The SimCity (1989) codebase was open sourced by EA under the name "Micropolis" as part of the OLPC (One Laptop Per Child) project. It's a C codebase, but large parts of it were ported from Assembly.

I wanted to push how long I can run the current generation of LLMs unattended if they are working with a clear spec and verifiable tasks, so I decided to port it to TypeScript and make it run in the browser.

I used exclusively gpt-5.3-codex with thinking set to high. After 4 days of work (most of it just letting Codex run unattended), this is what I ended up with.

My main innovation was creating a harness that calls the C code and uses fast-check to assert that my code behaves the same as the source for arbitray input, allowing the model to work unattended until it reaches a complete solution.

Surprisingly this got a lot of attention on Twitter, including

• Garry Tan - https://x.com/garrytan/status/2021241956806922370
• Greg Brockman - https://x.com/gdb/status/2021272681237361027
• Sriram Krishnan - https://x.com/sriramk/status/2021268764923244550

So I decided to publish the codebase.

Originally my goal was to port the game and then make it multiplayer (humans and LLMs) as a fun/art project. I might still do that in the future, but I'll freeze this codebase as a reference. PRs likely won't be accepted.

• Node.js >= 24
• pnpm >= 10.28 (Corepack recommended)

corepack enable
pnpm install
pnpm dev

• apps/web: Vite + React app using TanStack Router (file-based routes live in src/routes)
• packages: Ports of the core Micropolis engine, tiles, harness, etc

pnpm dev
pnpm build
pnpm lint
pnpm typecheck
pnpm test
pnpm format

The repo includes a Codex-driven orchestrator at scripts/auto-orchestrator.mjs. I change it for each long-running task, so it just contains whatever made sense for the last iteration.

Inspect queue + drift:

pnpm auto:queue
pnpm auto:drift

Run unattended (default runtime cap: 24 hours):

pnpm auto:run -- --max-runtime-minutes 1440

Useful flags:

• --once: complete exactly one task and stop.
• --dry-run: plan/selection only; no git push or PR changes.
• --streams <id,...>: run only specific stage stream ids (stage-0 ... stage-4).
• --no-tests: remove pnpm test from --checks if present.
• --max-retries-per-task <n>: retries before a task is marked blocked.
• --model <name>: pass a specific model to codex exec.

The orchestrator stores state/logs in .automation/. Create .automation/STOP to halt the loop after the current iteration.

Remote caching is ready to wire up when you want it:

pnpm dlx turbo login
pnpm dlx turbo link

Then set TURBO_TOKEN and TURBO_TEAM in CI secrets.

This project is a modified version of the Micropolis source released by Electronic Arts under the GNU General Public License v3 (or later). The full GPLv3 text is in LICENSE. The original Micropolis license notice and additional terms (including trademark restrictions) are preserved in ref/micropolis/README.