Assembly loading issue with Claude Code

[WazzaF]

The assembly loading issue is likely a bug where Unity MCP is trying to load the wrong assembly. Editor scripts go in "Assembly-CSharp-Editor" but runtime MonoBehaviour scripts should be in "Assembly-CSharp".

[dsarno]

@WazzaF can you give some more detail on when you’re running into this? Like an example prompt you’re giving the MCP, and the errors you’re getting either from your AI client or the unity console or both? That would help us isolate the issue better.

[WazzaF]

Thanks @dsarno , please see the report below for detail:

Unity MCP Assembly Loading Error Report

Issue Summary

The Unity MCP server has an assembly loading bug where it attempts to load "Assembly-CSharp-Editor" instead of "Assembly-CSharp" when working with runtime MonoBehaviour scripts through MCP tool calls.

Error Details

Primary Error Message

System.IO.FileNotFoundException: Could not load the file 'Assembly-CSharp-Editor'
   at System.Reflection.Assembly.LoadFrom(String assemblyFile)
   at UnityMCP.ComponentResolver.ResolveComponent(String componentName)
   at UnityMCP.GameObjectManager.AddComponent(GameObject target, String componentName)

Context

• Environment: Unity 6.2, Windows 10/11
• Project: Unity project with custom MonoBehaviour scripts
• Trigger: Adding custom script components to GameObjects via MCP tools
• Expected Behavior: Should load from "Assembly-CSharp" (runtime assembly)
• Actual Behavior: Attempts to load from "Assembly-CSharp-Editor" (editor assembly)

Technical Analysis

Root Cause

The MCP server appears to have incorrect assembly resolution logic that:

1. Defaults to looking in editor assemblies for runtime components
2. Does not fall back to runtime assemblies when editor assembly fails
3. May be using editor-specific reflection paths during runtime operations

Assembly Structure in Unity

Correct Assembly Mapping:
├── Assembly-CSharp (Runtime)
│   ├── MonoBehaviour scripts
│   ├── ScriptableObject scripts  
│   └── Runtime-only classes
└── Assembly-CSharp-Editor (Editor Only)
    ├── Editor scripts
    ├── Custom inspectors
    └── Editor utilities

Reproduction Steps

1. Create a custom MonoBehaviour script (e.g., PlayerController.cs)
2. Place script in Assets/Scripts/ folder (runtime location)
3. Use MCP tool to create GameObject
4. Attempt to add the custom component via MCP
5. Error occurs during component resolution

Workarounds Currently Used

1. Manual Assignment: Create GameObject via MCP, manually assign components in Inspector
2. Built-in Components Only: Use Unity's built-in components through MCP
3. Script Creation First: Create scripts via MCP, then manual component assignment

Expected Fix

The assembly resolution should:

1. First attempt to load from Assembly-CSharp (runtime assembly)
2. Fall back to Assembly-CSharp-Editor only for editor-specific components
3. Provide clear error messages distinguishing between assembly types

Sample Code That Fails

// This script exists in Assets/Scripts/PlayerController.cs
using UnityEngine;

public class PlayerController : MonoBehaviour 
{
    public float speed = 5.0f;
    
    void Update() 
    {
        // Runtime logic here
    }
}

MCP Call:

{
    "action": "add_component",
    "target": "Player",
    "component_name": "PlayerController"
}

Result: FileNotFoundException: Could not load the file 'Assembly-CSharp-Editor'

Additional Context

• Project has been built successfully with ./build.sh
• Scripts compile without errors in Unity Editor
• Manual component assignment works perfectly
• Issue only occurs when using MCP automation tools

System Information

• Unity Version: 6.2
• Platform: Windows
• .NET Version: .NET Standard 2.1
• Project Type: 3D URP

This error significantly impacts development workflow automation and requires manual intervention for every custom component assignment.

[dsarno]

@WazzaF this is helpful thank you!

[dsarno]

This should be fixed in latest version. Please give a try and let us know.