Sims3-MCP

Let your favorite AI play The Sims 3.

Sims3-MCP is an MCP server that connects an AI assistant (Claude Code, Codex, Claude Desktop, or any other MCP client) to a running copy of The Sims 3. The AI can look at your household, check your Sims' needs, click things the same way you would ("Have Quick Meal → Cereal", "Call Sim → Chat"), run the clock, answer pop-up dialogs, build skills and careers, shop in buy mode, and run cheats.

"My Sim is exhausted and needs the bathroom. Take care of her, then find her a job in the Medical career."

It was inspired by pevers/sims-mcp for The Sims 1.

What it can do

Related MCP server: unreal-engine-mcp

How it works (the "mailbox")

The Sims 3 has no API that outside programs can talk to, and its mod sandbox blocks network and file access. So the two sides talk through a shared mailbox in the game's memory:

 Your AI ──MCP──> Python server ──(writes request into game memory)──┐
                                                                      ▼
                         The Sims 3:  [ MAILBOX  request → | ← response ]
                                                                      ▲
                       Sims3Mcp script mod ──(checks it every game tick, runs the request
                                              with the game's own functions, writes the answer)

1. A small script mod (Sims3Mcp.package) reserves a 1 MB block of memory inside the game and stamps a unique label on it.

2. The Python MCP server finds that block by scanning the game's memory (ReadProcessMemory / WriteProcessMemory, the same calls trainers use).

3. Python drops a JSON request into the mailbox. On the next game tick the mod picks it up and runs it using the game's own functions, the same ones the UI uses when you click something. It then writes the answer back.

Because the mod goes through the real game logic, everything behaves as if a player did it: the UI updates, events fire, and saves stay safe. There's no DLL injection and no hard-coded memory addresses. If you restart the game or load a different save, the server finds the new mailbox by itself.

Requirements

• Windows, with The Sims 3 on patch 1.67 or 1.69 (Steam, EA app, Origin or disc). It works with just the base game.

• Python 3.10+

• .NET Framework 4.x, which comes with Windows. It provides csc.exe for compiling the mod.

Installation

1. Get the code and install the Python server

git clone https://github.com/Critical-Reynolds/Sims3-MCP.git
cd Sims3-MCP
python -m venv .venv
.venv\Scripts\python -m pip install -e .

2. Build and install the mod

The mod is compiled against your own copy of the game, so no EA files are shipped in this repo.

.venv\Scripts\python tools\extract_refs.py    # reads the game's script DLLs (doesn't modify the game)
.venv\Scripts\python tools\build_mod.py       # compiles the mod -> build\Sims3Mcp.package
.venv\Scripts\python tools\install_mod.py     # copies it into Documents\Electronic Arts\The Sims 3\Mods

Tip: If your game isn't in C:\Program Files\EA Games\The Sims 3, run tools\extract_refs.py --game "D:\Path\To\The Sims 3".

Tip: You need to have started the game at least once, so its Documents folder exists.

3. Clear the script cache (important!)

With the game closed, delete:

Documents\Electronic Arts\The Sims 3\scriptCache.package

The game caches its list of scripts and may ignore a new mod until you do this. Do it again every time you update the mod.

4. Start the game

Load a save. After a few seconds you should see the notification "Sims3MCP 0.1.0 connected".

Add it to your favorite AI

In every case the server is just this command:

<path-to>\Sims3-MCP\.venv\Scripts\python.exe -m sims3_mcp

Replace C:\path\to\Sims3-MCP below with wherever you cloned the repo.

Claude Code

One command, and it's available in every project:

claude mcp add sims3 --scope user -- C:\path\to\Sims3-MCP\.venv\Scripts\python.exe -m sims3_mcp

Then start claude and ask "What's my Sim doing right now?". Run /mcp inside Claude Code to check the connection.

Codex (OpenAI Codex CLI)

codex mcp add sims3 -- C:\path\to\Sims3-MCP\.venv\Scripts\python.exe -m sims3_mcp

Or add it by hand to %USERPROFILE%\.codex\config.toml:

[mcp_servers.sims3]
command = 'C:\path\to\Sims3-MCP\.venv\Scripts\python.exe'
args = ["-m", "sims3_mcp"]

Claude Desktop

Open Settings → Developer → Edit Config and add this to claude_desktop_config.json, then restart Claude Desktop:

{
  "mcpServers": {
    "sims3": {
      "command": "C:\\path\\to\\Sims3-MCP\\.venv\\Scripts\\python.exe",
      "args": ["-m", "sims3_mcp"]
    }
  }
}

Anything else (Cursor, VS Code, Gemini CLI, …)

Any MCP client that supports stdio servers works. Point it at the command above.

Things to try

• "Look at my household and tell me what everyone needs."

• "My Sim is hungry. Make her something from the fridge, then let time run until she's done."

• "Find a job for her in the Medical career."

• "Have her call a friend and chat, then show me her relationships."

• "Buy a cheap bookshelf and put it in the living room."

• "Put a lamp on the desk, then build a small 4×4 room behind the house and auto-roof it."

• "Run motherlode, then save the game as 'AI Run'."

The AI is told to change your game only when you ask. Still, back up your saves before experimenting!

Tool list

That's 49 tools in all.

Targets for list_interactions / do_interaction can be:

• an object id

• a Sim's name

• lot:<id> (travel)

• home

• terrain

• self

Troubleshooting

Development

• python -m sims3_mcp.cli look: send one raw command to the mod and print the JSON.

• pip install -e .[dev] then pytest: unit tests for the package format and the mailbox protocol (uses a simulated game).

• tools\decompile.sh decompiles the game's assemblies into build\src with ILSpy, for finding game APIs.

• Mod source is in mod\src (C#, compiled for the game's .NET 2.0 Mono). Server source is in server\sims3_mcp.

Things learned the hard way

• Script assemblies (S3SA) are XOR-chained. Each byte is XORed with table[seed], and seed advances by the ciphertext byte. The table is not modified while decoding. s3pe writes an all-zero table, which means the assembly is stored as plaintext.

• The game only tunes assemblies marked [assembly: Tunable]. Tuning is what runs a mod's [Tunable] static constructor.

• Mono's heap is PAGE_EXECUTE_READWRITE. Memory scans must include those pages.

• A modal dialog puts the task that opened it to sleep until it closes. Anything that might open one runs on its own task, never on the mailbox polling task.

License & disclaimer

MIT. See LICENSE.

This is a fan project and is not affiliated with or endorsed by Electronic Arts or Maxis. The Sims is a trademark of Electronic Arts Inc. No game files are included in this repository; the mod is built against your own installed copy.