vercel/ai · error · InvalidArgumentError

Invalid argument for parameter enumValues: Enum values must

Error message

Invalid argument for parameter enumValues: Enum values must be strings.

What it means

Enum output values are serialized into a string enum schema for the model, so every entry must be a string. validateObjectGenerationInput iterates the enum array and throws InvalidArgumentError on the first non-string element. This is a deliberate eager check rather than letting the provider fail later.

Source

Thrown at packages/ai/src/generate-object/validate-object-generation-input.ts:136

    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

Convert entries to strings, e.g. enum: Object.values(MyEnum).map(String)
Use `as const` string literal arrays instead of TS numeric enums
Validate array elements are strings before the call (see validationCode)

Example fix

// before
enum Size { S, M, L }
await generateObject({ model, output: 'enum', enum: Object.values(Size) });
// after
const SIZES = ['S','M','L'] as const;
await generateObject({ model, output: 'enum', enum: SIZES });

Defensive patterns

Strategy: validation

Validate before calling

if (output === 'enum' && enumValues?.some(v => typeof v !== 'string')) {
  throw new Error('All enum values must be strings');
}

Type guard

function isStringEnum(values: unknown): values is string[] {
  return Array.isArray(values) && values.every(v => typeof v === 'string');
}

Try / catch

try {
  return await generateObject(args);
} catch (e) {
  if (InvalidArgumentError.isInstance(e) && e.parameter === 'enumValues') {
    args.enum = (args.enum ?? []).map(String);
    return generateObject(args);
  }
  throw e;
}

Prevention

Use `as const` string literal arrays for enums
Avoid passing TS numeric enums directly; convert with Object.values(...).map(String)
Validate element types before calling

When it happens

Trigger: generateObject({ output: 'enum', enum: [1, 2, 3] }) or any array containing numbers, booleans, objects, or null mixed with strings, passed to generateObject/streamObject.

Common situations: Numeric TypeScript enums (`enum Color {Red}` yields numeric values) passed directly; enum arrays derived from Object.values() of non-string maps; untyped JSON config used as the enum source.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/779c232cff7df881. Report an issue: GitHub.