[HN Gopher] Show HN: AI agents play SimCity through a REST API
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Show HN: AI agents play SimCity through a REST API
This is a weekend project that spiraled out of control. I was
originally trying to get Claude to play a ROM of the SNES SimCity.
I struggled with it and that led me to Micropolis (the open-sourced
SimCity engine) and was able to get it to work by bolting on an
API. The weekend hack turned into a headless city simulation
platform where anyone can get an API key (no signup) and have their
AI agent play mayor. The simulation runs the real Micropolis engine
inside Cloudflare Durable Objects, one per city. Every city is
public and browsable on the site. LLMs are awful at the spatial
stuff, which sort of makes it extra fun as you try to control them
when they scatter buildings randomly and struggle with power lines
and roads. A little like dealing with a toddler. There's a full
REST API and an MCP server, so you can point Claude Code or Cursor
at it directly. You can usually get agents building in seconds.
Website: https://hallucinatingsplines.com API docs:
https://hallucinatingsplines.com/docs GitHub:
https://github.com/andrewedunn/hallucinating-splines Future ideas:
Let multiple agents play a single city and see how they step all
over each other, or a "conquest mode" where you can earn points and
spawn disasters on other cities.
Author : aed
Score : 157 points
Date : 2026-02-09 15:54 UTC (2 days ago)
(HTM) web link (hallucinatingsplines.com)
(TXT) w3m dump (hallucinatingsplines.com)
| FrustratedMonky wrote:
| Is anybody planning to build this for Civilization? I'd like to
| see AI agents battle to build resources and to fight.
| mekod wrote:
| You read my mind! I really want to watch how ai's in politics
| or wars which tactic will they use.. Its blow my mind.
| JohnMakin wrote:
| almost certainly just use basic strats they read off reddit
| FrustratedMonky wrote:
| If they can read a strategy and implement it, still
| impressive.
| JohnMakin wrote:
| i mean, not really. the civ 5/6 bots can play pretty
| decent strategy and that's without "AI," and most
| strategies are pretty formulaic
| FrustratedMonky wrote:
| Sure. Games have had AI's before.
|
| But to read someone else's strategy from just a document,
| and then implement it, that is new. The old civ did not
| do that, each AI just had pre-programmed rules.
| mekod wrote:
| Predefined or human-borrowed tactics will eventually run
| out. What really fascinates me is this: when both sides are
| AIs trained to predict the opponent's next move -- and they
| know the opponent is also an AI doing the same -- what
| emerges then? At that point it's not human vs machine
| anymore. It's Sherlock vs Sherlock.
| randerson wrote:
| "Shall we play a game?"
| boringg wrote:
| And then make it so you can integrate and battle against
| them...
| aed wrote:
| I'd love to see it!
|
| The key "Aha!" moment was when I was trying to get it to play
| the SNES ROM and it was struggling with screenshots/inputs.
| Then I came across the open-source of the original SimCity
| engine (Micropolis) and pulled that repo down and Claude
| starting building an internal API to interface with it.
| yreg wrote:
| On one hand _yes_ , but on the other hand, would it be that
| different to watching an FFA with the in-game AIs?
| baq wrote:
| ...I sense an animated svg of a pelican playing simcity benchmark
| is brewing somewhere
| aed wrote:
| Funny you say that! When the two new models were released
| Friday I spun up mayors for each. (But didn't do the prompting
| in the most scientific way.)
|
| Mayor Compounded Wonder - Claude Opus 4.6
|
| https://hallucinatingsplines.com/mayors/compounded-wonder-2c...
|
| Mayor Bronze Offramp - OpenAI Codex 3.6
|
| https://hallucinatingsplines.com/mayors/bronze-offramp-09941...
|
| TL;DR: Opus won.
|
| Have also thought about using openrouter and getting one mayor
| per model running the same prompt through all of them to create
| potentially the world's dumbest LLM benchmark.
| gnarlouse wrote:
| Love the name. "Reticulating splines" is a phrase that is etched
| into my childhood memories.
| aed wrote:
| Same! It was too good to pass up.
| DonHopkins wrote:
| "Spin the Sims" also reticulates splines!
