Automatically reorders all humanoid bones to be the first child to make VRChat's IK happy.

HierarchyFix.cs

// originally by https://github.com/brandonvdongen/PFCTools
using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;
using VRC.SDK3.Avatars.Components;

// ReSharper disable once CheckNamespace
[InitializeOnLoad]
public class HierarchyFix : AssetPostprocessor {
    private static readonly HumanBodyBones[] Limbs = {
        HumanBodyBones.LeftUpperArm,
        HumanBodyBones.LeftLowerArm,
        HumanBodyBones.RightUpperArm,
        HumanBodyBones.RightLowerArm,
        HumanBodyBones.LeftLowerLeg,
        HumanBodyBones.RightLowerLeg
    };

    static HierarchyFix()
    {
        // force a reorder on scene start, since these chances don't stick with packed prefabs after restarts or scene changes
        EditorApplication.delayCall += ReorderHierarchy;
        
        EditorSceneManager.sceneOpened += (scene, mode) => {
            EditorApplication.delayCall += ReorderHierarchy;
        };
    }


    private static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths) {

        var hasModelFile = false;
        foreach (var asset in importedAssets) {
            if (asset.ToLower().EndsWith(".blend") || asset.ToLower().EndsWith(".fbx")) {
                hasModelFile = true;
            }

            if (!hasModelFile) continue;
            EditorApplication.delayCall -= ReorderHierarchy;
            EditorApplication.delayCall += ReorderHierarchy;
            break;
        }
    }
    /// <summary>
    /// Reorders all humanoid bones to be the first child.
    /// </summary>

    [MenuItem("Tools/Fix Hierarchy Order")]
    private static void ReorderHierarchy() {
        var descriptors = Object.FindObjectsOfType<VRCAvatarDescriptor>();
        foreach (var descriptor in descriptors) {
            var animator = descriptor.GetComponent<Animator>();
            if (animator == null) return;
            foreach (var bone in Limbs) {
                var boneTransform = animator.GetBoneTransform(bone);
                if (boneTransform == null) continue;
                var ind = boneTransform.GetSiblingIndex();
                if (ind == 0) continue;
                Debug.Log($"Avatar: {descriptor.gameObject.name}: Humanoid Bone: {bone} is child {ind} instead of 0, fixing...", boneTransform);
                boneTransform.SetAsFirstSibling();
            }
        }
    }
}