"found undefined tag handle while parsing a node at line 21 column 5 (Psych::SyntaxError)"
I got this error on parsing unity3d prefab file with parse_stream method.

Psych.parse_stream(yaml, filename: path) do |node|
       puts node
end

line 21 is " --- !u!224 &224286004637740336"
I can insert a new line "%TAG !u! tag:unity3d.com,2011: " before every "---" to fix it.

source.gsub(/^(?<!%TAG !u! tag:unity3d.com,2011:\n)^(--- !u![0-9]+ &[0-9]+)$/, "%TAG !u! tag:unity3d.com,2011:\n\\1")

But I hope there is a solution in API level.

%YAML 1.1
%TAG !u! tag:unity3d.com,2011:
--- !u!1 &1007462957078648
GameObject:
  m_ObjectHideFlags: 0
  m_CorrespondingSourceObject: {fileID: 0}
  m_PrefabInstance: {fileID: 0}
  m_PrefabAsset: {fileID: 0}
  serializedVersion: 6
  m_Component:
  - component: {fileID: 224286004637740336}
  - component: {fileID: 222770377023727128}
  - component: {fileID: 114746315234884256}
  m_Layer: 5
  m_Name: Icon
  m_TagString: Untagged
  m_Icon: {fileID: 0}
  m_NavMeshLayer: 0
  m_StaticEditorFlags: 0
  m_IsActive: 1
--- !u!224 &224286004637740336
RectTransform:
  m_ObjectHideFlags: 0
  m_CorrespondingSourceObject: {fileID: 0}
  m_PrefabInstance: {fileID: 0}
  m_PrefabAsset: {fileID: 0}
  m_GameObject: {fileID: 1007462957078648}
  m_LocalRotation: {x: -0, y: -0, z: -0, w: 1}
  m_LocalPosition: {x: 0, y: 0, z: -1.3}
  m_LocalScale: {x: 0.7, y: 0.7, z: 0.9996802}
  m_Children: []
  m_Father: {fileID: 224567198966254662}
  m_RootOrder: 0
  m_LocalEulerAnglesHint: {x: 0, y: 0, z: 0}
  m_AnchorMin: {x: 0.5, y: 0.5}
  m_AnchorMax: {x: 0.5, y: 0.5}
  m_AnchoredPosition: {x: 45.3, y: -79.2}
  m_SizeDelta: {x: 132, y: 132}
  m_Pivot: {x: 0.5, y: 0.5}