Gemini Image

Gemini Image Generation Skill

You generate images by calling Gemini CLI with the nanobanana extension. Follow these rules precisely.

Model Selection

Nanobanana Commands

Nanobanana provides these slash commands inside the Gemini CLI prompt:

File Output

Nanobanana saves images to nanobanana-output/ in the current working directory with auto-generated filenames. After generation, rename the file to match our naming convention:

Format: {subject}-{style}-{timestamp}.png

• subject: 1-5 word kebab-case description (e.g. orange-cat, tech-logo)
• style: abbreviated style tag:
  • photo — realistic photograph
  • illus — illustration / digital art
  • logo — logo / icon design
  • pixel — pixel art
  • water — watercolor
  • flat — flat design / vector style
  • 3d — 3D render
  • sketch — pencil / line sketch
  • anime — anime / manga style
  • abstract — abstract art
• timestamp: YYYYMMDDHHMMSS format

Example: orange-cat-water-20260219150030.png

When retrying or iterating on the same image, reuse the same filename to overwrite.

Generating an Image

1. Read references/prompt-guide.md from the skill directory for prompt engineering guidance.

2. Craft an effective English prompt based on the user's request and the prompt guide. Even if the user writes in another language, the prompt sent to Gemini must be in English.

3. Get the current timestamp for the output filename:

   date +%Y%m%d%H%M%S
   

4. Run the generation command (timeout 180s):

   # Default (quick) mode:
   gemini -e nanobanana -p '/generate "<english_prompt>"' --yolo --sandbox false
   
   # HQ mode:
   NANOBANANA_MODEL=gemini-3-pro-image-preview gemini -e nanobanana -p '/generate "<english_prompt>"' --yolo --sandbox false
   

5. Find and verify the output:

   ls -lt nanobanana-output/*.png | head -1
   file nanobanana-output/<filename>.png
   

6. Rename to our naming convention:

   mv nanobanana-output/<generated_name>.png nanobanana-output/<subject>-<style>-<timestamp>.png
   

7. Report the result to the user: show the image using Read tool, confirm the file path, and briefly describe what was generated.

Editing an Existing Image

When the user wants to modify an existing image:

1. Confirm the source image path exists. If the image is not in nanobanana-output/, copy it there first.

2. Run the edit command (timeout 180s):

   gemini -e nanobanana -p '/edit <filename_in_nanobanana_output> "<edit_instructions>"' --yolo --sandbox false
   

3. Verify, rename, and report as above.

Specialized Commands

Icons

gemini -e nanobanana -p '/icon "<description>" --sizes="64,128,256" --type=app-icon --style=modern' --yolo --sandbox false

Patterns

gemini -e nanobanana -p '/pattern "<description>" --type=seamless --style=geometric' --yolo --sandbox false

Diagrams

gemini -e nanobanana -p '/diagram "<description>" --type=flowchart --complexity=detailed' --yolo --sandbox false

Error Handling

• If Gemini returns a safety filter error, inform the user and suggest rephrasing.
• If the model is unavailable, fall back to the other model and retry once.
• If the first generate_image call fails, nanobanana typically retries automatically. If it still fails, retry the entire command once.
• If the command times out, inform the user and suggest simplifying the prompt.
• If no output file appears in nanobanana-output/, check for error messages in stderr.

Important Notes

• Always use -e nanobanana to load the extension.
• Always use --yolo flag — required for headless auto-approval of tool calls.
• Always use --sandbox false — ensures Gemini can write to the working directory.
• Nanobanana calls the Gemini image API directly via its MCP server — it does NOT use Gemini CLI's built-in write_file.
• The NANOBANANA_GEMINI_API_KEY env var must be set for the extension to work.