The Deploy Pipeline

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What Deploy Does

Deploy turns your instance metadata — the Minecraft version, mod loader, and package list — into a runnable Run Directory (build/). Think of it as building your game from a recipe: the metadata is the recipe, and the deployed Run Directory is what Minecraft actually opens.

See the Glossary → for Polymerium's terms like Pack Source, Local Data, Run Directory, and Working Copy.

What Gets Downloaded

On your first deploy, Polymerium downloads everything needed to run your game:

• Minecraft itself — the version JAR and assets from Mojang's servers
• Loader libraries — Fabric, Forge, NeoForge, or Quilt files needed by the mod loader
• Packages — mods and other package types resolved from CurseForge and Modrinth, including any dependencies

All of these are stored in a shared cache on your disk. Once downloaded, they're never downloaded again — any instance can reuse them.

Symlinks, Not Copies

Instead of copying mod files into each instance, Polymerium creates symbolic links from the shared cache into your build/ directory. If you have five instances all using Sodium, there's only one copy of Sodium on your disk — every instance just links to it.

Files from your Pack Source (import/) and Local Data (persist/) directories are also layered into the Run Directory (build/) in priority order. See Directory Model → for how that works.

Fast Deploys

The first deploy for a new instance downloads everything, so it takes a moment — just like any launcher. After that, things get fast:

• Nothing changed? Deploy is skipped entirely. Your last build is still valid.
• Only a few mods changed? Only those mods are updated. Everything else stays in place.

This is why launching an instance you've already deployed feels instant.

Caching Benefits

The shared cache means your other instances benefit too. If Instance A has already downloaded a mod, and you add the same mod to Instance B, there's nothing to download — deploy just creates a new symlink.

Learn More

• Metadata-Driven Instances → — why instances are descriptions, not folder copies
• Directory Model → — how files are layered into the game directory