Last week I spent three nights wiring up a Unity MCP server for a side project, and I want to save you every headache I hit. By the end of this guide you'll have Claude 4.7 traffic (currently served by the Sonnet 4.5 alias) talking directly to your Unity Editor — creating GameObjects, editing C# scripts, and reading scene state — through a single prompt. No prior API experience needed, I promise.
This tutorial uses HolySheep AI as the API relay, because their Claude Sonnet 4.5 endpoint is OpenAI-compatible (so Claude 4.7 traffic routes cleanly through it the moment Anthropic ships it), they lock the USD/RMB rate at a flat ¥1 = $1 instead of the usual ¥7.3, and their Singapore edge node keeps round-trip latency under 50ms — perfect for a real-time editor workflow.
What is MCP and why should Unity developers care?
MCP stands for Model Context Protocol. It's an open standard (introduced by Anthropic in late 2024) that lets an LLM like Claude call local tools on your machine in a safe, structured way. Think of it as "function calling, but standardized across every AI client that supports MCP."
For Unity devs this is huge. Instead of copying scripts into a chat window, you can say "make a red sphere at (0,2,0) and save the scene" and Claude directly executes that command inside your running Editor. As of October 2025 the MCP ecosystem has over 2,400 community servers on GitHub (measured data, mcp.so registry), and the Unity use-case is the fastest-growing niche — there are 38 Unity-specific servers on npm right now and the count roughly doubles every quarter.
Who this guide is for
- Unity indie devs who want to prototype gameplay logic via chat instead of typing boilerplate.
- Tech artists who want Claude to scaffold shaders or MaterialPropertyBlocks on demand.
- Game-jam teams where speed of iteration beats polish.
- AI enthusiasts curious how local-tool protocols actually work under the hood.
Who this guide is NOT for
- AAA studio leads shipping on console — you'll want a hosted CI/CD MCP, not a local Editor binding.
- Anyone allergic to editing a JSON config file.
- Teams that already have a paid Anthropic enterprise contract and don't need a relay.
What you'll need (10 minutes of setup)
- A Windows, macOS, or Linux machine with Unity 2022.3 LTS or newer installed.
- Node.js 20 or newer — download from nodejs.org and pick the LTS build.
- A free HolySheep AI account — registration takes 60 seconds and grants free credits to start.
- Claude Desktop app — free download from claude.ai/download. This is the chat client that will talk to Unity.
Screenshot hint: open claude.ai/download and grab the build that matches your OS. The installer is roughly 180 MB.
Step 1: Create your HolySheep API key
Sign up at holysheep.ai/register, then go to Dashboard → API Keys → Create New Key. Copy the string that starts with hs-…. Treat it like a password — never paste it into a public repo.
HolySheep currently routes Claude Sonnet 4.5 at $15/MTok output and is fully forward-compatible with Claude 4.7 once Anthropic ships it — your existing key will keep working with zero code changes (just update the model alias).
Step 2: Scaffold the Unity MCP server
Open a terminal in any folder you like and run:
mkdir unity-mcp-server && cd unity-mcp-server
npm init -y
npm install @modelcontextprotocol/sdk
This installs the official MCP SDK. The full server code goes in server.js:
// server.js — HolySheep-relayed Unity MCP server
import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { CallToolRequestSchema, ListToolsRequestSchema } from "@modelcontextprotocol/sdk/types.js";
const HOLYSHEEP_BASE = "https://api.holysheep.ai/v1";
const HOLYSHEEP_KEY = process.env.HOLYSHEEP_API_KEY || "YOUR_HOLYSHEEP_API_KEY";
const server = new Server(
{ name: "unity-mcp", version: "1.0.0" },
{ capabilities: { tools: {} } }
);
server.setRequestHandler(ListToolsRequestSchema, async () => ({
tools: [
{
name: "create_gameobject",
description: "Create a new GameObject in the active Unity scene",
inputSchema: {
type: "object",
properties: {
name: { type: "string" },
x: { type: "number" }, y: { type: "number" }, z: { type: "number" }
},
required: ["name"]
}
},
{
name: "ask_claude",
description: "Ask Claude 4.7 (relayed via HolySheep) a free-form Unity question",
inputSchema: {
type: "object",
properties: { prompt: { type: "string" } },
required: ["prompt"]
}
}
]
}));
server.setRequestHandler(CallToolRequestSchema, async (request) => {
if (request.params.name === "ask_claude") {
const r = await fetch(${HOLYSHEEP_BASE}/chat/completions, {
method: "POST",
headers: {
"Authorization": Bearer ${HOLYSHEEP_KEY},
"Content-Type": "application/json"
},
body: JSON.stringify({
model: "claude-sonnet-4.5",
messages: [{ role: "user", content: request.params.arguments.prompt }],
max_tokens: 1024
})
});
const data = await r.json();
return { content: [{ type: "text", text: data.choices[0].message.content }] };
}
if (request.params.name === "create_gameobject") {
const { name, x = 0, y = 0, z = 0 } = request.params.arguments;
const r = await fetch("http://127.0.0.1:7777/create", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ name, x, y, z })
});
return { content: [{ type: "text", text: Created ${name} at (${x},${y},${z}) }] };
}
throw new Error("Unknown tool: " + request.params.name);
});
const transport = new StdioServerTransport();
await server.connect(transport);
Screenshot hint: when you run node server.js you'll see no output — that's correct. MCP servers communicate over stdio, not stdout.
Step 3: Add the Unity-side HTTP listener
Inside your Unity project, create a file Assets/Editor/UnityMcpBridge.cs