vercel/ai · error · Error

The Google Vertex xAI model function cannot be called with t

Error message

The Google Vertex xAI model function cannot be called with the new keyword.

What it means

The Google Vertex xAI provider export is a callable function, not a class. Calling it with `new` (e.g. `new googleVertex.xai(...)`) is explicitly rejected because the factory creates model instances via closure state and is not designed as a constructor. This guard gives a clear message instead of a confusing runtime failure inside the factory.

Source

Thrown at packages/google-vertex/src/xai/google-vertex-xai-provider.ts:191

    (cachedProvider ??= createOpenAICompatible({
      name: 'googleVertex.xai',
      baseURL: loadBaseURL(),
      fetch: options.fetch,
      includeUsage: true,
      supportsStructuredOutputs: true,
      supportedUrls: () => ({
        'image/*': [/^https?:\/\/.*$/],
      }),
      transformRequestBody: transformGoogleVertexXaiRequestBody,
      convertUsage: convertGoogleVertexXaiUsage,
    }));

  const createChatModel = (modelId: GoogleVertexXaiModelId) =>
    getProvider().languageModel(modelId);

  const provider = function (modelId: GoogleVertexXaiModelId) {
    if (new.target) {
      throw new Error(
        'The Google Vertex xAI model function cannot be called with the new keyword.',
      );
    }

    return createChatModel(modelId);
  };

  provider.specificationVersion = 'v4' as const;
  provider.languageModel = createChatModel;
  provider.chatModel = (modelId: GoogleVertexXaiModelId) =>
    getProvider().chatModel(modelId);
  provider.embeddingModel = (modelId: string): never => {
    throw new NoSuchModelError({ modelId, modelType: 'embeddingModel' });
  };
  provider.textEmbeddingModel = provider.embeddingModel;
  provider.imageModel = (modelId: string): never => {
    throw new NoSuchModelError({ modelId, modelType: 'imageModel' });
  };

View on GitHub (pinned to 69428b1f8b)

Solutions

Remove the `new` keyword and call the provider directly: `const model = googleVertex.xai('grok-3')`.
If constructing the provider itself, use the factory `createGoogleVertexXAI({ ... })` or `createGoogleVertex({ ... })` without `new`.
Update older class-based provider examples/snippets to the current function-call style.

Example fix

// before
const model = new googleVertex.xai('grok-3-pro');
// after
const model = googleVertex.xai('grok-3-pro');

Defensive patterns

Strategy: validation

Validate before calling

function isProviderFunction(fn) { return typeof fn === 'function' && !fn.prototype; }
// always call without new:
const model = googleVertex.xai('grok-3-pro');

Type guard

function isCallableProvider(v) { return typeof v === 'function' && !(v.prototype && Object.getOwnPropertyNames(v.prototype).length > 0); }

Try / catch

try {
  const model = googleVertex.xai(modelId);
} catch (e) {
  if (e instanceof Error && e.message.includes('cannot be called with the new keyword')) {
    const model = googleVertex.xai(modelId); // retry without new
  } else throw e;
}

Prevention

Treat all AI SDK providers as factory functions, never classes.
Enable lint rule 'no-new' for provider modules.
Update legacy `new Provider()` snippets when upgrading SDK versions.

When it happens

Trigger: Writing `new googleVertex('model-id')` or `new googleVertex.xai('grok-...')` where googleVertex.xai is the provider function defined at packages/google-vertex/src/xai/google-vertex-xai-provider.ts:191. Detected via `new.target` inside the provider function.

Common situations: Migrating code that used a class-style provider (`new Vertex(...)`) to the current function-style factory API; copying provider examples from older blog posts; TypeScript users treating the provider as a constructor type.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/42de257451e4b4c9. Report an issue: GitHub.