MCP for Unity - Developer Guide

Contributing

Branch off beta to create PRs. The main branch is reserved for stable releases.

Before proposing major new features, please reach out to discuss - someone may already be working on it or it may have been considered previously. Open an issue or discussion to coordinate.

Local Development Setup

1. Point Unity to Your Local Server

For the fastest iteration when working on the Python server:

1. Open Unity and go to Window > MCP for Unity
2. Open Settings > Advanced Settings
3. Set Server Source Override to your local Server/ directory path
4. Enable Dev Mode (Force fresh server install) - this adds --refresh to uvx commands so your changes are picked up on every server start

2. Switch Package Sources

You may want to use the mcp_source.py script to quickly switch your Unity project between different MCP package sources [allows you to quickly point your personal project to your local or remote unity-mcp repo, or the live upstream (Coplay) versions of the unity-mcp package]:

python mcp_source.py

Options:

1. Upstream main - stable release (CoplayDev/unity-mcp)
2. Upstream beta - development branch (CoplayDev/unity-mcp#beta)
3. Remote branch - your fork's current branch
4. Local workspace - file: URL to your local MCPForUnity folder

After switching, open Package Manager in Unity and Refresh to re-resolve packages.

Tool Selection & the Meta-Tool

MCP for Unity organizes tools into groups (Core, VFX & Shaders, Animation, UI Toolkit, Scripting Extensions, Testing). You can selectively enable or disable tools to control which capabilities are exposed to AI clients — reducing context window usage and focusing the AI on relevant tools.

Using the Tools Tab in the Editor

Open Window > MCP for Unity and switch to the Tools tab. Each tool group is displayed as a collapsible foldout with:

• Per-tool toggles — click individual tool toggles to enable or disable them.
• Group checkbox — a checkbox embedded directly in each group's foldout header (next to the group title) enables or disables all tools in that group at once without expanding or collapsing the foldout.
• Enable All / Disable All — global buttons to toggle all tools.
• Rescan — re-discovers tools from assemblies (useful after adding new [McpForUnityTool] classes).
• Reconfigure Clients — re-registers tools with the server and reconfigures all detected MCP clients in one click, applying your changes without navigating back to the Clients tab.

How Changes Propagate

Tool visibility changes work differently depending on the transport mode:

HTTP mode (recommended):

1. Toggling a tool calls ReregisterToolsAsync(), which sends the updated enabled tool list to the Python server over WebSocket.
2. The server updates its internal tool visibility via mcp.enable()/mcp.disable() per group.
3. The server sends a tools/list_changed MCP notification to all connected client sessions.
4. Already-connected clients (Claude Desktop, VS Code, etc.) automatically receive the updated tool list.

Stdio mode:

1. Toggles are persisted locally but cannot be pushed to the server (no WebSocket connection).
2. The server starts with all groups enabled. After changing toggles, ask the AI to run manage_tools with action='sync' — this pulls the current tool states from Unity and syncs server visibility.
3. Alternatively, restart the server to pick up changes.

The manage_tools Meta-Tool

The server exposes a built-in manage_tools tool (always visible, not group-gated) that AIs can call directly:

When You Need to Reconfigure

After toggling tools on/off, MCP clients need to learn about the changes:

• HTTP mode: Changes propagate automatically via tools/list_changed. Most clients pick this up immediately. If a client doesn't, click Reconfigure Clients on the Tools tab, or go to Clients tab and click Configure.
• Stdio mode: The server process needs to be told about changes. Either ask the AI to call manage_tools(action='sync'), or restart the MCP session. Click Reconfigure Clients to re-register all clients with updated config.

Running Tests

All major new features (and some minor ones) must include test coverage. It's so easy to get LLMs to write tests, ya gotta do it!

Python Tests

Located in Server/tests/:

cd Server
uv run pytest tests/ -v

Unity C# Tests

Located in TestProjects/UnityMCPTests/Assets/Tests/.

Using the CLI (requires Unity running with MCP bridge connected):

cd Server

# Run EditMode tests (default)
uv run python -m cli.main editor tests

# Run PlayMode tests
uv run python -m cli.main editor tests --mode PlayMode

# Run async and poll for results (useful for long test runs)
uv run python -m cli.main editor tests --async
uv run python -m cli.main editor poll-test <job_id> --wait 60

# Show only failed tests
uv run python -m cli.main editor tests --failed-only

Using MCP tools directly (from any MCP client):

run_tests(mode="EditMode")
get_test_job(job_id="<id>", wait_timeout=60)

Code Coverage

cd Server
uv run pytest tests/ --cov --cov-report=html
open htmlcov/index.html