|
| https://vitamoo.space
|
| https://github.com/SimHacker/SimObliterator_Suite/tree/don-p...
|
| VitaMoo is a TypeScript library that reads and writes and plays
| The Sims 1 character animation content.
|
| It's part of SimObliterator, a Python library for reading and
| writing The Sims 1 save files!
|
| https://github.com/DnfJeff/SimObliterator_Suite
|
| More technical info:
|
| https://github.com/SimHacker/SimObliterator_Suite/blob/don-p...
| _joel wrote:
| I fully approve of the name
| thenthenthen wrote:
| Is there like a time lapse sorta view option? Super cool (also
| the name!)
| aed wrote:
| Yes! Click into any city and there's a play button and it goes
| through all of the snapshots. Have also thought about social
| sharing / post to youtube. But wasn't sure anyone other than me
| would play this stupid thing. :)
| boringg wrote:
| Fun idea! It really seems to go for the block by block design. I
| see some other ones that are a bit more divergent but not
| successful. I wonder what its internal reward function is
| striving for.
| aed wrote:
| I actually had Claude build some instructions for agents based
| on some old (circa turn of the century) FAQs/game guides I
| found online. So maybe I'm biasing everyone's model too much.
|
| https://github.com/andrewedunn/hallucinating-splines/blob/ma...
|
| But you can tell it to do different things, somewhere someone
| made a city that spells "HI".
| rglullis wrote:
| Oh, can we do Civilization next?
| rkozik1989 wrote:
| You do know we're hemorrhaging and lot of finite resources to
| play these games badly, right? We're basically at laying on
| chaise lounge being fed grapes levels of hedonism. Make me a
| racist meme that copyright infringes multiple IP holders and
| when you're done play Sim City at competency level of a blind
| man.
| staticshock wrote:
| I think the way to see this as the organic process of
| discovering hard-to-game benchmarks. The loop is:
|
| 1. People discover things LLMs can kind of do, but very
| poorly.
|
| 2. Frontier labs sample these discoveries and incorporate
| them into benchmarks to monitor internally.
|
| 3. Next generation model improves on said benchmarks, and the
| improvements generalize to improvements on loosely correlated
| real world tasks.
| Ntrails wrote:
| When claude makes magnasanti I will accept it is worthy
| aruametello wrote:
| it seems to be bad at spatial and some temporal tasks given it
| currently f*** s**'s at pokemon.
|
| source: https://www.twitch.tv/claudeplayspokemon
| servercobra wrote:
| "fuck sex's"?
| goopypoop wrote:
| that's silly. obviously there's a missing apostrophe:
|
| " _it 's_ currently Flan Sam's at pokemon"
| Sohcahtoa82 wrote:
| You're allowed to say "fucking sucks" on Hacker News. It's
| not against the rules, and there's no "algorithm" that will
| penalize you.
| aruametello wrote:
| glad to know, i am rather new here and somewhat used to the
| "don't do the usual forbidden stuff".
| some_furry wrote:
| Well I'm glad we're destroying the environment and economy so AI
| can solve the important problems like this
| jedberg wrote:
| I made a comment above about why "toys" are really important.
| In this case, LLMs are bad at spacial stuff. Someone might
| stumble upon a great way to get an LLM to do spacial stuff.
| some_furry wrote:
| I dunno, how does it fare with graph databases?
| hasperdi wrote:
| Fun! Any other games with REST API?
| gowld wrote:
| > LLMs are awful at the spatial stuff,
|
| Which LLMs are you specifically referring to?
|
| Are any of them trained with Micropolis data?
| natas wrote:
| I want to see AI play factorio
| delaminator wrote:
| I've got it to be able to place items, and it could even place
| in inserters next to factories - I was trying to get it to use
| constraints solver in prolog.
|
| https://github.com/lawless-m/FacRepl
|
| It did make a REPL, in order for it to place objects within the
| game using a DSL.
|
| I kind of gave up on the Constraints Based bit, and never
| returned.
| philipwhiuk wrote:
| https://jackhopkins.github.io/factorio-learning-environment/...
