Prefab and Prefab Variant Format

Prefabs are Unity YAML files (%YAML 1.1). They can be created and modified by hand following the rules below. FileIDs are large unique positive integers — pick random 9–18 digit values that don't collide with existing anchors in the file.

Regular Prefab

Contains all GameObjects and components serialized as top-level YAML documents. Each object has a unique &fileID anchor. The root GameObject has m_Father: {fileID: 0} on its Transform.

Key object types:

• !u!1 — GameObject
• !u!4 — Transform
• !u!114 — MonoBehaviour

MonoBehaviour blocks reference their script by GUID (from the .cs.meta file):

m_Script: {fileID: 11500000, guid: <script-guid>, type: 3}
m_EditorClassIdentifier: AssemblyName::ClassName

Serialized fields appear directly under the MonoBehaviour by their C# field name:

_variable: 0

Prefab Variant

A prefab variant file contains a single !u!1001 PrefabInstance block instead of the full hierarchy. It stores only the delta from the base prefab:

--- !u!1001 &<fileID>
PrefabInstance:
  m_Modification:
    m_TransformParent: {fileID: 0}
    m_Modifications:
    - target: {fileID: <source-object-fileID>, guid: <base-prefab-guid>, type: 3}
      propertyPath: _variable
      value: 42
      objectReference: {fileID: 0}
    m_RemovedGameObjects:
    - {fileID: <source-fileID>, guid: <base-prefab-guid>, type: 3}
    m_AddedGameObjects: []
    m_AddedComponents: []
  m_SourcePrefab: {fileID: 100100000, guid: <base-prefab-guid>, type: 3}

• target.fileID refers to the object's fileID inside the base prefab
• target.guid is the GUID from the base prefab's .meta file
• m_RemovedGameObjects lists GameObjects from the base that are stripped in this variant
• To override a field: add an entry to m_Modifications with the correct propertyPath

MCP Workflow (preferred)

When Unity Editor is connected via UnityMCP, use MCP tools instead of hand-writing YAML:

• Create prefab: manage_asset (action create) or manage_gameobject + manage_prefabs (action create_from_gameobject)
• Instantiate prefab in scene: manage_gameobject (action create, param prefab_path)
• Edit prefab fields: manage_components (action set_property) targeting the prefab asset
• Create prefab variant: manage_prefabs (action create_variant)

Fall back to the manual YAML workflow below only when MCP is unavailable.

Manual Workflow

Create a prefab

1. Write the .prefab file with a %YAML 1.1 header and one block per object (root GameObject + Transform + components). Refer to the Regular Prefab section above for the structure.
2. Create a matching .prefab.meta file with a unique GUID:

fileFormatVersion: 2
guid: <new-guid>
PrefabImporter:
  externalObjects: {}
  userData: 
  assetBundleName: 
  assetBundleVariant: 

The GUID must be a 32-character lowercase hex string, unique across the project.

Create a prefab variant

1. Write the .prefab file containing only a single PrefabInstance block (see Prefab Variant section above).
2. Create a matching .prefab.meta file with a unique GUID (same format as above).
3. The m_SourcePrefab.guid must match the base prefab's GUID from its .meta file.

Edit a prefab

• To change a serialized field value: find the MonoBehaviour block for that component and edit the field value directly.
• To add a component: add a new block with a unique fileID and add its fileID to the owning GameObject.m_Component list.
• To modify a variant override: edit the value in the matching m_Modifications entry; do not change target.fileID or target.guid.
• To add a new override to a variant: append a new entry to m_Modifications using the target object's fileID from the base prefab and the base prefab's GUID.