# Adding Custom Tools to MCP for Unity
MCP for Unity supports auto-discovery of custom tools using decorators (Python) and attributes (C#). This allows you to easily extend the MCP server with your own tools.
Be sure to review the developer README first:
| [English](README-DEV.md) | [简体中文](README-DEV-zh.md) |
|---------------------------|------------------------------|
---
# Part 1: How to Use (Quick Start Guide)
This section shows you how to add custom tools to your Unity project.
## Step 1: Create a PythonToolsAsset
First, create a ScriptableObject to manage your Python tools:
1. In Unity, right-click in the Project window
2. Select **Assets > Create > MCP For Unity > Python Tools**
3. Name it (e.g., `MyPythonTools`)
## Step 2: Create Your Python Tool File
Create a Python file **anywhere in your Unity project**. For example, `Assets/Editor/MyTools/my_custom_tool.py`:
```python
from typing import Annotated, Any
from fastmcp import Context
from registry import mcp_for_unity_tool
from unity_connection import send_command_with_retry
@mcp_for_unity_tool(
description="My custom tool that does something amazing"
)
async def my_custom_tool(
ctx: Context,
param1: Annotated[str, "Description of param1"],
param2: Annotated[int, "Description of param2"] | None = None
) -> dict[str, Any]:
await ctx.info(f"Processing my_custom_tool: {param1}")
# Prepare parameters for Unity
params = {
"action": "do_something",
"param1": param1,
"param2": param2,
}
params = {k: v for k, v in params.items() if v is not None}
# Send to Unity handler
response = send_command_with_retry("my_custom_tool", params)
return response if isinstance(response, dict) else {"success": False, "message": str(response)}
```
## Step 3: Add Python File to Asset
1. Select your `PythonToolsAsset` in the Project window
2. In the Inspector, expand **Python Files**
3. Drag your `.py` file into the list (or click **+** and select it)
**Note:** If you can't see `.py` files in the object picker, go to **Window > MCP For Unity > Tool Sync > Reimport Python Files** to force Unity to recognize them as text assets.
## Step 4: Create C# Handler
Create a C# file anywhere in your Unity project (typically in `Editor/`):
```csharp
using Newtonsoft.Json.Linq;
using MCPForUnity.Editor.Helpers;
namespace MyProject.Editor.CustomTools
{
[McpForUnityTool("my_custom_tool")]
public static class MyCustomTool
{
public static object HandleCommand(JObject @params)
{
string action = @params["action"]?.ToString();
string param1 = @params["param1"]?.ToString();
int? param2 = @params["param2"]?.ToObject<int?>();
// Your custom logic here
if (string.IsNullOrEmpty(param1))
{
return Response.Error("param1 is required");
}
// Do something amazing
DoSomethingAmazing(param1, param2);
return Response.Success("Custom tool executed successfully!");
}
private static void DoSomethingAmazing(string param1, int? param2)
{
// Your implementation
}
}
}
```
## Step 5: Rebuild the MCP Server
1. Open the MCP for Unity window in the Unity Editor
2. Click **Rebuild Server** to apply your changes
3. Your tool is now available to MCP clients!
**What happens automatically:**
- ✅ Python files are synced to the MCP server on Unity startup
- ✅ Python files are synced when modified (you would need to rebuild the server)
- ✅ C# handlers are discovered via reflection
- ✅ Tools are registered with the MCP server
## Complete Example: Screenshot Tool
Here's a complete example showing how to create a screenshot capture tool.
