vercel/ai · error · Error

Gemini image models do not support generating a set number o

Error message

Gemini image models do not support generating a set number of images per call. Use n=1 or omit the n parameter.

What it means

Gemini image models on Google Vertex generate one image per request; the provider throws this plain Error in doGenerate when `n` is provided and greater than 1. It fails fast client-side because the underlying Gemini image API has no count parameter like Imagen's sampleCount.

Source

Thrown at packages/google-vertex/src/google-vertex-image-model.ts:87

      size,
      aspectRatio,
      seed,
      providerOptions,
      headers,
      abortSignal,
      files,
      mask,
    } = options;
    const warnings: Array<SharedV4Warning> = [];

    if (mask != null) {
      throw new Error(
        'Gemini image models do not support mask-based image editing.',
      );
    }

    if (n != null && n > 1) {
      throw new Error(
        'Gemini image models do not support generating a set number of images per call. Use n=1 or omit the n parameter.',
      );
    }

    if (size != null) {
      warnings.push({
        type: 'unsupported',
        feature: 'size',
        details:
          'This model does not support the `size` option. Use `aspectRatio` instead.',
      });
    }

    const userContent: Array<
      | { type: 'text'; text: string }
      | {
          type: 'file';
          data:

View on GitHub (pinned to 69428b1f8b)

Solutions

Set n to 1 or omit the n parameter entirely.
Call generateImage multiple times (or Promise.all) to obtain several images.
Use an Imagen model on Vertex if single-call multi-image generation (sampleCount) is needed.

Example fix

// before
await generateImage({ model: vertex.image('gemini-2.0-flash-exp-image'), prompt, n: 4 });
// after
const images = await Promise.all([1, 2, 3, 4].map(() => generateImage({ model: vertex.image('gemini-2.0-flash-exp-image'), prompt })));

Defensive patterns

Strategy: validation

Validate before calling

if (typeof n === 'number' && n > 1 && isGeminiImageModel(modelId)) {
  throw new Error('Gemini image models generate 1 image per call; loop calls instead.');
}

Try / catch

try {
  await generateImage({ model, prompt, n });
} catch (e) {
  if (e instanceof Error && e.message.includes('set number of images')) {
    // retry with n = 1 or fan out parallel calls
  } else throw e;
}

Prevention

Clamp n to 1 (or omit it) whenever the model id starts with 'gemini'.
Centralize image generation behind a helper that adapts n per provider capability.

When it happens

Trigger: generateImage({ model: vertex.image('<gemini image model>'), prompt, n: 2 }) (or any n > 1) — the check `if (n != null && n > 1)` throws before the request is sent.

Common situations: Migrating from Imagen or OpenAI DALL·E code that requested multiple images per call; loops that fan out a shared options object containing n: 4; expecting batch generation semantics that Gemini image models don't offer on Vertex.

Related errors

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/efcbacfc0078235c. Report an issue: GitHub.