Set Up Authlib Injector

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What is Authlib Injector?

Authlib Injector is a Java agent that intercepts Minecraft's authentication calls and redirects them to a third-party Yggdrasil-compatible server. This is commonly used by Chinese Minecraft communities for custom authentication.

Polymerium has built-in support for this — no manual Java agent configuration needed.

Setup

Step 1: Add an Authlib Account

1. Go to Accounts in the sidebar.

2. Click Add Account → Authlib Injector.

3. Enter the server URL of the Yggdrasil authentication server.

   Example: https://auth.example.com/api/yggdrasil

4. Polymerium fetches the server's metadata and validates the connection.

5. Enter your credentials (username/email + password) for that server.

6. The account is saved.

Step 2: Bind the Account to Your Instance

1. Open the instance's properties.
2. Set the account override to your Authlib account.

Or set the Authlib account as your default account if you use it for most instances.

Step 3: Deploy and Launch

1. Click Deploy (or Launch — it deploys automatically).
2. Polymerium injects the -javaagent flag and prefetched server metadata into the launch command.
3. The game authenticates against the third-party server instead of Mojang.

How It Works

When launching an instance with an Authlib account:

1. Polymerium downloads the latest Authlib Injector JAR (if not cached).
2. Fetches and caches the server's Yggdrasil metadata.
3. Adds -javaagent:authlib-injector.jar={server_url} to the JVM arguments.
4. The game process loads Authlib Injector, which patches authentication in-memory.

Server Compatibility

Polymerium works with any server that implements the Yggdrasil protocol, including:

• authlib-injector compatible servers
• Most Chinese community auth servers (LittleSkin, etc.)
• Custom Yggdrasil implementations

Troubleshooting