Built-in image_gen is missing basic image controls like transparent background, size, quality, and masks

What variant of Codex are you using?

Codex CLI / Codex App with the built-in image_gen tool

What feature would you like to see?

What feature would you like to see?

I’d like the built-in image_gen tool to expose the basic image-generation controls that already
exist in the image API / CLI fallback, especially:

• background (transparent / opaque / auto)
• size
• quality
• n
• mask
• input_fidelity

Why this matters

The built-in tool is useful, but right now it feels too limited for real asset work.

A few common cases are hard to do reliably without these controls:

• generating a real transparent-background PNG
• choosing the output size / aspect ratio
• doing masked edits
• staying inside the subscription-based Codex flow instead of falling back to API-key-based workflows

What I ran into

I tested the built-in image_gen tool with prompts like:

• background: transparent
• transparent background

The image looked like it had a transparent-style background, but the saved file was still a normal
RGB PNG with no alpha channel.

So in practice, prompt wording alone does not seem to be enough when you need actual transparency.

Why I think this is missing in the built-in path

From the public Codex repo, the built-in image_generation tool appears to expose only

Relevant files:

• codex-rs/tools/src/tool_spec.rs
• codex-rs/tools/src/tool_registry_plan.rs
• codex-rs/skills/src/assets/samples/imagegen/references/image-api.md

What I’d expect

If the built-in image tool is meant to be used for real image generation and editing, it should
expose the main controls needed for that work, especially:

• transparency
• output dimensions
• quality
• masks
• edit fidelity

The biggest one for me is background=transparent, because that is hard to replace with prompt
wording.

Additional context

Related issues / discussions:

• #8758
• #14895
• Discussion #592

Additional information

_No response_