Guides / Building & Resources

A Bedrock skin pack is four files, and two of its failure modes are silent

Two UUIDs that must differ, a lang file that is not optional, and skin keys that strip punctuation — so 'My Skin!' and 'My Skin?' become one skin with no error.

2026-08-01

A .mcpack is a zip with a different extension, and a skin pack inside one needs exactly four kinds of file:

manifest.json      a header and a skin_pack module, each with its own UUID
skins.json         one entry per skin: texture, geometry, and a KEY
texts/en_US.lang   the display names — skins.json holds only keys
*.png              the textures themselves

Three of the four are easy to get right. The interesting failures all come from the relationship between them.

The two UUIDs must differ

manifest.json carries a UUID in its header and another in its skin_pack module:

{
  "format_version": 1,
  "header":  { "name": "…", "uuid": "…", "version": [1, 0, 0] },
  "modules": [ { "type": "skin_pack", "uuid": "…", "version": [1, 0, 0] } ]
}

They are two separate identities — the pack, and the thing inside the pack — and reusing one value for both is a real error even though nothing complains at build time.

There is a second, subtler consequence: a re-import with the same UUIDs is treated as the same pack, and Minecraft silently keeps the copy it already has. Editing a pack, rebuilding it with the same identifiers and re-importing looks like it did nothing. Every build from this page gets fresh UUIDs for exactly that reason, which is also why re-importing an edited pack needs a fresh download rather than a re-zip of the old one.

The lang file is not optional

skins.json never contains a display name. It contains a key, and texts/en_US.lang maps that key to text:

skinpack.<PackKey>=My Pack
skin.<PackKey>.<SkinKey>=My First Skin

Ship the pack without the lang file and nothing errors — every skin simply appears in the game under its raw key. This is the most common reason a hand-assembled pack looks broken while validating fine.

Keys strip punctuation, and duplicates collapse

A key must be alphanumeric. Everything else is removed:

The first two rows are the trap. Two skins named distinctly by a human become one key, and a duplicate key does not error — it silently collapses into a single skin in game. You upload twelve skins, get eleven, and nothing tells you which one went missing.

The builder here defends against that by appending a counter to any key it has already used, so the second MySkin becomes MySkin1. Worth knowing if you assemble a pack by hand: the check you need is on the stripped names, not the ones you typed.

A name that is entirely punctuation leaves nothing at all, so it falls back to Skin plus its position in the list.

Slim needs the right geometry string

Each entry names a model, and there are two:

• geometry.humanoid.custom — classic, 4-pixel arms
• geometry.humanoid.customSlim — slim, 3-pixel arms

Getting this wrong is the usual cause of an Alex-proportioned skin turning up with Steve's blocky arms. The texture is fine; the model is the one being asked for. The pack builder infers it by sampling the fourth arm column, but a hand-written skins.json has to say it explicitly, and the two strings differ by four characters at the end.

Entries also carry "type": "free", and a cape is an optional extra PNG referenced from the same entry — the cape travels inside the skin pack rather than as a separate download.

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