Custom GPT endpoint cannot use Imagen (The built-in image_gen tool described by the skill is not exposed in this session)

What version of Codex CLI is running?

codex-cli 0.142.5

What subscription do you have?

max 20

Which model were you using?

gpt-5.5

What platform is your computer?

macos

What terminal emulator and version are you using (if applicable)?

VS Code

Codex doctor report

What issue are you seeing?

Essentially if I set Codex with custom endpoint and a custom key, which relays to my codex 20x subscription the Imagen Tool is then not available through codex and in the session. So I can't use my subscriptions quota to create images.

I get the following

I’ll use the imagegen skill instructions here. For a single dot, the skill itself says simple shapes are better made directly unless
• Explored
  └ Read SKILL.md (imagegen skill)

────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

• The built-in image_gen tool described by the skill is not exposed in this session. I can still produce a dot deterministically, which
  is also the path the skill recommends for a simple shape.

────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

• I can’t use the built-in image_gen tool here because it isn’t exposed in this session. Per the imagegen skill, a simple dot is better
  drawn directly

What steps can reproduce the bug?

install the latest version of codex.

update .codex/.config.toml with different provider baseurl

[model_providers.OpenAI]
name = "OpenAI"
base_url = "...."
wire_api = "responses"
requires_openai_auth = true

and update .codex/auth.json with API key for your provider.

What is the expected behavior?

image_gen tool should be available

Additional information

_No response_