ck3agent

Let Claude play Crusader Kings III. A library loaded into the game exposes its state to Claude through MCP, the protocol Claude uses to call tools. Claude sets a ruler's strategy. The game's own AI carries it out.

What you get

• Watch Claude rule. Load an observer save; Claude takes a ruler, plays it and explains each move. You interrupt and redirect it in plain language.

• Play against Claude. Load your own campaign; Claude takes a rival or a neighbour while you keep playing your character. Claude can never take your character.

• Run it unattended. ck3agent run plays a goal for N cycles and records a transcript and a manifest.

• Several rulers at once. One Claude, one session, one posture per ruler; the same game clock.

Related MCP server: ck3-strategy-advisor

What you need

• Linux x86_64 with glibc 2.34 or newer: Ubuntu 22.04 or later, Debian 12 or later, Fedora (tested on 44), Arch.

• Crusader Kings III 1.18.3.1, the native Linux build from Steam.

• Python 3.11 or newer.

• Claude Code and a Claude plan or API key. Every round of play (a cycle: observe, decide, act, let a game month pass) is a paid model call, about $0.20 to $2 at API prices depending on how much Claude reads; an evening of play is a few dollars, and a subscription plan counts it against your usage limit.

Will it work for me?

Install

sha256sum -c SHA256SUMS --ignore-missing     # in the download directory
tar xzf ck3agent-2026.12.6-linux-x86_64.tar.gz && cd ck3agent-2026.12.6-linux-x86_64
./install.sh                                 # installs the bridge and the ck3agent command

Before you launch: have a non-Ironman single-player save ready. ck3agent launch gives the game its own user-data directory, ~/.local/state/ck3agent/game/ (ck3agent config prints it): the save is copied there, and the game's saves and autosaves land there. Your own ~/.local/share/Paradox Interactive/Crusader Kings III/ is read, never written. Claude cannot play your character: use an observer save (start a new game, choose Observe, save), or let Claude take an AI ruler in your own campaign. Getting started covers both.

ck3agent launch --continue      # the save CK3 played last, copied; --save PATH loads a specific one
cd ~/ck3agent-play && claude

Documentation

Help and contributing

Something broke: troubleshooting, then an issue; the forms ask for ck3agent doctor output. Questions and show-and-tell: Discussions. Security problems: SECURITY.md, never a public issue.

Contributions are welcome under the DCO: CONTRIBUTING.md, CODE_OF_CONDUCT.md. Windows and macOS: port plan. Built on Zydis, RapidJSON, stb_image and the MCP Python SDK (THIRD_PARTY_LICENSES.md).

Status and license

Alpha, versioned by date (CHANGELOG.md). Each release supports one game build and refuses all others. Code Apache-2.0, docs CC BY 4.0, third party THIRD_PARTY_LICENSES.md, SECURITY.md, CITATION.cff.

Crusader Kings® and Paradox Interactive® are registered trademarks of Paradox Interactive AB; Claude is a trademark of Anthropic, PBC. ck3agent is not affiliated with or endorsed by either. It ships no game assets and no game code beyond the short byte patterns in re/offsets/signatures.h that locate functions at load; it works by reading and writing the memory of a copy of the game you own, with offsets found by reverse engineering its Linux binary. Paradox's EULA and Steam's terms are between you and them: single-player only, never Ironman, achievement or multiplayer games.