Migrate from Other Launchers

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Migrate Automatically (Recommended)

Polymerium can import instances directly from another launcher's data directory — no export step needed. Supported launchers:

• MultiMC and Prism Launcher
• CurseForge App
• ATLauncher
• Modrinth App

Step 1: Open the Migration Wizard

• In Settings, find Migrate from Launcher.
• Or on the Instances page, click Migrate from another launcher.

Step 2: Pick a Launcher and Its Data Directory

Select the launcher you're migrating from. The data directory is pre-filled with that launcher's default location — change it if your launcher uses a custom directory.

Step 3: Scan and Select Instances

Click Scan. Polymerium lists every instance it finds with its name, Minecraft version, and loader. Instances that can't be read are flagged with the reason, and you can select or skip each one.

Step 4: Migrate

Click Migrate. For each selected instance, Polymerium:

1. Identifies mods — .jar files in mod, resource pack, and shader pack folders are matched against CurseForge and Modrinth. Identified files become tracked package references (no re-download if already cached); the rest are copied as-is.
2. Copies everything else — configs, worlds, screenshots, options, and other files are copied verbatim into the new instance's Run Directory (build/).
3. Registers the instance — the profile (Minecraft version, loader, package list) is created only after the file transfer completes, so a failed instance never leaves a half-created profile behind. Corrupt instances are reported and migration continues with the rest.

Your old launcher and its instances are untouched. Then continue with After Migrating.

What the Wizard Transfers

Accounts and World Data

Worlds are tied to the player UUID, not to the username or the account shown in the launcher. Signing in with a different account doesn't lose the world files — the world still loads, but with a fresh player: the inventory, spawn position, and progression attached to the old UUID stay behind.

The world files are never deleted — the data simply stays under the old UUID. To keep your progress, sign in with the same account you played with before migrating. For third-party auth servers, see Set Up Authlib Injector →.

Launcher Directory Layouts

If your launcher has no adapter, you migrate manually — and the first step is finding where its data lives. Launchers come in two families:

For a per-instance launcher, an instance's game data is the folder containing mods/, saves/, and options.txt. For a flat launcher, it's the single .minecraft folder. Once you know which folder is an instance's game data, the manual procedure below applies to any launcher.

Manual Migration (Any Launcher)

When there's no adapter — typically the official launcher — migration is manual but straightforward:

Step 1: Create a Matching Instance

1. Click + in Polymerium.
2. Choose New Instance and select the same Minecraft version you were playing.
3. If you use mods, select the same mod loader (Fabric, Forge, NeoForge, or Quilt) and a compatible version.

Not sure which loader or version? In a per-instance launcher, check the instance's metadata file. In the official launcher, open .minecraft/versions/ — a folder like 1.21.4-fabric-0.16.9 means Fabric 0.16.9 on 1.21.4; plain version folders like 1.21.4 mean vanilla.

Step 2: Copy the Game Data

Your mods, saves, configs, screenshots, and settings all live together in the launcher's game data folder (see Launcher Directory Layouts above). That folder is what Minecraft treats as its game home — and so is the instance's Run Directory (build/). Copy the folder's contents straight into build/, keeping the same structure, and everything loads exactly as it did in your old launcher.

1. Open the launcher's game data folder.
2. Copy mods/, resourcepacks/, shaderpacks/, saves/, screenshots/, options.txt, and servers.dat — everything the game generated or you added — into build/, preserving the folder structure.
3. Skip versions/, assets/, and libraries/ — Polymerium supplies the Minecraft version, assets, and libraries from its shared cache; copying them only duplicates data.

Optional — track files as packages. Files copied into build/ aren't tracked, so they can't update as packages. If you'd rather track some of them, and they exist on CurseForge or Modrinth, add them as package references instead:

• Import asset from file — drop the .jar in on the instance page; Polymerium identifies it and adds it as a reference.
• Add Package — search CurseForge or Modrinth in this dialog and add manually.

Both work one file at a time, so for a large mod list copying the files as described above is faster — convert individual files when you want them tracked.

With the files in place, continue with After Migrating to deploy, sign in, and protect your data.

Troubleshooting

• Worlds are missing: make sure you copied saves/ into build/ and deployed after copying.
• Settings didn't carry over: check that options.txt is in build/.
• Mods aren't loading: verify the mod loader version matches. A Fabric mod won't load on Forge, and vice versa.

After Migrating

Regardless of which path you took — automatic or manual — finish with these steps:

1. Add a game account — credentials never transfer. Open Accounts and sign in with the same account you used before — see Accounts and World Data.
2. Deploy and play — launch the instance; Polymerium deploys automatically.
3. Protect your data — migrated files land in the Run Directory (build/), which reset clears. Move anything you want to keep — worlds, screenshots, configs — into Local Data (persist/) so it survives resets. See Directory Model.

Alternative: Import an Exported Modpack

If your launcher can export a modpack — Prism Launcher and MultiMC can export MultiMC, Modrinth, or CurseForge format — you can import the archive instead:

1. Export the instance from the other launcher (MultiMC format is the most faithful for this purpose).
2. In Polymerium, click + → Import from file and select the archive. Polymerium auto-detects the format, resolves mods, and creates the instance. After importing, continue with After Migrating.

This path also works when you only have a modpack archive — e.g. downloaded from a website — rather than a launcher installation.

Things to Know

• Polymerium uses symlinks — make sure Developer Mode is enabled on Windows.
• Mod files are shared — you won't re-download mods already in CurseForge/Modrinth format; Polymerium resolves them from its cache.
• Your old launcher instances are untouched — migration and import create new instances; nothing is deleted or modified in your previous launcher.