/nano-banana

name: nano-banana
description: Generate or edit images with Google's Nano Banana 2 (`gemini-3.1-flash-image-preview`). Use when the user asks to generate an image, edit an image, or create a picture. Supports 512 / 1K / 2K / 4K resolutions.

Nano Banana 2

Text-to-image and image edit via Google's **Nano Banana 2** model, aka `gemini-3.1-flash-image-preview`. Fast, cheap, high-quality. Returns PNG.

When to use

• User asks for an image, picture, photo, thumbnail, cover art, poster, icon, etc.
• User provides an input image and wants it modified (background change, object removal, style swap)
• You need a quick visual for a blog post, README, demo, or slide

When NOT to use

• User wants a video — Nano Banana is image-only
• User wants a pixel-perfect UI mockup — use a design tool
• User needs a specific licensed stock photo — use a stock service

Setup

1. Get a Gemini API key: https://aistudio.google.com/app/apikey 2. Export it:

   export GEMINI_API_KEY="YOUR_KEY_HERE"
   # or
   export GOOGLE_API_KEY="YOUR_KEY_HERE"

3. Install `uv` if missing:

   command -v uv || curl -LsSf https://astral.sh/uv/install.sh | sh

First run auto-installs `google-genai` and `pillow` via `uv`'s inline script deps.

Generate

uv run ~/.claude/skills/lich-skills/skills/nano-banana/scripts/generate_image.py \
    --prompt "A cinematic rainy Tokyo alley, neon reflections, 35mm, low angle" \
    --filename "tokyo-alley.png" \
    --resolution 1K

Output is saved to `tokyo-alley.png` in the current directory unless the filename is absolute.

Edit

Pass an input image with `--input-image`. The model sees the image + your instruction.

uv run .../generate_image.py \
    --prompt "Remove the crowd behind the subject. Keep framing and grading unchanged." \
    --filename "cleaned.png" \
    --input-image "./portrait.png" \
    --resolution 2K

When editing, the script auto-matches output resolution to the input's dimensions unless you explicitly pass `--resolution`.

Flags

| Flag | Default | Purpose | |------|---------|---------| | `--prompt`, `-p` | required | Image description or edit instruction | | `--filename`, `-f` | required | Output path (PNG) | | `--input-image`, `-i` | — | Source image for editing | | `--resolution`, `-r` | `1K` | One of `512`, `1K`, `2K`, `4K` | | `--api-key`, `-k` | env | Overrides `GEMINI_API_KEY` / `GOOGLE_API_KEY` | | `--model`, `-m` | `gemini-3.1-flash-image-preview` | Rarely needed |

Resolution mapping

Map loose user language to a resolution flag:

| User says | Use | |-----------|-----| | (nothing) | `1K` | | "small", "thumbnail", "quick draft" | `512` | | "normal", "medium" | `1K` | | "high quality", "large" | `2K` | | "4k", "print", "wallpaper" | `4K` |

Prompt template (generation)

Subject: <subject>.
Style: <photo | illustration | 3D | painting | pixel art | ...>.
Composition: <shot type, angle>.
Lighting: <lighting description>.
Background: <background>.
Avoid: <what should NOT be in the image>.

Prompt template (editing)

Change only: <the specific edit>.
Keep subject, framing, lighting, palette, text, and overall style unchanged.

Being explicit about what to keep is the difference between a clean edit and a total re-render.

Environment

• Key lookup order: `--api-key` → `GEMINI_API_KEY` → `GOOGLE_API_KEY`
• Output format: PNG (RGBA converted to RGB on white background)
• Never prints the API key or image bytes to stdout

Fast checks

command -v uv
test -n "$GEMINI_API_KEY" || test -n "$GOOGLE_API_KEY"
# If editing:
test -f ./input.png

Common failures

| Error | Fix | |------|-----| | `Error: No API key provided.` | Export `GEMINI_API_KEY` or pass `--api-key` | | `Error loading input image:` | Bad path or unreadable file | | `Error: No image was generated in the response.` | Prompt too vague or unsafe — rewrite | | `403` / `quota` | Wrong key, region-locked, or over quota |

Red flags

• Hardcoding the API key in any script or example
• Committing generated images to git without checking size
• Re-running the same prompt 10 times hoping for luck — instead, rewrite the prompt with the template above

Verification

• [ ] `$GEMINI_API_KEY` or `$GOOGLE_API_KEY` set in env, not in files
• [ ] A test generation produces a PNG at the expected path
• [ ] `gitleaks detect --source .` clean