vercel/ai · error · InvalidArgumentError

Invalid argument for parameter enumValues: Enum values are r

Error message

Invalid argument for parameter enumValues: Enum values are required for enum output.

What it means

In 'enum' output mode, the `enum` array is the only way to describe the allowed values, so it is mandatory. validateObjectGenerationInput throws InvalidArgumentError when enum output is requested but enumValues is null or undefined. The library cannot build the response schema without it.

Source

Thrown at packages/ai/src/generate-object/validate-object-generation-input.ts:127

    if (schemaDescription != null) {
      throw new InvalidArgumentError({
        parameter: 'schemaDescription',
        value: schemaDescription,
        message: 'Schema description is not supported for enum output.',
      });
    }

    if (schemaName != null) {
      throw new InvalidArgumentError({
        parameter: 'schemaName',
        value: schemaName,
        message: 'Schema name is not supported for enum output.',
      });
    }

    if (enumValues == null) {
      throw new InvalidArgumentError({
        parameter: 'enumValues',
        value: enumValues,
        message: 'Enum values are required for enum output.',
      });
    }

    for (const value of enumValues) {
      if (typeof value !== 'string') {
        throw new InvalidArgumentError({
          parameter: 'enumValues',
          value,
          message: 'Enum values must be strings.',
        });
      }
    }
  }
}

View on GitHub (pinned to 69428b1f8b)

Solutions

Pass a non-empty `enum: string[]` array when using output: 'enum'
Ensure the variable holding the enum values is populated before the call
Fall back to output: 'object' with a schema if values are not known statically

Example fix

// before
await generateObject({ model, output: 'enum' });
// after
await generateObject({ model, output: 'enum', enum: ['small','medium','large'] });

Defensive patterns

Strategy: validation

Validate before calling

if (output === 'enum' && (!Array.isArray(enumValues) || enumValues.length === 0)) {
  throw new Error('enum output requires a non-empty enum array');
}

Type guard

function hasEnumValues(args: { output?: string; enum?: unknown[] }): args is { output: 'enum'; enum: string[] } {
  return args.output !== 'enum' || Array.isArray(args.enum) && args.enum.length > 0;
}

Try / catch

try {
  return await generateObject(args);
} catch (e) {
  if (InvalidArgumentError.isInstance(e) && e.parameter === 'enumValues') {
    throw new Error('Provide `enum: string[]` when using output: enum');
  }
  throw e;
}

Prevention

Always supply `enum` when output is 'enum'
Check the enum source array is loaded before the call
Type the options with discriminated unions so TS enforces the enum field

When it happens

Trigger: Calling generateObject({ output: 'enum' }) or streamObject({ output: 'enum' }) without an `enum` property, or passing an explicitly undefined/null enum.

Common situations: Dynamically constructed options where the enum array fails to load (empty config, failed fetch) and arrives as undefined; TypeScript types bypassed via `as any`; migration from schema-based output where `enum` was forgotten.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/b71320b1de42d6b8. Report an issue: GitHub.