### Python File (`Assets/Editor/ScreenShots/Python/screenshot_tool.py`)
```python
from typing import Annotated, Any
from fastmcp import Context
from registry import mcp_for_unity_tool
from unity_connection import send_command_with_retry
@mcp_for_unity_tool(
description="Capture screenshots in Unity, saving them as PNGs"
)
async def capture_screenshot(
ctx: Context,
filename: Annotated[str, "Screenshot filename without extension, e.g., screenshot_01"],
) -> dict[str, Any]:
await ctx.info(f"Capturing screenshot: {filename}")
params = {
"action": "capture",
"filename": filename,
}
params = {k: v for k, v in params.items() if v is not None}
response = send_command_with_retry("capture_screenshot", params)
return response if isinstance(response, dict) else {"success": False, "message": str(response)}
```
### Add to PythonToolsAsset
1. Select your `PythonToolsAsset`
2. Add `screenshot_tool.py` to the **Python Files** list
3. The file will automatically sync to the MCP server
### C# Handler (`Assets/Editor/ScreenShots/CaptureScreenshotTool.cs`)
```csharp
using System.IO;
using Newtonsoft.Json.Linq;
using UnityEngine;
using MCPForUnity.Editor.Tools;
namespace MyProject.Editor.Tools
{
[McpForUnityTool("capture_screenshot")]
public static class CaptureScreenshotTool
{
public static object HandleCommand(JObject @params)
{
string filename = @params["filename"]?.ToString();
if (string.IsNullOrEmpty(filename))
{
return MCPForUnity.Editor.Helpers.Response.Error("filename is required");
}
try
{
string absolutePath = Path.Combine(Application.dataPath, "Screenshots", filename);
Directory.CreateDirectory(Path.GetDirectoryName(absolutePath));
// Find the main camera
Camera camera = Camera.main;
if (camera == null)
{
camera = Object.FindFirstObjectByType<Camera>();
}
if (camera == null)
{
return MCPForUnity.Editor.Helpers.Response.Error("No camera found in the scene");
}
// Create a RenderTexture
RenderTexture rt = new RenderTexture(Screen.width, Screen.height, 24);
camera.targetTexture = rt;
// Render the camera's view
camera.Render();
// Read pixels from the RenderTexture
RenderTexture.active = rt;
Texture2D screenshot = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);
screenshot.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
screenshot.Apply();
// Clean up
camera.targetTexture = null;
RenderTexture.active = null;
Object.DestroyImmediate(rt);
// Save to file
byte[] bytes = screenshot.EncodeToPNG();
File.WriteAllBytes(absolutePath, bytes);
Object.DestroyImmediate(screenshot);
return MCPForUnity.Editor.Helpers.Response.Success($"Screenshot saved to {absolutePath}", new
{
path = absolutePath,
});
}
catch (System.Exception ex)
{
return MCPForUnity.Editor.Helpers.Response.Error($"Failed to capture screenshot: {ex.Message}");
}
}
}
}
```
### Rebuild and Test
1. Open the MCP for Unity window
2. Click **Rebuild Server**
3. Test your tool from your MCP client!
---
# Part 2: How It Works (Technical Details)
This section explains the technical implementation of the custom tools system.
## Python Side: Decorator System
### The `@mcp_for_unity_tool` Decorator
The decorator automatically registers your function as an MCP tool:
```python
@mcp_for_unity_tool(
name="custom_name", # Optional: function name used by default
description="Tool description", # Required: describe what the tool does
)
```
**How it works:**
- Auto-generates the tool name from the function name (e.g., `my_custom_tool`)
- Registers the tool with FastMCP during module import
- Supports all FastMCP `mcp.tool` decorator options: <https://gofastmcp.com/servers/tools#tools>
**Note:** All tools should have the `description` field. It's not strictly required, however, that parameter is the best place to define a description so that most MCP clients can read it. See [issue #289](https://github.com/CoplayDev/unity-mcp/issues/289).
