Simon Willison Builds a Browser RTS Mainly With Claude Opus 5.5

A Command & Conquer style game, built mainly by Claude Opus 5.5.

@simonw built Battle Drones, a Command & Conquer style RTS that runs in the browser. He built it mainly using Claude Opus 5.5, and it originally included device-to-device multiplayer over an audio link.

LLMs can play it too.

Besides human against in-game AI and human against human, Battle Drones supports two LLMs playing each other. The game pauses every 5s so they can think about the game state and queue up commands. He shared a replay of Haiku 5.5 against GPT-6 Luna.

Claude generated all of the 3D models, in Three.js. A model explorer lists them with their animation states. His favorite is the giant “Goliath” stomping mech.

One subscription, one month.

Asked how many Claude Code Max 200 subscriptions it would take to build in a week, @simonw said two should do it. He used one, spread over a month of prompting.

From audio link to online play.

The audio multiplayer came with a warning. It plays a high-pitched noise throughout the game, “annoying at best, and at worst might give you an actual headache.” Natalie asked: “why do my teeth hurt?”

That bug inspired a fix. @simonw added proper online multiplayer through a tiny @flydotio hosted WebSockets relay server. Now a player can start an online game and send a friend a link to join.