Generate Images with Google Imagen

google-imagen (Google Image Generation)

This example generates images with Gemini. The directory keeps its existing name so init --example google-imagen continues to work.

npx promptfoo@latest init --example google-imagen
cd google-imagen

Prerequisites

Choose Google AI Studio for the native Gemini API or Vertex AI for Google Cloud authentication.

Option 1: Google AI Studio (Quick Start)

• Get an API key from Google AI Studio.
• Use a billing-enabled project with access to the selected image model.

Option 2: Vertex AI

• Use a Google Cloud project with billing and the Vertex AI API enabled.
• Authenticate with gcloud auth application-default login.
• Check the Vertex Gemini 3.1 Flash Image model card for model availability. The example uses the global endpoint.

Setup

For Google AI Studio:

export GOOGLE_API_KEY=your-api-key

### Windows Command Prompt:
### set GOOGLE_API_KEY=your-api-key
### Windows PowerShell:
### $env:GOOGLE_API_KEY="your-api-key"

Run without GOOGLE_PROJECT_ID or GOOGLE_CLOUD_PROJECT set: a configured project selects Vertex AI in this adapter.

For Vertex AI:

gcloud services enable aiplatform.googleapis.com
gcloud auth application-default login
export GOOGLE_PROJECT_ID=your-project-id

### Windows Command Prompt:
### set GOOGLE_PROJECT_ID=your-project-id
### Windows PowerShell:
### $env:GOOGLE_PROJECT_ID="your-project-id"

Environment Variables

• GOOGLE_API_KEY - Native Gemini API key.
• GOOGLE_PROJECT_ID - Google Cloud project used by the Vertex example.

Available Models

Imagen Models (legacy google:image: prefix)

The old Imagen configurations have been migrated to Gemini image generation. Native Imagen 4 reached its August 17, 2026 shutdown. Google Cloud separately discontinued Imagen 3 and 4 models on June 30, 2026.

Use google:gemini-3.1-flash-image for the replacement. The google:image: prefix selects Imagen's predict adapter; putting a Gemini model after that prefix does not migrate the protocol.

Gemini Native Image Generation

The default config uses google:gemini-3.1-flash-image at 1K resolution. promptfooconfig-gemini.yaml compares these native Gemini API models:

• google:gemini-3.1-flash-lite-image - 1K only; no Google Search grounding.
• google:gemini-3.1-flash-image - Supports 1K, 2K, and 4K output.
• google:gemini-3-pro-image - Supports 1K, 2K, and 4K output.
• google:gemini-2.5-flash-image - Legacy comparison until its October 2, 2026 native shutdown; use 3.1 Flash Image for new configs. Does not support imageSize.

Use the stable IDs above. Google shut down the gemini-3.1-flash-image-preview and gemini-3-pro-image-preview aliases on June 25, 2026. Check current native pricing for the model and resolution you select; Vertex has separate pricing and availability.

Running the Example

For Google AI Studio:

npx promptfoo@latest eval

For Vertex AI, use the OAuth configuration:

npx promptfoo@latest eval -c promptfooconfig-advanced.yaml

Notes

• Gemini image generation uses generateContent with TEXT and IMAGE response modalities.
• Configure imageAspectRatio and imageSize. Imagen's aspectRatio, safetyFilterLevel, seed, personGeneration, and addWatermark options are not interchangeable with this adapter's settings.
• Responses can contain both text and images. The assertions inspect context.providerResponse.images so a valid text-and-image response passes and a text-only response fails.

Advanced Configuration

promptfooconfig-advanced.yaml demonstrates Vertex AI with an explicit project and 2K image generation. It sets apiKeyRequired: false because Google Cloud OAuth supplies authentication.

Gemini Native Image Generation

For a comparison of native image models or a Google Search-grounded image, run:

npx promptfoo@latest eval -c promptfooconfig-gemini.yaml
npx promptfoo@latest eval -c promptfooconfig-gemini-grounding.yaml

The grounding config uses google:gemini-3.1-flash-image with tools: [{ googleSearch: {} }]. See Google's GenerateContent image guide for supported input, output, and grounding options.