Multi-Version Minecraft Support - Testing Protocol Compatibility

Understanding Protocol Versioning

Every Minecraft version speaks a different network protocol. A 1.20 client can't talk to a 1.21 server -- they're fundamentally incompatible at the packet level. Normally this means players must run the exact version your server is on, which fractures your player base every time Mojang ships an update.

The ViaVersion suite fixes this by translating packets on the fly, so a single server can accept clients spanning from 1.8 all the way to the latest release.

The ViaVersion Suite

ViaVersion isn't one plugin -- it's three, each covering a different direction of translation.

ViaBackwards requires ViaVersion. ViaRewind requires both. Install all three for maximum coverage.

# Download the suite
wget https://github.com/ViaVersion/ViaVersion/releases/download/5.0.0/ViaVersion-5.0.0.jar
wget https://github.com/ViaVersion/ViaBackwards/releases/download/5.0.0/ViaBackwards-5.0.0.jar
wget https://github.com/ViaVersion/ViaRewind/releases/download/3.0.0/ViaRewind-3.0.0.jar

# Place all three in the plugins folder
mv ViaVersion-5.0.0.jar ViaBackwards-5.0.0.jar ViaRewind-3.0.0.jar plugins/

Version Support Matrix

Configuration

ViaVersion config.yml

checkforupdates: true
send-supported-versions: false
block-protocols: []
force-json-transform: false
suppress-metadata-errors: false
debug: false
minimize-cooldown: true
team-colour-fix: true
cache-values: false
character-width-fix: false

The settings that matter most: keep debug off in production (it has a significant performance cost), enable team-colour-fix to fix scoreboard colors across versions, and turn on minimize-cooldown for a better combat feel on pre-1.9 clients.

ViaBackwards config.yml

suppress-conversion-warnings: false
fix-fire-animation: true
shield-blocking: true
hologram-patch: true
sound-conversion: true

Version-Specific Limitations

Not everything translates perfectly. Players on older versions will see approximations for content that didn't exist in their version.

Proxy Multi-Version Support

On networks using Velocity or BungeeCord, installing ViaVersion on the proxy is the way to go. It handles translation in one place instead of on every backend server, and it lets backends run different Minecraft versions behind a single entry point.

mv ViaVersion-5.0.0.jar velocity/plugins/
mv ViaBackwards-5.0.0.jar velocity/plugins/
mv ViaRewind-3.0.0.jar velocity/plugins/

mv ViaVersion-5.0.0.jar bungeecord/plugins/
mv ViaBackwards-5.0.0.jar bungeecord/plugins/
mv ViaRewind-3.0.0.jar bungeecord/plugins/

A typical proxy network might have a 1.21 lobby, a 1.20.4 survival world, and a 1.19 creative world. With ViaVersion on the proxy, any client version can move between all three backends without reconnecting.

ViaFabricPlus (Client-Side)

ViaFabricPlus is a client-side Fabric mod that lets a modern client connect to virtually any server version -- including Alpha, Beta, and even snapshots. One client installation can test connectivity against every version your server supports, which makes it invaluable for quick smoke tests.

Testing Multi-Version Support

Just installing the plugins isn't enough -- you need to actually test it.

What to Test

Cover these areas across your supported client versions:

• Connection: Client joins without errors, spawns normally, server console is clean.
• Movement: Walking, sprinting, jumping, and swimming feel correct. Some rubber-banding on 1.8 clients is expected due to physics prediction differences.
• Blocks and items: Newer content appears as placeholders on legacy clients. Collision and breaking should still work.
• Combat: The combat differences between 1.8 and 1.9+ are the #1 source of player complaints on multi-version servers. 1.8 clients use spam-clicking; 1.9+ clients use attack cooldowns. Both should register hits, but the feel will always differ. Consider OldCombatMechanics if you want to normalize this.
• Chat: Messages and commands work. Avoid hex colors (&#FF5555) in messages sent to pre-1.16 clients; stick to legacy color codes (&c) for universal compatibility.
• World height: On 1.18+ servers, clients below 1.18 can't see blocks outside the classic y=0-255 range. Build important structures within that safe zone.
• Entities: New mobs render as substitute mobs on pre-1.19 clients. Hitboxes and server-side AI remain correct.

Minimum Test Matrix

Pick representative versions from each protocol era:

If all six connect and the core areas above work, your Via suite is configured correctly.

Automating Tests with Bots

Manually launching six different client versions after every server update gets old fast. Bots solve this.

SoulFire includes ViaFabricPlus integration, so a single SoulFire instance can connect as any protocol version. You can point it at your server, set the target protocol version (e.g., 1.16.5), launch a handful of bots, and confirm they connect without protocol errors. Repeat for each version in your test matrix.

For load testing, connect bots across multiple versions simultaneously -- for example, 5 bots each on 1.21, 1.20.4, 1.19, 1.16.5, 1.12.2, and 1.8.9 (30 bots total). Let them stay online for 15-30 minutes and monitor TPS. If the server holds 19-20 TPS with no protocol errors, your setup is solid.

Common Issues

Performance

ViaVersion's overhead is real but often overstated. The honest answer is: for most servers under 100 players, you won't notice it. The packet translation cost is negligible compared to what chunk loading, entity ticking, and world generation already demand from your CPU.

Where it does get expensive: bridging large version gaps. Translating packets between 1.20 and 1.21 is trivial, but a 1.8 client connecting to a 1.21 server means every packet passes through years of protocol changes. The nms-player-ticking feature (enabled by default) simulates player ticks for 1.9+ behavior on older servers, and this shows up in timings reports -- though it's really just calling internal server methods that would run anyway. If you see ViaVersion in your spark profiles, try setting nms-player-ticking to false, but know that it'll break eating, bows, and potions for cross-version players.

Memory-wise, ViaVersion 5.x significantly reduced its netty buffer allocations compared to earlier versions, so make sure you're on the latest release. If you're using ViaVersion on a 1.12 server with block connections for pre-1.13 clients, set block-connection-method to world instead of packet -- the packet mode has noticeably higher memory and CPU overhead.

For proxy networks, running ViaVersion on the proxy (Velocity/BungeeCord) rather than on each backend is the best move for performance. It centralizes the translation work and keeps your game servers focused on what they're good at.

What to Watch Out For

The things that'll actually bite you in production:

• World height on pre-1.18 clients. If your spawn has a basement at y=-10, anyone on 1.17 or below just falls into the void. Keep critical builds within y=0-255.
• Combat system mismatch. This generates more tickets than everything else combined. Document what players on each version should expect, or standardize with OldCombatMechanics.
• Block placeholders. A build made entirely of copper and deepslate looks like a stone box to someone on 1.12.2. If you care about aesthetics for legacy players, stick to older block palettes in public areas.

Keep your Via plugins up to date -- the team ships performance and compatibility fixes constantly. And when you hit a weird edge case (you will), the ViaVersion Discord is genuinely one of the better Minecraft support communities. They've seen it all.