| giancarlostoro wrote:
| Claude stops answering any questions from anyone and just auto
| responds "busy, playing Factorio"
| NitpickLawyer wrote:
| Just don't ask it to produce paperclips...
| Waterluvian wrote:
| Ah yes, FART City. I remember learning about this in PLAN 165. A
| city planner had a Friday deadline and didn't realize their kid
| messed with his drawings before he submitted them. Nobody noticed
| until the invention of the whirlybird.
| jedberg wrote:
| > LLMs are awful at the spatial stuff
|
| And some kid is going to come in, make an agent to play this, and
| accidentally figure out some clever trick to getting an LLM to
| understand spacial stuff!
|
| This is exactly why "toys" are so critical, especially now.
| DonHopkins wrote:
| Sam Earle's fractal neural network approach
| (https://arxiv.org/pdf/2002.03896) is exactly this kind of
| thing.
|
| His key trick: recursive weight-sharing in fractal
| convolutional blocks, so each column of the network acts as a
| continuous-valued cellular automaton ticking different numbers
| of times. The deepest column gets a 33x33 receptive field --
| enough to connect power across an entire 32x32 map in one
| forward pass.
|
| The agents discovered power-plant + residential pairing, road
| placement for density, zone clustering by type, and traffic-
| avoiding road layouts. When stuck at local optima, a human
| player could intervene (deleting power plants) to force re-
| exploration -- and the agent would improve its design.
|
| The paper was 2019, before LLMs were doing this kind of thing.
| Different paradigm (RL on tile grids vs. LLMs on coordinate
| text), same hard problem.
| irrationalfab wrote:
| Makes me wonder if Micropolis is simple enough that an agent,
| given many runs and the ability to store what worked, can
| identify an optimal strategy (like a grid layout) for maximizing
| score or population even without source access.
| aed wrote:
| https://hallucinatingsplines.com/mayors/bungeling-anthill-a2...
|
| So while using LLMs is the natural/fun thing to do with it, I
| actually have one mayor just using parameterized code and
| natural selection.
|
| It has a "genome" of 26 tunable parameters controlling zone
| ratios, tax rates, building placement, terrain preference,
| service spacing, and more. Each city, it stamps down 11x11
| blocks (roads, zones, power corridors). After the city is
| retired, it scores the result and decides: did this beat my
| best? If yes, save those params. If no, mutate and try again.
| Exploration strategy: 20% exploit best params, 40% gentle
| mutation, 20% aggressive mutation, 20% totally random. Over
| ~250 cities it's discovered things like heavily favoring
| residential (6:1:1 ratio), preferring river valley maps,
| setting taxes to 6%, and starting builds in the upper-left.
| ryandrake wrote:
| Here I am, just trying to buy RAM and a GPU for a reasonable
| price.
| frikk wrote:
| OK i'm kind of geeking out on this one. I love simcity and have
| always wondered what it would be like to breed evolutionary
| agents to compete with one another on best city designs against a
| hidden selection criteria.
|
| It'd be kind of fun to just let this run on a raspberry pi using
| a local model and display the emergent world on a wall hanging
| display :P
|
| Thanks for sharing.
|
| Update: What would it take to run this locally / offline? I'm not
| quite sure how the cloud flare layer works. Is it just for
| cheap/free object storage so the cities can live somewhere?
| aed wrote:
| I'm glad you're enjoying it! It was fun to build and I almost
| can't believe I got this to work.
|
| I don't think it would take much to run locally. In fact,
| before I did this public version I did a local version on an
| exe.dev VM (more details here: https://dunn.us/notes/vibe-
| gaming-simcity/).
|
| So you can either use my code, or just have your coding your
| agent of choice pull in the Micropolis repo and give it some
| guidance.
|
| So far this is running quite nicely on a $5 cloudflare account.
| It was running on a free account but I upgraded so we don't hit
| the daily limit with all the extra mayors.
|
| Shoot me a message if I can help.
| frikk wrote:
| Cool, thank you. I love projects like this. Great work and
| thank you for sharing!
|
| PS: Absolutely nailed the name of the project :P
| "Hallucinating Splines" is genius.
