Claude Fable 5 Autonomously Builds LAAS, a Procedural 4×4 km 3D Open World in the Browser

Anthropic's Claude Fable 5 model was tasked with building LAAS — a fully procedural 4×4 km open world running in the browser via three.js WebGPURenderer and raw WGSL compute. The project contains no pre-made image, model, or audio assets; every mesh, texture, and light is generated by code at boot from a single seed parameter. Roughly 99% of the ~21,000 lines of strict TypeScript across 90+ commits was written by Fable 5 across long autonomous sessions, with human input limited to high-level creative direction and motion feedback that the model could not self-evaluate from static output.

The scope is substantial: hydraulic-eroded terrain, six procedurally grown tree species with per-instance unique meshes, around 190,000 trees and 450,000 understory instances culled into indirect GPU draws, four-cascade shadow maps with PCSS, a Hillaire LUT atmosphere, raymarched volumetric clouds, screen-space reflections, temporal AA, and a full walk/fly exploration mode. Fable 5 also handled its own QA — booting the world headlessly via Playwright, sampling pixels, diffing frames, profiling GPU passes, and logging diagnostics to a persistent STATUS.md file used as durable memory between sessions. The project serves as a concrete benchmark of frontier model code generation and autonomous engineering capability.