Summary

The manage_components tool with set_property action fails when trying to set reference-type fields (e.g., Button component references) on MonoBehaviour components. The tool validation rejects the input format for reference properties.

Environment

• Unity MCP Tool: Latest version
• Unity Version: 2022.3.62f1
• Use Case: Configuring UI button component references in scene

Problem Description

Attempted Operation

Trying to set the button field (a Button component reference) on a custom MenuPanelButton MonoBehaviour:

Tool: manage_components
Action: set_property
Target: GameObject with instanceID
Component Type: Custom MonoBehaviour class
Properties: {"button": {"instanceID": "29690"}}

Code Example

public abstract class MenuPanelButton : MonoBehaviour
{
    [SerializeField] protected Button button;
    // ... other fields
}

Attempted Tool Calls

Attempt 1: Direct instanceID reference

{
  "action": "set_property",
  "target": "29690",
  "component_type": "Watermelon.SquadShooter.MenuInventoryButton",
  "properties": {
    "button": {"instanceID": "29690"}
  }
}

Error:

1 validation error for call[manage_components]
properties
  Input should be a valid dictionary [type=dict_type, input_value='{"button": {"instanceID": "29690"}}', input_type=str]

Attempt 2: String instanceID

{
  "action": "set_property",
  "target": "29690",
  "component_type": "Watermelon.SquadShooter.MenuInventoryButton",
  "properties": {
    "button": "29690"
  }
}

Result: No error, but field not set (likely treated as string, not reference)

Expected Behavior

The tool should accept a reference format like:

{
  "properties": {
    "button": {"instanceID": 29690}
  }
}

Or:

{
  "properties": {
    "button": {"gameObjectInstanceID": 29690}
  }
}

And correctly set the component reference on the target MonoBehaviour.

Current Workaround

Created an Editor script using reflection to set reference fields:

var buttonField = typeof(MenuPanelButton).GetField("button", 
    BindingFlags.NonPublic | BindingFlags.Instance);
buttonField.SetValue(menuButton, buttonComponent);

Suggested Solution

1. Add support for reference-type properties in set_property action
2. Define clear format for component/GameObject references (e.g., {"instanceID": 12345})
3. Add documentation examples for setting reference fields
4. Consider adding a dedicated set_reference action for reference-type fields

Impact

This limitation prevents automated scene configuration via MCP tools, requiring manual Editor scripting or manual Inspector configuration for any reference-type fields.

Additional Context

• Primitive types (int, float, string, bool) work correctly with set_property
• Only reference types (Component, GameObject, Transform, etc.) have this issue
• The validation error suggests the tool expects a flat dictionary, not nested objects

Reproduction Steps

1. Create a MonoBehaviour with a reference field:

   public class TestScript : MonoBehaviour
   {
       [SerializeField] private Button myButton;
   }

2. Add the script to a GameObject in scene
3. Try to set myButton reference via manage_components tool
4. Observe validation error or silent failure

Related Use Cases

• Setting up UI component references
• Configuring scene object dependencies
• Automated testing setup
• Batch scene configuration