What is mcp-unity AGENTS.md?

A project guide for AI agents working on the MCP Unity package, describing how its Node.js server and Unity Editor components communicate.

Use it when editing MCP tool or resource names, Unity bridge code, the Node server, connection settings, or main-thread execution behavior.

It provides the key architecture, file locations, defaults, and naming rules needed to change the package without breaking communication between its parts.

How st-VALVe/mcp-unity configures Codex and OpenCode

Reuse

Borrowing it

Nothing to install: this file belongs to st-VALVe/mcp-unity. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/st-VALVe/mcp-unity/main/AGENTS.md

Clone the repo

git clone --depth 1 https://github.com/st-VALVe/mcp-unity

Made for: Codex, OpenCode.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/instructions/st-valve/mcp-unity/agents-md/github.svg)](https://agentmods.dev/instructions/st-valve/mcp-unity/agents-md)

Your own site

<a href="https://agentmods.dev/instructions/st-valve/mcp-unity/agents-md"><img src="https://agentmods.dev/badge/instructions/st-valve/mcp-unity/agents-md/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/instructions/st-valve/mcp-unity/agents-md"><img src="https://agentmods.dev/badge/instructions/st-valve/mcp-unity/agents-md.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 3,162 This file is loaded in full into every session.

When invoked 3,162 The same file — it is already loaded in full.

Security scan A 1 finding. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.03162	$0.03162
Opus 5.5	$0.01265	$0.01265
Sonnet 5.5	$0.00632	$0.00632
Haiku 4.5	$0.00316	$0.00316

Measured 2mo ago against content hash 8e44780c54e2, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

mcp-unity AGENTS.md scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Runs shell commandslowCapability

Expected in a hook, worth knowing in a rule or an instructions file.

Real UIAutomation interaction with native dialogs cannot run in standard headless CI (session 0). Unit tests mock `child_process.spawn` via DI; the live PowerShell path requires a manual run on an interactive Windows run

AGENTS.md · 193 lines

How it starts

The opening of the file, as written. The whole thing — 193 lines — stays where its author put it; the contents beside it link to each section on GitHub.

MCP Unity — AI Agent Guide (MCP Package)

Purpose (what this repo is)

MCP Unity exposes Unity Editor capabilities to MCP-enabled clients by running:

Unity-side “client” (C# Editor scripts): a WebSocket server inside the Unity Editor that executes tools/resources.
Node-side “server” (TypeScript): an MCP stdio server that registers MCP tools/resources and forwards requests to Unity over WebSocket.

How it works (high-level data flow)

MCP client ⇄ (stdio / MCP SDK) ⇄ Node server (Server~/src/index.ts)
Node server ⇄ (WebSocket JSON-RPC-ish) ⇄ Unity Editor (Editor/UnityBridge/McpUnityServer.cs + McpUnitySocketHandler.cs)
Tool/Resource names must match exactly across Node and Unity (typically lower_snake_case).

Key defaults & invariants

Unity WebSocket endpoint: ws://localhost:8090/McpUnity by default.
Config file: ProjectSettings/McpUnitySettings.json (written/read by Unity; read opportunistically by Node).
Execution thread: Tool/resource execution is dispatched via EditorCoroutineUtility and runs on the Unity main thread. Keep synchronous work short; use async patterns for long work.

Repo layout (where to change what)

/
├── Editor/                       # Unity Editor package code (C#)
│   ├── Tools/                    # Tools (inherit McpToolBase)
│   ├── Resources/                # Resources (inherit McpResourceBase)
│   ├── UnityBridge/              # WebSocket server + message routing
│   ├── Services/                 # Test/log services used by tools/resources
│   └── Utils/                    # Shared helpers (config, logging, workspace integration)
├── Server~/                      # Node MCP server (TypeScript, ESM)
│   ├── src/index.ts              # Registers tools/resources/prompts with MCP SDK
│   ├── src/tools/                # MCP tool definitions (zod schema + handler)
│   ├── src/resources/            # MCP resource definitions
│   └── src/unity/mcpUnity.ts      # WebSocket client that talks to Unity
└── server.json                   # MCP registry metadata (name/version/package)

Read the full file on GitHub · 193 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

2mo ago First seen · 193 lines · 3,162 tokens per session scan A 8e44780c54e2

Subscribe to this mod's changes

mcp-unity AGENTS.md is an instructions file published in the GitHub repository st-VALVe/mcp-unity (10 stars, last pushed 5mo ago), licensed MIT. It adds 3,162 tokens to every session, about $0.0126 per session on Opus 5.5. A static security scan graded it A with 1 finding (runs shell commands). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Game development

mcp-unity AGENTS.md next to 6 instructions from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
mcp-unity AGENTS.mdst-VALVe/mcp-unity	Instructions file	10	5mo ago	MIT	A	3,162	Codex, OpenCode
vscode buildNext.instructions.mdmicrosoft/vscode	Instructions file	195k	yesterday	MIT	A	9,719	GitHub Copilot
next.js AGENTS.mdvercel/next.js	Instructions file	143k	yesterday	MIT	A	7,461	Codex, OpenCode
langchain AGENTS.mdlangchain-ai/langchain	Instructions file	149k	yesterday	MIT	A	4,680	Codex, OpenCode
vscode oss-third-party-notices.instructions.mdmicrosoft/vscode	Instructions file	195k	yesterday	MIT	A	5,516	GitHub Copilot
vscode copilot-instructions.mdmicrosoft/vscode	Instructions file	195k	yesterday	MIT	A	3,093	GitHub Copilot
deepseek-harness AGENTS.mddeepseek-ai/deepseek-harness	Instructions file	274k	2d ago	MIT	A	4,313	Codex, OpenCode

mcp-unity: Instructions file for Codex