BrowseCraft is a build-only Minecraft assistant built around a single-shot voxel.exec pipeline. The model writes JavaScript against a small voxel DSL, the server executes that code against a headless world, and the Fabric mod applies the resulting absolute block diff in game. Execution runs through just-bash's js-exec sandbox, with a local patch in patches/just-bash@2.13.0.patch to fix the published package's broken worker path.

Architecture

flowchart LR
  A["Minecraft Fabric Mod"] -->|"POST /v1/chat + worldContext"| B["TypeScript Backend"]
  B -->|"single model call"| C["Anthropic via Vercel AI SDK"]
  B -->|"executeCode(world, code)"| D["sim package"]
  D -->|"world diff"| B
  B -->|"build.apply"| A
  A -->|"build.result"| B
  B -->|"chat.response"| A
  D -->|"JSONL manifests"| E["veRL / TRL / downstream Python consumers"]

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Repo Layout

• sim/: TypeScript simulator, task generation, grading, collection, curriculum, and export
• backend/: TypeScript HTTP/WebSocket server
• mod/: Fabric client for snapshot capture, chat UI, and block application

Quickstart

1. Install workspace dependencies.

   cd ~/BrowseCraft
   pnpm install

2. Run package tests.

   cd ~/BrowseCraft
   pnpm --filter @browsecraft/sim test
   pnpm --filter @browsecraft/backend test
   cd ~/BrowseCraft/mod && gradle test

3. Start the backend and the client.

   cd ~/BrowseCraft
   pnpm --filter @browsecraft/backend dev

   cd ~/BrowseCraft/mod
   gradle runClient

4. In game, use /chat <message> and /session new|list|switch <id>.

Training Data Pipeline

Generate deterministic tasks:

cd ~/BrowseCraft
pnpm --filter @browsecraft/sim generate-tasks --mode build --seed 45 --count 2 --output sim/runs/build_seed45.jsonl

Collect trajectories:

cd ~/BrowseCraft
pnpm --filter @browsecraft/sim collect --mode build --model claude-sonnet-4-5 --seed 45 --per-tier 2 --output sim/runs/build.jsonl
pnpm --filter @browsecraft/sim collect --mode text_qa --model claude-sonnet-4-5 --seed 45 --per-tier 2 --output sim/runs/text_qa.jsonl
pnpm --filter @browsecraft/sim collect --mode creative --model claude-sonnet-4-5 --seed 45 --count 10 --output sim/runs/creative.jsonl

Export stage manifests:

cd ~/BrowseCraft
pnpm --filter @browsecraft/sim export --input sim/runs/all_episodes.jsonl --output-dir sim/runs/manifests

This emits:

• spatial-sft.jsonl
• spatial-grpo.jsonl
• creative-sft.jsonl
• creative-grpo.jsonl

seed=45 is the deterministic validation seed and is locked by the fixtures in sim/test/fixtures.