Everything for Legacy Minecraft in one place

Everything you need to boot a Beta 1.7.3 server. When a release has two jars, take the one without original- in its name.

Every plugin from RetroMC and BetaMC, grouped and searchable, is on the Plugins page. A few staples are below.

• All plugins RetroMC and BetaMC plugins for Beta 1.7.3, grouped by what they do: core, protection, moderation, economy, Discord, monitoring and developer tools. plugins.html
• FundamentalsSource Essentials replacement built for Poseidon (FundamentalsCore and FundamentalsImport). No prebuilt releases yet, and mainly written for RetroMC. github.com/retromcorg/Fundamentals
• Essentials (RetroMC fork) RetroMC's fork of the classic Essentials suite of server commands. github.com/retromcorg/Essentials
• JohnyPerms UUID-aware permissions plugin that replaces PermissionsEx. github.com/retromcorg/JohnyPerms
• WorldEdit The map editor and mod, forked from EngineHub/WorldEdit for Beta 1.7.3. github.com/retromcorg/WorldEdit
• WorldGuard Protects a server and lets players claim areas. Forked from EngineHub/WorldGuard. github.com/retromcorg/WorldGuard
• LWC Private locks for chests, furnaces and doors (RetroMC's fork). github.com/retromcorg/LWC

Compatibility: plugins built for the original 2011 CraftBukkit often load on Poseidon, but not always, because Poseidon's API differs. Test every jar on a local server first, and only download jars from sources you trust.

Find people to play with and see who is online right now. Player counts on the featured servers refresh live when the data source is reachable.

The official launcher handles Beta poorly. Beta Evolutions is the main pick here; the rest are alternatives that fix crashes, skins, sound and login, and some let modern clients join old servers.

• Beta EvolutionsMain pick RetroMC's Beta 1.7.3 launcher and client. Start here: downloads and setup are on its site. evolutions.retromc.org
• Betacraft LauncherActive Launcher for every Pre-Classic to 1.5.2 version, with fixes for crashes, skins and sound plus a built-in server list. betacraft.uk/downloads
• Betacraft Launcher (source) GPL-3.0 source and issue tracker. Windows, Linux and macOS. github.com/betacraftuk/betacraft-launcher
• Prism Launcher Open-source multi-instance launcher. Each instance has its own settings and mods, so it suits Babric setups. prismlauncher.org
• MultiMC Classic multi-instance launcher. Several Beta servers recommend it alongside the Betacraft launcher. multimc.org
• Babric Prism instance Prism/MultiMC instance for running Beta 1.7.3 with the Babric loader. github.com/babric/prism-instance
• ViaProxy Standalone proxy that lets newer clients join servers from Classic, Alpha, Beta and Release versions. github.com/ViaVersion/ViaProxy
• Release2Beta Proxy and Bukkit plugin that lets 1.12.2+ clients join beta servers. Poseidon has built-in IP forwarding support for it. github.com/DirtPowered/Release2Beta

Beta worlds use the old McRegion (.mcr) format. Many modern tools only read Anvil (.mca), so convert first.

• NBTExplorer Edit level.dat and other NBT data. Lists support for .mcr and .mca region files. github.com/jaquadro/NBTExplorer
• McRegion-Anvil-Converter Multithreaded McRegion to Anvil converter. About 19x faster on 32 threads in RetroMC's benchmark. github.com/retromcorg/McRegion-Anvil-Converter
• BlueMap for Beta 1.7.3 A BlueMap build for Beta 1.7.3 to publish a 3D web map of your world. github.com/retromcorg/BlueMap
• Minecraft Overviewer Renders worlds into browsable web maps. RetroMC converts worlds to Anvil first so Overviewer can read them. overviewer.org
• MCA Selector Visually select, export or delete chunks and regions. Supports Anvil worlds (1.2.1+), so convert Beta worlds first. github.com/Querz/mcaselector
• legacytracker Run your own tracker for a network of legacy servers. github.com/retromcorg/legacytracker

Client-side and full-game mods for Beta 1.7.3, mostly built on Babric, a Fabric loader for the Beta era.

• Mods for Beta 1.7.3 Babric and StationAPI mods, libraries, skin fixes and interface tweaks, on their own page. mods.html
• Babric Brings Fabric to Beta Minecraft: installer, barn mappings and a b1.7.3 intermediary. babric.github.io
• Babric on GitHub Loader, Loom plugin, installer, stitch, mappings and the Prism instance. github.com/babric
• StationAPI General-purpose API for Fabric Loader mods on legacy versions. The develop branch targets Beta 1.7.3 on Babric. github.com/ModificationStation/StationAPI
• StationAPI on Modrinth Latest StationAPI releases to download. modrinth.com/mod/stationapi
• StationAPI wiki Guides for making mods with StationAPI. github.com/ModificationStation/StationAPI
• OptiBabric Run OptiFine on Babric. Add the OptiFine zip to your instance's mods, never into minecraft.jar. github.com/mineLdiver/OptiBabric
• Better than Adventure Heavily modified Beta 1.7.3 with its own launcher, release API and mod ecosystem. BTA mods target BTA, not plain b1.7.3. github.com/Better-than-Adventure
• Beta Evolutions 2 mod template ModLoader mod example for Beta Evolutions 2. github.com/retromcorg/Beta-Evolutions-2-Mod-Template

Read the manuals, then ask the people who run these projects.