"Rigging an AI-generated (rigless) character mesh"

• AI character arrives as a static textured mesh (no armature).
• Blender automatic weights fail with: Bone Heat Weighting: failed to find solution.
• All vertex groups empty — even for a tiny 2-bone rig on a cleaned single-island mesh.
• Euclidean "nearest bone" weights look plausible in the viewport but cheeks follow arms, etc. on chibi proportions.

Bone-heat auto-weights fail wholesale on many AI-generated meshes. Topology/island quirks that look fine visually still break the heat solver.

Euclidean nearest-bone assignment is not enough — chibi proportions put cheek verts closer to arm bones than the head bone in 3D space, even when they should belong to the head along the surface.

Also: Blender's OBJ importer puts the Y-up→Z-up fix on the object transform — mesh.vertices[i].co stays file-local Y-up. Any scripted landmark/seed math must use obj.matrix_world @ v.co or seeds are silently wrong.

Proven local lane (headless Blender):

1. Copy the mesh out of any regenerating AI delivery folder (re-runs overwrite).
2. Hand-build (or script) an armature — pelvis as true root; no extra roots above it.
3. Scripted geodesic weights (along-surface Dijkstra) to per-bone seed tubes:
   • top-3 bones
   • inverse-power blend
   • ~10 diffusion passes
   • cap 4 influences
4. Scripted keyframe clips; keep quaternion keys hemisphere-continuous (if dot(prev, new) < 0: negate).
5. Export FBX per fbx-export-recipe.
6. Animated .vmdl using citizen as schema reference; scale with ModelModifier_ScaleAndMirror 0.3937 (cm sources), not per-file import_scale on the mesh alone (that leaves AnimFile data unscaled).

Blender meters → FBX centimeters (apply_unit_scale) → ScaleAndMirror 0.3937 → engine inches

AnimationList wants default_root_bone_name = "pelvis"; set bone_cull_type = "None" to keep helper bones. Schema reference: addons/citizen/Assets/models/citizen/citizen.vmdl + its prefabs/*.vmdl_prefab.

Runtime: play sequences on SkinnedModelRenderer (Sequence.Name = "..."). Enable crossfade-without-animgraph for smooth switches. Do not rely on SetBoneTransform / SetIk for secondary motion on a sequence-only rig — see setbonetransform-silently-noop.

Geodesic distance follows the mesh surface, so influence regions match anatomical adjacency instead of 3D proximity through empty space. Cap + diffusion keeps skinning engine-friendly (≤4 influences). ScaleAndMirror is the citizen convention that scales mesh and animations together; splitting scale across import_scale vs anim files is what produces correctly sized meshes with microscopic (or gigantic) motion.