Codex Imagegen

Codex Imagegen: Illustrated Asset Generation via Codex CLI

Generate polished, professionally-illustrated raster art (mascots, hero images, logos, sprite sheets, product mockups) by driving Codex CLI's $imagegen skill. Use this instead of hand-drawing shapes in SVG/CSS/SwiftUI when the asset needs to look designed, not programmatically constructed.

Scope check (do this first)

This skill wraps a multi-minute, credit-billed Codex run and is the default handler for image-generation requests:

• Architecture/flow/sequence diagram → tell the user to use docs-diagram instead and stop here.
• Otherwise, continue: the cost gate in Phase 2 decides whether to confirm before running.

Ground rules

• Single-quote the whole prompt. $imagegen must reach codex literally, not be shell-expanded: a double-quoted or unquoted prompt containing $imagegen breaks.
• Never fabricate a model ID. gpt-5.6-sol is this recipe's known-good default at time of writing; if codex rejects it, check codex --version/codex exec --help for a current one rather than guessing a replacement.
• Never claim chroma removal or file output succeeded without viewing the pixels. sips hasAlpha: yes proves nothing; open the PNG.
• Stop before an expensive run and confirm when the brief is a batch (multiple assets), a consistency-critical asset (recurring character/mascot), or explicitly says "final"/"official": this is real spend and multi-minute wall-clock time, not a free retry.

Phase 1: Environment check

codex --version

• Not found → tell the user to install Codex CLI and stop.
• Version older than 0.144 → gpt-5.6-sol will 400 with "requires a newer version"; tell the user to upgrade before continuing. See references/troubleshooting.md for the auth/version gotcha table.
• Run from the target repo's root so codex's relative move-file paths land in the right workspace.

Phase 2: Gather the brief and budget effort

Mine the request for: subject, purpose (README hero, app mascot, icon set, sprite sheet), style, exact save path(s), and whether transparency is required.

Pick model_reasoning_effort:

xhigh burns significant tokens/credits: don't default to it. If unclear which bucket the request falls in, ask.

Confirm before running when this is a batch, a consistency-critical/official asset, or the user said "final": state the design brief, model, effort, exact output paths, and estimated run length (minutes) and get a go-ahead before invoking. For a single obvious low-stakes placeholder, proceed straight to Phase 3.

Phase 3: Invoke

Run from the workspace root, prompt single-quoted, non-interactive:

codex exec --full-auto -m gpt-5.6-sol -c model_reasoning_effort="<xhigh|high>" \
  'Use the $imagegen skill to create <asset>. <detailed design brief — subject,
   style, composition, palette, "transparent background" if needed>.
   Move the final PNGs into the workspace at these exact paths: <path1>, <path2>.'

If transparency is required, say so explicitly in the brief ("transparent background"): the skill's built-in flow handles chroma-key removal on its own; don't ask for it as a separate step.

For a consistent character across multiple images, put the full shared design brief in every asset's prompt, and generate pose/mood variants as edits of the first approved image rather than independent generations: one built-in image_gen call per asset is the skill's own rule, so batches are sequential, not parallel.

Phase 4: Run it in the background, don't pipe the output

codex exec has no background flag, background it yourself and never pipe through tail/head (the pipe buffers everything until exit, so nothing streams while it runs):

codex exec --full-auto -m gpt-5.6-sol -c model_reasoning_effort="high" \
  -o /tmp/codex-imagegen-final.md \
  '...' > /tmp/codex-imagegen-run.log 2>&1 &

• -o final.md captures just the final summary.
• Poll /tmp/codex-imagegen-run.log for progress; a multi-asset brief can run for many minutes (sequential generation + edit chains + chroma removal + reasoning between steps).
• File presence in the target paths is not "done." Assets can appear incrementally before the run finishes (chroma removal/verification still pending): wait for the process to exit before treating the run as complete.
• After exit, cat the -o output file for the summary and verify the promised paths actually exist (ls).

Phase 5: QC every asset

Open each PNG before wiring it into anything:

• Transparent background actually transparent (not just alpha-flagged): check by viewing the image, not by trusting a metadata field.
• Edges clean, no chroma-key spill (a pink/magenta or green fringe around the subject).
• Consistent character/style across a batch.

Codex can claim chroma removal succeeded while the key color is still visibly there: never report success on the CLI's word alone.

Notes

• Chroma-key flow: the built-in image_gen tool has no native alpha, so the skill generates on a flat chroma-key background then strips it locally. Never pass --despill/--spill-cleanup on pink/magenta-family subject art: despill desaturates those colors toward gray.
• True native alpha (no chroma-key step) needs the CLI fallback (gpt-image-1.5 --background transparent) plus OPENAI_API_KEY: codex will ask before downgrading to it; don't force it.
• ChatGPT-account auth rejects API-only models; if -m gpt-5.6-sol fails on an auth error rather than a version error, see references/troubleshooting.md.
• Regenerating with the same output filenames is cheap; re-integrating a bad asset into the app isn't: favor a re-run over shipping a QC failure.
• If a chroma removal comes out visibly wrong, don't just report failure: the raw generation and the fix procedure are in references/troubleshooting.md (load it when you actually hit this).

Worked examples

1. Single placeholder icon (low stakes, no confirmation gate)

$ codex --version → codex-cli 0.146.0
Effort: high (basic placeholder, single asset) — proceeding without a confirm gate.

$ codex exec --full-auto -m gpt-5.6-sol -c model_reasoning_effort="high" \
    -o /tmp/codex-imagegen-final.md \
    'Use the $imagegen skill to create a minimalist gear icon, flat vector style,
     single accent color on transparent background. Move the final PNG to
     assets/icons/gear.png.' > /tmp/codex-imagegen-run.log 2>&1 &

[poll log until process exits]
$ ls assets/icons/gear.png → found
[open assets/icons/gear.png — background transparent, edges clean]
Done: assets/icons/gear.png

2. Official mascot, transparency-critical (confirmation gate + chroma QC)

Brief: official product mascot, xhigh effort (consistency-critical), transparent
background, 3 pose variants, saves to assets/mascot/{idle,wave,jump}.png.
Estimated run: several minutes across 3 sequential generations. Proceed? (yes/no)

[user: yes]
$ codex exec --full-auto -m gpt-5.6-sol -c model_reasoning_effort="xhigh" \
    -o /tmp/codex-imagegen-final.md \
    'Use the $imagegen skill to create <full shared mascot design brief>, idle pose,
     transparent background. Move the final PNG to assets/mascot/idle.png.' \
    > /tmp/codex-imagegen-run.log 2>&1 &
[wait for exit, then generate wave/jump as edits of the approved idle.png]

[open each PNG] idle.png: clean alpha, no spill. wave.png: faint magenta fringe
on the left arm — chroma removal didn't fully clear. Re-running wave.png only
before shipping.

3. Scope-check redirect (not this skill's job)

User: "draw an architecture diagram of our microservices"

This is a diagram, not an illustrated raster asset — use the docs-diagram
skill instead. Stopping here.

Reference Files

• references/troubleshooting.md: version/auth gotcha table, manual chroma-key recovery procedure. Load when a version/auth error blocks Phase 1, or a chroma removal needs a manual fix after Phase 5.