

Error message

" + operationId + " (reserved word) cannot be used as method name

What it means

The scalaz client generator's toOperationId() rejects operationIds that are Scala reserved words, because the generated Scalaz/http4s client method could not be declared with that name in Scala. The RuntimeException names the offending word.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalazClientCodegen.java:219

    public String getName() {
        return "scalaz";
    }

    @Override
    public String getHelp() {
        return "Generates a Scalaz client library (beta) that uses http4s";
    }

    @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);
    }

    private static abstract class CustomLambda implements Mustache.Lambda {
        @Override
        public void execute(Template.Fragment frag, Writer out) throws IOException {
            final StringWriter tempWriter = new StringWriter();
            frag.execute(tempWriter);
            out.write(formatFragment(tempWriter.toString()));
        }

        public abstract String formatFragment(String fragment);
    }

    private class EnumEntryLambda extends CustomLambda {
        @Override

View on GitHub (pinned to fcec517be3)

Solutions

  1. Rename the operationId to a non-reserved word, ideally verb+noun (fetchType, buildObject)
  2. Check the generator's reserved word list (reservedWords() in the Scala codegens) when picking ids
  3. Add a CI lint step that rejects Scala reserved words in operationIds for Scala targets

Example fix

# before
paths:
  /schema:
    get:
      operationId: type
# after
paths:
  /schema:
    get:
      operationId: getType
Defensive patterns

Strategy: validation

Validate before calling

# Reject Scala reserved words as operationIds (Python):
SCALA_RESERVED = {'abstract','case','catch','class','def','do','else','extends','false','final',
                  'finally','for','forSome','if','implicit','import','lazy','match','new','null',
                  'object','override','package','private','protected','return','sealed','super',
                  'this','throw','trait','true','try','type','val','var','while','with','yield'}
for oid in all_operation_ids(spec):
    assert oid not in SCALA_RESERVED, f'operationId {oid!r} is a Scala reserved word'

Try / catch

try {
    new DefaultGenerator().opts(input).generate();
} catch (RuntimeException e) {
    // '<word> (reserved word) cannot be used as method name' - rename that operationId and rerun
}

Prevention

When it happens

Trigger: -g scalaz on a spec with operationId: type / object / return / package / trait / match / implicit / lazy, etc. Single-word ids are not changed by camelize(operationId, LOWERCASE_FIRST_LETTER), so they reach the check intact.

Common situations: Operation ids mirroring domain verbs that collide with Scala keywords; specs shared across language generators where only the Scala one fails; keyword lists differing between Java and Scala (e.g. 'type', 'object', 'trait', 'implicit' fail here though they are legal elsewhere).

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/83a706dd0a5ec4e5. Report an issue: GitHub.