Agent-Skills.md

Agent Skills: Gemini image generator

Generate images using Google Gemini with customizable options

UncategorizedID: stevenmays/dotfiles/gemini-image-generator

Repository

stevenmays/dotfiles
stevenmaysLicense: MIT
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Install this agent skill to your local

pnpm dlx add-skill https://github.com/stevenmays/dotfiles/tree/HEAD/skills/gemini-image-generator

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skills/gemini-image-generator/SKILL.md

Skill Metadata

Name
gemini-image-generator
Description
Generate or edit raster images with Google's Nano Banana model through the Antigravity CLI's built-in generate_image tool, run in a subagent. Use when the user names Gemini, Nano Banana, or Antigravity. Not for SVG, icons, or code diagrams. The default image path is codex-image-generator.

Gemini image generator

The Antigravity CLI (agy) has a built-in generate_image tool backed by Google's Nano Banana model. This skill wraps one headless agy call around that tool, copies the image out of the Antigravity brain directory, and verifies the file. No API key is needed; agy uses its own Antigravity login.

Run it

Delegate the run to a general-purpose subagent pinned to model: "sonnet". A run takes about 15 to 30 seconds and the result is a file check, so the session must not block on it. Give the subagent the exact command and the exit codes below.

"${CLAUDE_PLUGIN_ROOT}/skills/gemini-image-generator/scripts/generate.sh" \
  --prompt "<image description>" \
  --output <path>.jpg \
  [--aspect 16:9] [--reference <image>]... [--open]
  • --prompt: describe subject, composition, action, location, and style. The Prompting section lists Google's guidance.
  • --output: the final path, ending in .jpg, .jpeg, or .png. The script creates the parent directory and refuses to replace an existing file.
  • --aspect: one of 1:1, 2:3, 3:2, 3:4, 4:3, 9:16, or 16:9. The default is 1:1.
  • --reference: passes a local image to edit, blend, or use as a style reference. Repeat it up to 3 times. Say in the prompt which role each reference plays and what must stay unchanged. In testing, a color-only edit kept the composition, background, and aspect ratio.
  • --model: the agy driver model that calls generate_image. The default gemini-3.8-flash-low is the cheapest slug and drives the tool in about 15 seconds. The driver never changes the image model.
  • --timeout: seconds before agy gives up. The default is 240.
  • --open: opens the verified image in Preview. Use it for one-off requests so the user sees the result. Skip it for assets that code consumes.

The script prints the absolute path on line 1. Line 2 holds the file description, byte size, elapsed seconds, driver model, and agy conversation ID. The exit codes are:

  • 2: bad usage, a missing reference, a fourth reference, or an existing file at --output.
  • 3: agy or jq is missing from PATH, or sips is missing when a format conversion is needed.
  • 4: agy exited nonzero. Stderr holds the log path and its last 20 lines.
  • 5: agy finished without leaving an image. A warning: agy denied line on stderr means headless mode blocked a tool that the driver tried instead of generate_image.

What this path can and cannot produce

The generate_image tool takes only a prompt, an image name, an aspect ratio, and reference paths. It has no resolution, format, or model parameter, so asking for "4K" or "transparent PNG" in the prompt changes nothing.

| | This skill (agy) | Direct Gemini API (not wrapped) | |---|---|---| | Model | Nano Banana 2 (gemini-3.1-flash-image); Nano Banana Pro (gemini-3-pro-image) where the account has it; no choice | Flash or Pro, chosen per call | | Resolution | 1K tier only: 1024x1024 at 1:1, 1264x848 at 3:2, 1376x768 at 16:9 | Flash 0.5K, 1K, 2K, 4K; Pro 1K, 2K, 4K | | Format | Always JPEG (JFIF, 300 DPI) | PNG or JPEG | | Transparency | None | None | | Aspect ratios | 7: 1:1, 2:3, 3:2, 3:4, 4:3, 9:16, 16:9 | 10: adds 21:9, 5:4, 4:5 | | References | Up to 3 | Up to 14 | | Cost | Nothing per image; Antigravity login and quota; no API key | Flash $0.067 at 1K, $0.101 at 2K, $0.151 at 4K; Pro $0.134 at 1K or 2K, $0.24 at 4K | | Watermark | SynthID, cannot be disabled | SynthID, cannot be disabled |

2K and 4K output, PNG output, the 21:9, 5:4, and 4:5 ratios, and more than 3 references need the direct Gemini API. This skill does not wrap that API. When a request needs one of them, tell the user the per-image price from the table before you switch paths.

A .png output is a container conversion by sips from the JPEG source. The conversion adds no alpha channel. When the user needs real transparency, offer codex-image-generator, which accepts a transparent-background request.

Reference images can be PNG, JPEG, or WebP. Gemini scales each one down to at most 3072x3072.

The tool saves under ~/.gemini/antigravity-cli/brain/<conversation_id>/. That directory is ephemeral, so only the copy at --output is durable.

Sources: Antigravity models, Nano Banana Pro in Antigravity, Gemini API image generation, Gemini API pricing, Firebase AI Logic input file requirements, Antigravity agent skills: image generation.

Prompting

Google's guidance for Nano Banana:

  • Open with "Create an image of", then describe the subject, composition, action, location, and style.
  • Describe what you want in positive terms: "an empty street", not "no cars".
  • Use camera language for framing, such as wide-angle, macro, or low-angle.
  • Quote in-image text verbatim and name the font or type style. Nano Banana Pro renders text best.
  • For a cutout, ask for a solid chroma-key green (#00FF00) background with flat lighting and sharp edges. Then key out the green in a separate step.
  • For an edit, name the change and list every element that must stay unchanged.

Sources: Gemini API image generation, Gemini image prompt guide.

Rules

  • One asset per call. For variants or a set, run the script once per asset with distinct prompts.
  • Never overwrite an existing asset the user did not ask to replace. Use a sibling name such as hero-v2.jpg.
  • Report the saved path and the final prompt. Do not paste the image into chat.
  • If agy is missing or not logged in, say so and stop.
  • Pin the driver model. Keep gemini-3.8-flash-low unless the user names another slug from agy models.
  • Never pass --dangerously-skip-permissions to agy. The generate_image tool needs no permission.
  • Use a .png output only when a consumer needs the PNG container, because the conversion adds no alpha.