Codex Pet Generator

Codex Pet Generator

Use this skill when the user wants to create, revise, validate, package, or install a custom Codex desktop pet.

Core Principles

• Animation direction first, image generation second, deterministic packing third.
• Optimize for tiny UI readability, not for how pretty a large strip looks in isolation.
• Treat each row as one loopable state machine, not eight poster poses.
• Keep canon anchors: one approved scale reference, one seated-body reference, one standing-body reference, and one prop-language reference.
• Every generated row must be marked approved or unapproved before it can advance.

Codex Format

Custom pets live at:

~/.codex/pets//
  pet.json
  spritesheet.png|webp

Required spritesheet:

• 1536x1872
• 8x9 grid
• 192x208 per frame
• transparent final background
• PNG or WebP

Required pet.json:

{
  "id": "",
  "displayName": "",
  "description": "",
  "spritesheetPath": "spritesheet.webp"
}

spritesheetPath is resolved relative to the pet.json file. Use the same `` as the parent directory name.

Row order is fixed:

1. idle
2. running-right
3. running-left
4. waving
5. jumping
6. failed
7. waiting
8. running
9. review

For row-by-row semantics, common failure modes, and tiny-UI design guidance, read [references/row-semantics.md](references/row-semantics.md) before prompting.

Workflow

1. Gather references and write a character bible in plain language.
2. Define all 9 row semantics before generating anything.
3. Pick canon anchors:

• one approved overall scale reference
• one approved seated reference
• one approved standing reference
• one approved prop-language reference

1. Generate one row strip at a time, or individual frames. Do not generate a full 72-frame sheet as the final asset.
2. Review each row with the acceptance ladder in [references/acceptance-checklist.md](references/acceptance-checklist.md).
3. Mark every candidate approved or unapproved. Only approved rows advance.
4. Extract individual frames into per-row directories. The skill provides written slicing guidance in [references/packing-and-slicing.md](references/packing-and-slicing.md), but the actual extraction is manual — either export frames directly from the generator, or cut strips with your image tool of choice. The bundled packer assumes per-frame inputs and does not slice strips for you.
5. Pack with scripts/pack_codex_pet.py.
6. Validate with scripts/validate_codex_pet.py.
7. Install into ~/.codex/pets/.
8. When iterating in the Codex UI, use a new pet id such as -v2 to avoid cached spritesheets.

Prompting Rules

Prompts should explicitly state:

• one row equals one loopable action
• frame 8 must hand off cleanly to frame 1
• frames are sequential animation states, not unrelated poster poses
• same character, same camera, same scale across the row
• exactly two arms and exactly two legs
• no cropped body parts
• no props outside the row-specific action
• final target is a tiny desktop pet sprite, so readability beats nuance

If a row is hard to read:

• do not merely “polish” it
• redesign the motion around bolder state changes
• prefer binary readable mechanics such as open/closed, upright/flat, connect/fail, tension/release, or in/out

High-risk guidance:

• Standing rows are high-risk for vertical stretch. Always anchor them to an approved compact standing canon.
• Directional rows often collapse into generic running. Reject them if the intended action semantics vanish.
• Gestures often fail when they rely on finger nuance alone. If needed, redesign them into chunkier hand states.
• Failure rows often become generic sadness. Prefer unmistakable failure silhouettes or prop logic.
• Jump rows often become athletic. Heavy toy-like characters usually need compression, shock, or flop logic instead.

Background and Alpha Rules

• Prefer generating with a removable flat background when possible.
• If you need chroma key, use flat #00ff00, no shadows, no gradients, and no green on the character.
• Remove the background locally before packing.
• Zero the RGB channels of fully transparent pixels before final install artifacts if your cleanup leaves preview garbage.

Approval Gate

Do not carry ambiguous rows forward.

• approved: chosen final candidate for that row
• unapproved: rejected, superseded, or still exploratory

Only approved rows should be sliced, packed, validated, and installed.

The acceptance ladder and rejection patterns live in [references/acceptance-checklist.md](references/acceptance-checklist.md).

Slicing and Packing Rules

Slicing is manual — the skill provides guidance, not an automated slicer. The packer takes per-frame inputs and assembles them into the final sheet using one uniform scale per row.

• Do not assume a generated row strip is cut into 8 equal visual columns.
• Do not slice approved strips into equal widths unless the spacing is known to be uniform.
• Prefer per-frame export or gap-aware manual slicing based on real empty space between poses.
• The packer uses one scale per row, not one scale per frame.
• Keep at least 8-12px of padding from cell edges (packer default is 10, hard cap is half-cell-minus-one).
• Keep exactly one directory per row index inside the rows root. Multiple matching directories (01-idle-explore, 01-idle-final) cause the packer to abort instead of silently picking one — this protects the approval gate.

See [references/packing-and-slicing.md](references/packing-and-slicing.md) for manual slicing workflow guidance.

Bundled Scripts

Pack approved rows into a final sheet:

python scripts/pack_codex_pet.py  

Validate the final sheet:

python scripts/validate_codex_pet.py 

If pet.json sits next to the spritesheet, the validator picks it up automatically and checks the install contract (id, displayName, description, spritesheetPath). To validate against a pet.json in a different location use --pet-json . To skip pet.json entirely use --no-pet-json.

Both scripts require Pillow. Install with pip install -r requirements.txt. Run from the bundle root, or use absolute paths.

Interpretation rule:

• validator errors are hard blockers — wrong dimensions, empty frames, missing alpha, fully-opaque sheet, malformed pet.json
• validator warnings still require visual review — fully-opaque single frame, edge contact, width/height drift, spritesheetPath mismatch
• width or height drift warnings may reflect real performance changes, but edge-contact warnings usually mean crop risk

Install Notes

• Install final assets under ~/.codex/pets//
• Keep pet.json and the spritesheet in the same folder
• Prefer a new id when testing revisions in the UI

If the UI looks broken, first suspect:

• bad row semantics
• bad slicing
• bad alpha cleanup
• bad packing

Do not assume the generator is the only failure point.

Source & license

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

• Author: wyddy7
• Source: wyddy7/codex-pet-generator
• License: MIT

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