Unity YAML Editing Agent Skill

YAML Editing - Safe Hand-Edit Rules

Advisory module. This is operational guidance for directly editing Unity serialized YAML text when the REST skills (and the Editor itself) cannot do the job. Hand-editing YAML is the last resort, not a shortcut.

> Mode: Documentation only — no REST skills to gate; load freely under any operating mode (Approval / Auto / Bypass).

When To Load / When NOT

Prefer REST first. For normal object/asset work, use the REST skills — they keep serialization valid for you:

• GameObjects / components → gameobject_*
• Prefabs → prefab_*
• ScriptableObjects → scriptableobject_*
• Scenes → scene_*
• Importers → importer_*
• Graphics / rendering → graphics_*

Only hand-edit YAML when REST cannot reach, i.e. one of:

• Compile failure — a script fails to compile, so the Editor cannot deserialize the component. Editor/REST APIs are unavailable; the only level left is the YAML text.
• .meta files — no REST skill touches .meta; GUID and importer fields live only in text.
• Hidden ProjectSettings fields — many ProjectSettings/*.asset fields have no public API or REST surface.
• Merge conflicts — REST runs sequentially and cannot perform a 3-way merge of .unity / .prefab.

If none of these apply, stop and use REST. Do not hand-edit YAML for convenience.

Core Concepts

• Object block / anchor: every serialized object is a block headed by --- !u!<classID> &<fileID>. <classID> is the Unity type id (e.g. 1 = GameObject, 4 = Transform, 114 = MonoBehaviour). <fileID> is the object's anchor, unique within this file.
• GUID vs fileID:
• GUID = asset-level identity, a 32-char hex string, stored in the asset's .meta. Identifies the whole asset (a script, prefab, texture, ...).
• fileID = sub-object identity inside a file (a specific component, the specific MonoBehaviour script class, ...).
• Serialized reference: a cross-object/asset reference is {fileID: N, guid: H, type: T}. guid is omitted for same-file references ({fileID: N}).
• m_Script: a MonoBehaviour points at its script via m_Script: {fileID: 11500000, guid: <scriptGUID>, type: 3}. fileID: 11500000 is the conventional MonoScript fileID; guid is the .cs file's GUID from its .meta; type: 3 means the reference targets an asset on disk. A wrong guid/fileID here is exactly what produces a "missing script / missing component".

Coverage Scenarios

1. Reference / fileID repair

When: a script was renamed or moved, so a prefab/scene m_Script guid/fileID no longer resolves; or a compile failure left a component as Missing and you must locate, delete, or re-point it at the YAML level (the Editor cannot deserialize a broken component, so REST/Editor API is unavailable).

How: read the script's current GUID from its .cs.meta. In the prefab/scene, find the offending m_Script: {fileID: 11500000, guid: ..., type: 3} and set guid to the correct value (keep fileID: 11500000, type: 3). To delete a Missing component, remove its whole --- !u!114 &<fileID> block and the matching {component: {fileID: <fileID>}} entry under the owner GameObject's m_Component list — leaving one without the other corrupts the file.

Danger: deleting the object block but not its m_Component reference (or vice versa) breaks the GameObject; changing a fileID that other objects reference orphans those references.

2. .meta / GUID safety

When: a .meta GUID looks like a pseudo-random GUID and risks third-party collision, or you must edit an importer field that REST does not expose.

How: for pseudo-GUID detection and the uuid4 repair flow, reuse /metacheck — it documents 5 pseudo-GUID heuristics (consecutive hex runs, interleaved increment, repeated chars, literal abcdef, wrong length), the uuid4 regeneration step, and the ValidationSkills.cs.meta collision lesson. Do not re-derive that here. Generate replacement GUIDs with python -c "import uuid; print(uuid.uuid4().hex)", never by hand.

Danger: changing a GUID without updating every reference. Before changing any GUID, grep the whole project for that GUID (.asset / .prefab / .unity / .meta) and update every reference point in the same change. Run /metacheck for a full GUID health audit before/after.

3. ProjectSettings patch

When: you need a ProjectSettings/*.asset field (YAML) that REST does not expose — e.g. a specific physics layer / collision matrix entry, or a hidden QualitySettings value.

How: git-back up (or confirm a clean tree) first, locate the exact field by key, change only its value, then let Unity reload and re-serialize to validate. Confirm the change round-trips (Unity rewriting the file the same way) rather than reverting or reformatting it.

Danger: editing collision-matrix or enum-backed fields by guessing the encoding can silently break settings; always verify Unity accepts the value on reload.

4. Merge conflict assistance

When: a .unity / .prefab has Git conflict markers and you must resolve them.

How: Unity YAML is line-mergeable because each object is an independent --- !u!<classID> &<fileID> block. Resolve per-block: for a block changed on only one side, take that side; for the same block changed on both sides, do a true 3-way merge of the conflicting fields. After resolving, ensure every fileID/guid referenced still has a defining block.

Danger: blindly accepting one side (--theirs/--ours) drops objects added on the other side or leaves dangling references. Never resolve by deleting whole conflict regions without checking the fileID references inside them.

Hard Rules

• Back up first: confirm a clean git tree (or commit/stash) before any YAML edit, so you can diff and revert.
• Don't touch structure unless repairing references: change values only; do not renumber or reorder anchor/fileID unless the explicit task is reference repair.
• Re-import to validate: after editing, trigger a Unity reimport/reload and confirm no new errors and the file round-trips.
• GUIDs are uuid4, never hand-written: python -c "import uuid; print(uuid.uuid4().hex)".
• Dry-run before bulk edits: grep the impact set first; know exactly how many references a change touches before making it.
• Change a GUID → change every reference: a GUID edit is not done until all referencing files are updated in the same change.

Routing

• If a REST skill can do it, use REST — gameobject_* / prefab_* / scriptableobject_* / scene_* / importer_* / graphics_* — and do not hand-edit.
• GUID health audit / pseudo-GUID detection / uuid4 repair → /metacheck.
• After editing skill docs or for doc/code consistency → /skillcheck.