node = hou.pwd()
input_geo = node.geometry()  # This is the writable geometry in a Python SOP
reference_geo = node.inputs()[1].geometry()  # Second input (read-only)

# Get reference agent from second input
if len(reference_geo.prims()) > 0:
    reference_agent = reference_geo.prims()[0]
    reference_def = reference_agent.definition()
    reference_rig = reference_def.rig()
    reference_shape_lib = reference_def.shapeLibrary()
    print(f"Reference rig found: {reference_rig}")
    print(f"Reference definition: {reference_def}")
    print(f"Reference shape library: {reference_shape_lib}")
else:
    print("Error: No agent in reference input (second input)")
    raise hou.NodeError("No agent in reference input")

# Process each agent in the input geometry
for i, agent in enumerate(input_geo.prims()):
    print(f"\n--- Agent #{i} ---")
    
    # Get original definition and info
    original_def = agent.definition()
    print(f"Original definition: {original_def}")
    print(f"Original rig: {original_def.rig()}")
    print(f"Original clips: {len(original_def.clips())}")
    print(f"Original layers: {len(original_def.layers())}")
    print(f"Original transform groups: {len(original_def.transformGroups())}")
    
    # Create a new frozen definition with the reference rig
    new_def = original_def.freeze(new_rig=reference_rig)
    print(f"New definition created: {new_def}")
    print(f"New rig: {new_def.rig()}")
    print(f"Clips preserved: {len(new_def.clips())}")
    print(f"Layers preserved: {len(new_def.layers())}")
    print(f"Transform groups preserved: {len(new_def.transformGroups())}")
    
    # Update the agent's definition in place
    # In a Python SOP, node.geometry() is directly modifiable
    agent.setDefinition(new_def)
    
    # Verify the change
    print(f"Updated agent: {agent}")
    print(f"New agent definition: {agent.definition()}")
    print(f"New agent rig: {agent.definition().rig()}")

Reference rig found: <hou.AgentRig rig:new_agent>
Reference definition: <hou.AgentDefinition 0x0000773c34602100>
Reference shape library: None

--- Agent #0 ---
Original definition: <hou.AgentDefinition 0x000077400e9f9780>
Original rig: <hou.AgentRig rig:agent1>
Original clips: 18
Original layers: 65
Original transform groups: 7
New definition created: <hou.AgentDefinition 0x0000773c3f3aeb00>
New rig: <hou.AgentRig rig:new_agent>
Clips preserved: 18
Layers preserved: 65
Transform groups preserved: 7
Updated agent: <hou.Agent prim #0 of geometry frozen at 0x773ff5d23c00>
New agent definition: <hou.AgentDefinition 0x0000773c3f3aeb00>
New agent rig: <hou.AgentRig rig:new_agent>