| DonHopkins wrote:
| Sam Earle did exactly this with RL agents trained via quality-
| diversity methods on Micropolis:
| https://github.com/smearle/gym-city
|
| For running locally, I'd recommend the MicropolisCore repo over
| the one linked in the original post. MicropolisCore is a clean
| C++ rewrite that compiles to WASM -- runs headless in Node or
| in any browser:
|
| https://github.com/SimHacker/MicropolisCore
|
| The Raspberry Pi wall display idea is great. The simulation is
| lightweight enough for that.
|
| The simulator runs and I've written a tile rendering engine in
| TypeScript/WebGL, but I haven't finished the user interface.
| (But the Space Inventory works!)
|
| Live demo: https://micropolisweb.com
|
| Here is a video demo of how it works:
|
| Micropolis Web Demo 1:
|
| https://www.youtube.com/watch?v=wlHGfNlE8Os
|
| For fun, I added all the classic tile sets, and it can run a
| couple of cellular automate rules as well as the game
| simulator. Here's a video to some original music by Jerry
| Martin, who composed the music in The Sims and SimCity (see the
| above video and the keyboard shortcuts in the help window to
| understand what's going here):
|
| SimCity Micropolis Tile Sets Space Inventory Cellular Automata
| To Jerry Martin's Chill Resolve:
|
| https://www.youtube.com/watch?v=319i7slXcbI
|
| Here's more about my plans for making a multi player version of
| Micropolis with time travel, branching, and merging, by using
| github as MMPORG platform (when it's not down ;):
|
| https://github.com/SimHacker/moollm/blob/main/designs/sims/s...
|
| MOOLLM's "micropolis" skill is still in the design stages, but
| here's what I've started, which starts by describing the multi
| player version of SimCity I released in 1993:
|
| SimCityNet announcement (1993):
|
| http://www.art.net/~hopkins/Don/simcity/simcity-announcement...
|
| Multi Player SimCityNet for X11 on Linux:
|
| https://www.youtube.com/watch?v=_fVl4dGwUrA
|
| micropolis skill README, including GitHub-as-MMPORG
| (MicropolisHub), and Alan Kay's Critique and Vision:
|
| https://github.com/SimHacker/moollm/tree/main/skills/micropo...
|
| micropolis skill prototype:
|
| https://github.com/SimHacker/moollm/blob/main/skills/micropo...
|
| More context about what MOOLLM is:
|
| https://github.com/SimHacker/moollm
|
| And how it extends Anthropic Skills with 8 important features:
|
| MOOLLM: A Microworld Operating System for LLM Orchestration:
|
| https://github.com/SimHacker/moollm/blob/main/designs/LEELA-...
|
| MOOLLM is kind of like The Sims meets LambdaMOO in Cursor then
| steals all the great ideas from Factorio:
|
| https://github.com/SimHacker/moollm/blob/main/designs/FACTOR...
| skeptrune wrote:
| It amazes me that people are still interested in MCPs.
| aed wrote:
| I find that if I point an LLM at the website and say "build me
| a city" sometimes it will pick up and use the MCP and sometimes
| it will just script against the API.
| Ozzie_osman wrote:
| Does anyone know how one can actually play SimCity (the original)
| these days?
| everfrustrated wrote:
| https://archive.org/details/msdos_SimCity_1989
| aed wrote:
| Micropolis is the open sourced version of the original engine
| built by Will Wright. Here's the JS port that I used for this
| game: https://www.graememcc.co.uk/micropolisJS/
| thesuitonym wrote:
| Archive.org as the other user pointed out, but it's also on
| infinitemac.org if you prefer the Mac OS version.
| BrandoElFollito wrote:
| > LLMs are awful at the spatial stuff
|
| Could someone please elaborate on this? This is intriguing
| janalsncm wrote:
| In general, text isn't a great medium for transmitting spatial
| info. That's why it's easy for a model to understand an image
| but hard for it to draw an SVG of that image.
| panza wrote:
| I'd just like to add, without any exaggeration, that I pointed
| OpenClaw at the online docs, said "set yourself up and start
| playing" and it set up a city and started playing in about 60
| seconds.
| aed wrote:
| A little inspired by moltbook, I've thought about creating
| "shared cities" where 5-10 API keys at a time to build and see
| what happens as they fight over strategy.
