Brass & Sigil Launcher

A private custom Minecraft Java Edition launcher for a small friend group.

C# / .NET 8 Avalonia CmlLib.Core Single-file Windows Private project

Download launcher View source code Developer portfolio

You'll need to be whitelisted on the server to actually join — message Matt with your Minecraft username.

About the project

Brass & Sigil is a private Minecraft modpack centred on the Create mod, aeronautics, tech and magic mods, with Distant Horizons for far-render exploration. This launcher is the desktop client built specifically to distribute the pack to a small friend group (under 50 players, no public release).

The launcher is a native Windows application written in C# on .NET 8, using the Avalonia UI framework. It ships as a single self-contained executable so friends can just download and run it — no installer, no .NET runtime to install, no separate config files.

What it does

Fetches a JSON manifest from a self-hosted server and syncs the modpack files (mods, configs, resourcepacks) to the player's local install directory, using SHA-1 hashing so only changed files are downloaded.
Authenticates the player with their own personal Microsoft account via the standard MSAL OAuth flow, so only legitimate Minecraft Java Edition owners can sign in.
Installs the right Minecraft version and Forge loader, then launches the game with the configured memory and the player's session.
Auto-updates the modpack on every launch when the manifest changes — players never have to manually install or update mods.

Technical details

Language

C# (.NET 8)

UI Framework

Avalonia 12

Minecraft auth

CmlLib.Core.Auth.Microsoft

Game launching

CmlLib.Core 4.x + Forge installer

Distribution

Single-file self-contained .exe

Audience

Private friend group (< 50)

Privacy & data

The launcher does not collect, store, or transmit any user data beyond what the standard Microsoft and Minecraft authentication flows require. Auth tokens are cached locally on the player's machine via the MSAL token cache — no telemetry, no analytics, no third-party services beyond Microsoft and Mojang.

The modpack manifest and mod files are served from a self-hosted Linux server that I personally operate.

Status

Active development. The launcher is functional end-to-end (manifest sync, Microsoft auth, Forge install, game launch) and is currently being prepared for distribution to a small group of friends.

Source code is publicly available, MIT-licensed, on my self-hosted Bitbucket: sijbers.uk:8443/.../brass-and-sigil-launcher.

NOT AN OFFICIAL MINECRAFT PRODUCT. NOT APPROVED BY OR ASSOCIATED WITH MOJANG OR MICROSOFT.