Component serialization can crash or disconnect UnityMCP when reading Photon Fusion components

[cyanxwh]

Summary

Reading GameObject or prefab component properties through UnityMCP can crash the Unity Editor or disconnect the UnityMCP session when the target contains Photon Fusion NetworkBehaviour components.

Photon Fusion is a popular Unity multiplayer game development framework used by many real-time multiplayer projects. Because Fusion components are commonly attached to player rigs, networked interactables, projectiles, and other gameplay prefabs, this issue can block UnityMCP inspection workflows in a broad class of multiplayer Unity projects.

Photon Fusion IL weaving injects fields/properties with unsafe internal types such as Fusion.NetworkBehaviourBuffer, Fusion.NetworkBehaviourCallbackBuffer, Fusion.Networked+Internals, and Fusion.Changed<T>. These types can be reached by the current reflection-based component serializer and are not safe to access or serialize.

Impact

This affects UnityMCP workflows that inspect scene objects or prefabs containing Fusion components, including:

• Fusion.NetworkObject
• Fusion.NetworkTransform
• Fusion.Addons.Physics.NetworkRigidbody3D
• Fusion.XR.Host.Rig.NetworkRig
• Project-specific NetworkBehaviour subclasses

Depending on the object and session state, the symptom can be an Editor crash or a UnityMCP session disconnect while awaiting the serialization result.

Expected Behavior

UnityMCP should skip unsafe Fusion-injected fields/properties and continue serializing safe component data.

Actual Behavior

UnityMCP may attempt to reflect and serialize unsafe Fusion-injected types, causing instability when accessing Fusion-related components.

Proposed Fix

Before reading component fields/properties, detect and skip unsafe types, including:

• known Fusion IL-weaved unsafe types
• generic containers wrapping those types
• arrays wrapping those types
• pointer/by-ref/ref-like types

A regression test should cover direct and nested unsafe Fusion-like types.

Validation

A fix was validated with:

• Unity 2022.3.62f1 EditMode targeted test
• Unity 6000.0.41f1 EditMode targeted test
• Manual UnityMCP validation against a real Photon Fusion project, including prefab-stage reads and scene-instantiated prefab reads