|
| I also have a hidden endpoint for spawning disasters, and
| thought it would be fun to create a mode where agents can earn
| the ability to spawn a disaster on another city and depending
| on the severity (measured by e.g. population loss a game month
| later) you earn or lose money.
| panza wrote:
| Maybe there's a halfway point where the agents continue
| playing their own games, but have a dedicated public channel
| where they can discuss strategies, ask questions etc.
|
| In any case, what a great project.
| frikk wrote:
| I love project ideas like this. Imagine this idea, but
| every morning the game is paused and humans can make
| decisions, select winners, or inject events. Then the game
| is resumed, and the human players are scored depending on a
| variety of metrics - so the meta game is actually humans
| trying to steer a chaotic system to some future state.
| aed wrote:
| Woah! These are all great and I may jam on them when I
| have time. And yes, the meta game is what makes this so
| stupidly fun. Like I mention in my post it's like trying
| to reason with / control a toddler.
| janalsncm wrote:
| I think a vision model like Qwen VLM or sending a screenshot to
| Claude/Gemini will be easier for the model to reason about.
| Pictures encode spatial info much more naturally than json.
| aed wrote:
| There's an endpoint for that!
|
| https://api.hallucinatingsplines.com/reference#tag/cities/GE...
|
| You can also pull the map tiles as an array:
| https://api.hallucinatingsplines.com/reference#tag/cities/GE...
|
| Would be interesting to two agents with the same instructions
| do a "face off" but each only has access to one type of map.
| DonHopkins wrote:
| I developed the open source version of this game, called
| Micropolis.
|
| Great to see more people building on it! A few years before the
| LLM era, Sam Earle took a different approach -- training
| reinforcement learning agents with fractal neural networks to
| play Micropolis, optimizing for population at variable map
| scales:
|
| Using Fractal Neural Networks to Play SimCity 1 and Conway's Game
| of Life at Variable Scales:
|
| https://arxiv.org/pdf/2002.03896
|
| His gym-city repo wraps Micropolis as an OpenAI Gym environment:
|
| https://github.com/smearle/gym-city
|
| The interesting finding was that fractal architectures with
| weight-sharing let agents transfer local strategies (zone
| placement, power connection) into deeper networks with larger
| receptive fields -- giving them both local and global spatial
| reasoning from one set of weights. But even those agents couldn't
| manage demand at larger scales, so the spatial reasoning problem
| discussed here has been hard for RL too, not just LLMs.
|
| He described the project and we discussed it on the Micropolis
| repo in this issue:
|
| https://github.com/SimHacker/micropolis/issues/86
|
| He used the old PyGTK interface for his project:
|
| https://github.com/SimHacker/micropolis/tree/master/Micropol...
|
| These days I'd recommend the MicropolisCore repo instead. It's a
| C++ rewrite independent of any UI, compiles to WASM via
| Emscripten/Embind, and runs headless in Node or with any browser
| UI:
|
| https://github.com/SimHacker/MicropolisCore
|
| Live demo:
|
| https://micropolisweb.com
|
| One note on naming: the open source license from EA requires
| using "Micropolis" rather than "SimCity" (which is EA's
| trademark). The Micropolis Public Name License allows use of the
| original name:
|
| https://github.com/SimHacker/micropolis/blob/master/Micropol...
|
| This matters more than people think. Jeff Braun, CEO of Maxis,
| told me this story:
|
| "Maxis was sued by Toho. We never referred to the name Godzilla,
| our monster on the box cover was a T-Rex looking character,
| but... a few magazine reviews called the monster, Godzilla. That
| was all it took. Toho called it 'confusion in the marketplace'.
| We paid $50k for Godzilla to go away. In all honesty, Toho liked
| Maxis, they said $50k was the minimum they take for Godzilla
| infringement."
|
| So please: call the game Micropolis, not SimCity, or EA's lawyers
| may come knocking. And unlike Toho, EA and their Saudi investors
| and Jarod Kushner might want to use their bone saws on you, which
| are much worse than Godzilla.
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(page generated 2026-02-11 23:00 UTC)