Write
A model is one .pet file with named sections: world, params, patches, pets, setup, step. Sliders and readouts are declared in the file, so there is no interface to wire up.
Read the language referenceAgent-based modelling in the browser
Agent-based models
you can read.
MathPets is a small language in the NetLogo tradition: patches on a grid, agents that move across them, links between agents, and rules that run each tick. Most models in the library are under 100 lines.
Runs in the browser. No account needed.
world: x: -60..60 y: -44..44 topology: box params: density = slider(62, 0..100) monitors: burned-trees = count patches where burned burning-trees = count patches where burning living-trees = count patches where tree patches: state: empty | tree | burning | burned = empty setup: patches: state := tree chance density else empty where (tree and px = min-x): state := burning step staged: patches: where burning: state := burned where (tree and any neighbors4 where burning): state := burning stop when (count patches where burning = 0) - step
- 0
- burning
- 0
- trees
- 0
- burned
- 0
The complete Fire model, 32 lines, running beside its source.
Run
Models compile and run in the browser. Each run has a seed, so it can be replayed exactly. Inspect any patch or agent on the board, and change parameters while it runs.
Browse the modelsEdit
Every model in the library opens with its source. Change a rule or a colour, apply it to the running model, and keep your version.
Follow the quickstartFrom the library
Four models, running live
Scroll to switch between them. Each one opens in the studio with its source.
Flocking
Reynolds' boids. Each bird turns away from a neighbour that is too close; otherwise it turns toward its neighbours' average heading and their centre.
No bird leads and nothing is global. Flocks form, split and merge, and leave one edge of the torus to come back on the other.
Open modelAnts
Ants that find food carry it home and drop a chemical on the way. It diffuses and evaporates, and searching ants follow its gradient.
Trails form to the nearest food pile first and fade once it is empty. The three piles are usually emptied in order of distance.
Open modelLenia
Bert Chan's continuous Game of Life: cells hold values between 0 and 1, neighbours are weighted by a ring-shaped kernel, and a smooth growth rule replaces birth and survival.
The seed settles into Orbium, a glider that keeps its shape and crosses the board at about half a cell per tick. Raise sigma from 0.016 to 0.02 and it splits until rings fill the board.
Open modelWolves and sheep
Sheep eat grass, wolves eat sheep, and grass regrows after a delay. Moving costs energy, births are random, and an animal dies when its energy runs out.
The populations cycle, with wolves lagging sheep. Switch model-version to sheep-wolves (no grass) and the cycles usually end in extinction.
Open modelFree course
A short course in
agent-based modelling
6 units, from patch rules to network dynamics. Lessons are narrated over a live model, with exercises that have you run it, tune it and edit its code.
- Narration synced to the model on screen
- Exercises on the board and in the code
- A short checkpoint at the end of each unit
- Units
- 6
- Lessons
- 27
- Self-paced
- ≈ 120 min
Syllabus6 units · 27 lessons
1A world of patchesGrids, state, and neighbours
22m2Rules that make patternsConway's Life, read straight off the code
18m3Pets that moveFlocking: three rules, no leader
22m4Food chains and life cyclesWolves, sheep, energy, die and hatch
23m5NetworksLinks, contagion, and graph structure
22m6Network dynamicsLinks, flow, and collaboration
16mDocumentation
The language reference, a quickstart that builds Fire and Ants line by line, and the physics, sound and X libraries.
Read the docsConnect an AI
Connect an MCP client such as Claude Code to the model you have open. It can search the docs, render previews and edit the draft, and its edits are highlighted as they land.
Set up a connection
Open-source core
The parser, compiler, deterministic runtime and VS Code extension are Apache-2.0 on GitHub. You can compile and run model files without this site.
Read about the core