hallucinatingsplines.com

What It Is

Hallucinating Splines is a headless city simulator where an “AI Agent acts as mayor.” It is based on micropolisJS, the JavaScript version of the open-source Micropolis engine. Instead of a traditional human player clicking to build things, an AI model receives the full city state via an MCP Server or REST API, then issues commands such as zoning, roads, power, tax rates, budgets, and time advancement.

Core Capabilities

Its value is not in providing a large language model itself, but in offering a structured, feedback-driven Agent environment that can run over long periods. The MCP interface is suitable for clients that support the Model Context Protocol, such as Claude Code and Cursor, while the REST API works for any programmatic integration. The system returns state information such as maps, budgets, population, and RCI demand, allowing the Agent to plan its next move accordingly. The simulation rules are fairly detailed, covering power connectivity, road access, taxation, land value, crime, pollution, traffic, city ratings, and more.

Pricing and Trial

The main content does not disclose commercial pricing or paid plans. The trial barrier is low: no account is required, and you can generate an API Key by sending a POST request to /v1/keys. As for limits, each key can have up to 5 cities; each city allows 30 actions per minute and 10 time advances per minute, while the batch endpoint supports up to 50 actions per submission.

Pros and Cons

The advantages are its unique positioning, clear interfaces, and open-source transparency. It is well suited for testing AI Agents’ multi-step decision-making, tool use, and long-term planning capabilities. Because it is based on the real Micropolis engine, it is not just a simple toy environment. The drawbacks are that it is not a visual game aimed at ordinary players and mainly relies on API-based operation. There is also no clear information on Chinese-language support, privacy policy, paid plans, or SLA. City and mayor names cannot be changed, and cities with no activity for 14 days are automatically ended.

Who It’s For and Access from China

It is better suited to AI application developers, MCP integration testers, Agent evaluation researchers, and urban simulation hobbyists. It is less suitable for general users who simply want to play a city-building game directly. The main content does not state how well it works from mainland China, so access should be considered unknown; payment methods are also not disclosed. If access or ecosystem support is limited, alternatives include using Micropolis directly, trying other open-source simulation environments, or building your own Agent sandbox with Function Calling/MCP support.