Codex Imagegen

Codex imagegen — delegate image generation, keep the code here

Claude Code cannot generate images. When a task needs generated or AI-edited images, delegate to the Codex CLI's built-in $imagegen tool (gpt-image): it runs on the operator's ChatGPT subscription — no API key, no per-call billing, i2i editing included. Division of labor: code and prompts are authored here; pixels come from Codex.

Invocation

• Scriptable default: codex exec -C -s workspace-write --skip-git-repo-check "" via execFileSync (array args, never shell strings). Timeout ~15 min per call; run batches sequentially (rate limits), support --slug re-runs and skip-existing + --force.
• Resolve the binary: $CODEX_BIN env → codex on PATH → /Applications/Codex.app/Contents/Resources/codex (macOS app bundles the CLI). Verify with --version before batching.
• MCP registration (claude mcp add --transport stdio codex -- codex mcp-server) is the interactive alternative; codex exec stays the default for reproducible pipelines.

Sandbox isolation — non-negotiable

Point -C at a scratch dir (os.tmpdir() subdir), NEVER the repo. The instruction embeds content-derived text (templates, user-editable YAML), so prompt injection through content must not reach a repo-writable agent — with -C repoRoot an injected instruction could rewrite package.json and land RCE on the next npm run dev. The wrapper script alone copies finished artifacts from the scratch dir into the repo.

Instruction anatomy (what actually controls quality)

1. Verbatim prompt block: wrap the exact prompt in --- delimiters with "Use EXACTLY this prompt, verbatim, without rewriting it" — otherwise Codex paraphrases and the published prompt no longer matches the sample it claims to produce.
2. Native sizes only: gpt-image emits 1024x1024 / 1536x1024 / 1024x1536. Other ratios: generate the nearest orientation → center-crop → resize (sips on macOS, sharp elsewhere).
3. Bleed instruction whenever cropping: "the image will be center-cropped to WxH; treat the outer Npx as expendable bleed — background only; ALL text and critical elements inside the surviving central region." Without it, edge-anchored text gets cut (bottom ribbons are the classic casualty).
4. i2i is two steps: generate the source image first, then EDIT mode referencing the input path ("treat 'the attached photo' as the input image at "). Keep the source artifact — before/after is the proof the edit works.
5. Close with "Save the final image to (PNG). Do not edit any other files."

Quality gate — generation is not done at exit 0

• Review every image visually before publishing. For Japanese text: character-by-character — models garble kanji, and a wrong glyph on a shipped banner is worse than none.
• Check the crop actually preserved all text, and (i2i) that the source subject/label survived the edit.
• Retakes are normal (budget 2–3 per image); re-run per slug, never the whole batch. Trial ONE image before any batch — composition bugs (missing bleed, wrong ratio) waste a full batch otherwise.
• Quota is the operator's ChatGPT plan: dozens of images per session is fine, plan accordingly.

Pipeline sketch

const work = path.join(os.tmpdir(), 'imagegen-work');           // sandbox, not the repo
execFileSync(codex, ['exec', '-C', work, '-s', 'workspace-write',
  '--skip-git-repo-check', instruction(prompt, workFile)], { timeout: 900_000 });
if (!existsSync(workFile)) throw new Error('not saved');
execFileSync('sips', ['-c', cropH, cropW, workFile]);            // center-crop to target ratio
execFileSync('sips', ['-z', finalH, finalW, workFile]);          // resize to target px
copyFileSync(workFile, repoTarget);                              // script does the repo write

Derive the filled prompt from the same single source the app renders (shared data + one fill function, parity-tested) — a stored copy of the prompt WILL drift from the template that generated it.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Syo-M
• Source: Syo-M/fable5_skills
• License: MIT

Install and usage instructions live in the source repository linked above.