### Auto-Discovery
Python tools are automatically discovered when:
- The Python file is added to a `PythonToolsAsset`
- The file is synced to `MCPForUnity/UnityMcpServer~/src/tools/custom/`
- The file is imported during server startup
- The decorator `@mcp_for_unity_tool` is used
### Sync System
The `PythonToolsAsset` system automatically syncs your Python files:
**When sync happens:**
- ✅ Unity starts up
- ✅ Python files are modified
- ✅ Python files are added/removed from the asset
**Manual controls:**
- **Sync Now:** Window > MCP For Unity > Tool Sync > Sync Python Tools
- **Toggle Auto-Sync:** Window > MCP For Unity > Tool Sync > Auto-Sync Python Tools
- **Reimport Python Files:** Window > MCP For Unity > Tool Sync > Reimport Python Files
**How it works:**
- Uses content hashing to detect changes (only syncs modified files)
- Files are copied to `MCPForUnity/UnityMcpServer~/src/tools/custom/`
- Stale files are automatically cleaned up
## C# Side: Attribute System
### The `[McpForUnityTool]` Attribute
The attribute marks your class as a tool handler:
```csharp
// Explicit command name
[McpForUnityTool("my_custom_tool")]
public static class MyCustomTool { }
// Auto-generated from class name (MyCustomTool → my_custom_tool)
[McpForUnityTool]
public static class MyCustomTool { }
```
### Auto-Discovery
C# handlers are automatically discovered when:
- The class has the `[McpForUnityTool]` attribute
- The class has a `public static HandleCommand(JObject)` method
- Unity loads the assembly containing the class
**How it works:**
- Unity scans all assemblies on startup
- Finds classes with `[McpForUnityTool]` attribute
- Registers them in the command registry
- Routes MCP commands to the appropriate handler
## Best Practices
### Python
- ✅ Use type hints with `Annotated` for parameter documentation
- ✅ Return `dict[str, Any]` with `{"success": bool, "message": str, "data": Any}`
- ✅ Use `ctx.info()` for logging
- ✅ Handle errors gracefully and return structured error responses
- ✅ Use `send_command_with_retry()` for Unity communication
### C#
- ✅ Use the `Response.Success()` and `Response.Error()` helper methods
- ✅ Validate input parameters before processing
- ✅ Use `@params["key"]?.ToObject<Type>()` for safe type conversion
- ✅ Return structured responses with meaningful data
- ✅ Handle exceptions and return error responses
## Debugging
### Python
- Check server logs: `~/Library/Application Support/UnityMCP/Logs/unity_mcp_server.log`
- Look for: `"Registered X MCP tools"` message on startup
- Use `ctx.info()` for debugging messages
### C#
- Check Unity Console for: `"MCP-FOR-UNITY: Auto-discovered X tools"` message
- Look for warnings about missing `HandleCommand` methods
- Use `Debug.Log()` in your handler for debugging
## Troubleshooting
**Tool not appearing:**
- **Python:**
- Ensure the `.py` file is added to a `PythonToolsAsset`
- Check Unity Console for sync messages: "Python tools synced: X copied"
- Verify file was synced to `UnityMcpServer~/src/tools/custom/`
- Try manual sync: Window > MCP For Unity > Tool Sync > Sync Python Tools
- Rebuild the server in the MCP for Unity window
- **C#:**
- Ensure the class has `[McpForUnityTool]` attribute
- Ensure the class has a `public static HandleCommand(JObject)` method
- Check Unity Console for: "MCP-FOR-UNITY: Auto-discovered X tools"
**Python files not showing in Inspector:**
- Go to **Window > MCP For Unity > Tool Sync > Reimport Python Files**
- This forces Unity to recognize `.py` files as TextAssets
- Check that `.py.meta` files show `ScriptedImporter` (not `DefaultImporter`)
**Sync not working:**
- Check if auto-sync is enabled: Window > MCP For Unity > Tool Sync > Auto-Sync Python Tools
- Look for errors in Unity Console
- Verify `PythonToolsAsset` has the correct files added
**Name conflicts:**
- Use explicit names in decorators/attributes to avoid conflicts
- Check registered tools: `CommandRegistry.GetAllCommandNames()` in C#
**Tool not being called:**
- Verify the command name matches between Python and C#
- Check that parameters are being passed correctly
- Look for errors in logs
