Matt
8e00526d27
Microsoft access tokens have a ~1 hour TTL. The launcher cached the MSession once at sign-in and re-used it on every Play click, so leaving the launcher open for >1 hour and then hitting Play sent stale credentials to MC -- Mojang's session check on the server then rejected the join with "Invalid session". Fix: in DoLaunchAsync, before assembling the MC launch command, call _auth.TryAuthenticateSilentlyAsync(). XboxAuthNet's silent flow uses the cached refresh token (~14 day TTL) to mint a fresh access token transparently. On success ApplySession swaps in the new MSession and we launch with valid credentials. On failure (refresh token also stale) the launcher prompts the player to sign in again -- they keep the launcher open, hit Sign In, no MC restart needed. No new dependencies, no UI change beyond the brief "Refreshing session..." status line, no perf cost beyond the ~500ms HTTP roundtrip to Microsoft each launch. Bump the launcher version in launcher/ModpackLauncher.csproj before publishing so old launcher installs see the upgrade banner.
Brass & Sigil Launcher
A custom Minecraft Java Edition launcher built for distributing the private "Brass & Sigil" modpack (Create + aeronautics + tech + magic + Distant Horizons) to a small friend group.
NOT AN OFFICIAL MINECRAFT PRODUCT. NOT APPROVED BY OR ASSOCIATED WITH MOJANG OR MICROSOFT.
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. SHA-1 hashing ensures only changed files are downloaded.
- Authenticates each player with their own personal Microsoft account via
the standard MSAL OAuth + Xbox Live + Minecraft Services chain, using the
open-source
CmlLib.Core.Auth.Microsoftlibrary without modification. - Installs the appropriate Minecraft Java Edition version and Forge mod loader, then launches the game with the player's authenticated session.
Stack
- C# / .NET 8
- Avalonia 12 -- desktop UI
- CmlLib.Core 4.x -- Minecraft install + launch
- CmlLib.Core.Auth.Microsoft 3.x + XboxAuthNet.Game.Msal -- Microsoft auth
- CmlLib.Core.Installer.Forge -- Forge support
Build
Requires the .NET 8 SDK.
dotnet build
To produce the shippable single-file executable (~46 MB):
dotnet publish -c Release -r win-x64 --self-contained true
Output: bin/Release/net8.0/win-x64/publish/ModpackLauncher.exe -- a single
file with no other dependencies, ready to send to a friend.
Configuration
The repo ships with a launcher-config.template.json. Copy it to
launcher-config.json and fill in real values before building:
Copy-Item launcher-config.template.json launcher-config.json
launcher-config.json is gitignored so local values (manifest URL, Azure
client ID) never get committed.
Fields
| Field | Purpose |
|---|---|
packName |
Display name shown in the launcher |
manifestUrl |
URL of the hosted manifest JSON |
installDirName |
Folder name under %APPDATA% for game files |
memoryMinMB / memoryMaxMB |
JVM memory defaults |
msalClientId |
Azure App Registration client ID for Microsoft auth |
The config is embedded into the exe at build time, so the launcher ships
as a single self-contained file. A sidecar launcher-config.json placed
beside the exe will override the embedded copy at runtime (handy for testing).
Manifest format
See manifest.example.json for the schema. Minimum:
{
"name": "Brass & Sigil",
"version": "1.0.0",
"minecraft": { "version": "1.20.1" },
"loader": { "type": "forge", "version": "47.2.0" },
"files": [
{ "path": "mods/example.jar", "url": "https://...", "sha1": "..." }
]
}
The launcher diffs the manifest against the local install dir using SHA-1
hashes and downloads only what has changed. Files removed from the manifest
are pruned from managed folders (mods/, config/, resourcepacks/,
shaderpacks/, kubejs/, defaultconfigs/).
Privacy
The launcher does not collect, store, or transmit any user data beyond what the standard Microsoft and Minecraft authentication APIs require. Auth tokens are cached locally via the MSAL token cache. No telemetry, no analytics, no third-party services beyond Microsoft and Mojang.
Local data is written to:
%APPDATA%\BrassAndSigil\-- launcher settings + log file%APPDATA%\<installDirName>\-- modpack and Minecraft installation
License
MIT -- see LICENSE.
Author
Matt Sijbers -- https://sijbers.uk/matt / project page