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