Watch
It films itself
Leave it alone and a documentary camera takes over. It cuts between hunts, kills, herds and wide vistas, and a narrator speaks when something big happens: a drought settling in, a stampede, a new pack taking a range.
Seasons turn every thirty seconds at normal speed. Grass greens and cures, snow settles in the hollows and melts, and a valley year passes in two minutes.
Follow
One life at a time
Click any animal for its story: where it was born, its pack, its traits set against the founders' and the species' now, a family tree, and an In memoriam card when it dies.
Every animal carries heritable speed, size, sight, boldness, herding, coat and litter genes, and the herds change under the wolves' pressure. A naturalist's notebook keeps the moments you witnessed, dated by valley and year.
Step in
Every change ripples
Send rain or drought, start a fire, loose a plague, cull the herd, bring wolves back or take them away. The grass answers first, then the grazers, then the hunters and the ravens that follow them.
Challenges set you a goal in a named world, from bringing wolves home to carrying every species through the dry years.
Measured
A balance you can check
The valley is a simulation first. The same rules run headless for centuries across many worlds, and the game only shows what they produce. At a hundred times speed you can watch the decades roll past with the population graph open.
- 20 worlds, 1,200 years eachThe current balance ran 20 worlds headless for 1,200 valley-years. Bison lasted in all 20, and every species was present at least nine tenths of the time in 19 of them.
- Booms about once a decadeBison rise and fall on a roughly ten-year cycle, and wolf numbers track them a year behind while holding steadier than their prey.
- Shocks healIn the shock suite, bison came back in every world after a 70% cull, a drought and a wolf wipeout. Every rule change has to pass these gates before it ships.
At the controls
A short field card
One valley-year is 2 minutes at 1x, 12 seconds at 10x. The world opens in late summer on a valley called Tallgrass; any other seed grows a different valley with its own history.
- Space
- pause
- 1 2 3
- speed: 1x, 10x, 100x
- N
- fly to the next event
- C
- cinematic documentary, with captions
- H
- ambient view, no interface
- V
- narrator on or off
- G E
- population graph, evolution chart
- P
- save a clean photo of the view
- F
- follow the selected animal
How it was made
Simulation first, then the picture
Tallgrass was built in October 2026 by Claude Code agents working through long unattended night and day runs, directed by Stewart Gregerson. Nothing on screen is hand-animated or hand-placed.
A deterministic valley
The ecosystem is a TypeScript simulation that runs from a seed, with no renderer. A batch runner plays dozens of worlds for centuries and charts every species, and a golden-hash test proves each new rule is off by default until it passes.
A Three.js renderer
Terrain, grass, weather and day and night are drawn in WebGL from the simulation's state. Animal meshes are retargeted to a shared leg rig by a Blender script; the cottonwoods grow from a port of ez-tree.
Blind critics
Every look change is captured from inside the browser by script. A critic that sees only the stills and charts, and Gemini watching video, review each round; over thirty rounds, every issue was triaged fix, defer or reject.
Generated sound
Wind, birds, thunder, the animals' calls, the music cues and the narrator's lines were generated with ElevenLabs and mixed live by the game's audio director.