Feature Lab
Live preview for Minecraft Bedrock worldgen JSON, inside the editor. Open a
features/ or feature_rules/ file, press a key, and see the blocks that file
would place — in a rotatable 3D view, next to the JSON that produced them.
What it does
- Previews a feature or a feature rule. Point it at a file and it loads the
whole behaviour pack around it — features, feature rules, structures, biomes
and any block definitions the pack ships — then places the one feature you
asked for into a small generated world and renders the result.
- Re-renders when you save. Saving re-reads only the file you saved, so the
preview comes back in tens of milliseconds rather than re-reading the pack.
- Explains what it refused to do. Placement failures that leave no visible
trace — an attach condition that did not match, a
may_replace list that
rejected a position, a delegate that declined — are listed in the panel's
Diagnostics section with the delegation chain that reached them, a count, and
a button that moves the camera to the position it happened at. Load-time
problems also appear as squiggles in the JSON.
- Follows references. Ctrl+click (Cmd+click) a
places_feature or a
features entry to jump to the file that defines it.
Getting started
- Open a folder containing a behaviour pack — anything with a
features/
directory in it.
- Open a feature or feature rule JSON file.
- Run Feature Lab: Preview Feature (
Ctrl+Alt+P, Cmd+Alt+P on macOS),
from the editor title bar, or from the right-click menu.
The panel opens beside the file. Everything in it is per-preview and lives only
as long as the panel does.
The panel
| Section |
What it is for |
| Feature / Rule |
Which feature or rule to place, where to place it, and how many times. A rule preview runs the rule's own distribution across the chunks it would decorate, instead of placing one feature at one point. |
| Environment |
The generated world to place into — a terrain preset and the size of the volume. Ten presets: plains, forest, desert, ocean, nether, end, three underground fills and an empty void. |
| Materials |
Which blocks the preset's terrain is made of, so a feature that only attaches to a specific block can be tested without editing the pack. |
| Biome |
A pack biome to run under, or a hand-written tag list, for features gated on biome tags. |
| Budget |
Per-run limits on writes, delegations and wall-clock time. These are the bench's own guard rails, not the game's: a feature that references itself in a loop truncates the preview instead of hanging it. |
| View |
Slice the volume by height, show or hide the surrounding terrain, colour cells by how many times they were written, and frame the camera. |
| Diagnostics |
Everything the run declined to do, and why. |
| Profiler |
Optional per-feature timing and write counts for a delegation chain, off by default. |
Settings
| Setting |
Default |
What it does |
featurelab.binaryPath |
(empty) |
Path to the engine executable. Leave empty to use the copy bundled with the extension. |
featurelab.blockTextures |
true |
Draw blocks with real Minecraft textures instead of flat per-block colours. The first preview offers once to fetch Mojang's sample resource pack (about 150 MB transferred, cached outside your pack); nothing is downloaded without an explicit yes, and declining is remembered. Set to false for flat colours and no question. See the project wiki's Block Textures in the Preview page for what gets downloaded, where it is cached, and how to remove it. |
featurelab.env |
plains |
Which environment preset a new panel starts on. |
featurelab.requestTimeoutMs |
30000 |
How long to wait for the engine before treating a request as failed. |
What a preview is, and is not
The preview is a bench, not the game. It places one feature into a world it
generated for the purpose, so it can show you a feature in isolation and tell
you why a placement failed. It does not generate a real chunk: no other feature
runs beside yours, and nothing in the panel proves the same JSON behaves
identically in Minecraft.
Where the two are known to differ, the tool says so — in a diagnostic when the
difference affects your file, and in this project's documentation set
otherwise. That set carries a page per feature type describing what every field
does, plus a coverage page listing each known gap and what it means for a pack.
It is the honest place to start when a preview and the game disagree.
Requirements
VS Code 1.85 or newer. The engine binary is bundled; nothing else to install.
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