Ability to reliably generate transparent images

[Alan01252]

What variant of Codex are you using?

codex-cli 0.122.0-alpha.10

What feature would you like to see?

Codex currently exposes the built-in Responses
image_generation tool, but the serialized tool spec
appears to only include:

  {
    "type": "image_generation",
    "output_format": "png"
  } 

This makes it hard to reliably generate transparent PNG
assets. Prompting for “transparent background”, “alpha
channel”, or “alpha=0 outside the subject” often still
produces an opaque RGB image with a fake checkerboard or
matte background.

The Responses image generation tool supports options such
as:

 {
    "type": "image_generation",
    "output_format": "png",
    "background": "transparent",
    "quality": "high"
  }

It would be useful if Codex exposed these options, either
through configuration or an enhanced built-in image tool
schema.

Use case:

I am building small web games and need generated sprites/
assets with real alpha transparency so they can be
composited over game backgrounds without halos,
checkerboards, or matte cleanup. This matters for workflows
where Codex generates image assets and then writes them into
a project.

Suggested implementation:

• Extend Codex’s ImageGeneration tool spec to include
  optional background, quality, and possibly size.
• Allow background: "transparent" when output_format is png
  or webp.
• Optionally expose a config default, e.g.
  image_generation.background = "transparent" and
  image_generation.quality = "high".
• Preserve the current default behavior if no config is set.

Why this matters:

Without the background parameter, users must rely on prompt-
only transparency requests, which are unreliable. The
fallback is chroma-key or checkerboard extraction, which
adds artifacts and extra cleanup. Real alpha output would
make Codex much better at producing usable game art, UI
assets, stickers, icons, and composited web visuals.

Additional information

No response