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Minecraft versions

RP Engine runs on Paper or Spigot 1.19.4 or newer, on Java 17 or newer.

Everything works on a current server. On an older one, most of the plugin works exactly the same and a few things are reduced — never silently. The plugin checks your server's version at startup and prints what it cannot do and what you get instead, so you never have to work out from the docs whether a feature reached you.

If you can choose, choose 1.20.3 or newer

1.19.4 is supported because that is where the entities placed models are made of arrived. But the two losses you would most notice — stacked bundles, and models that glide when they move — both sit just below 1.20.3, so those four versions buy back more than any other four.

What always works#

These do not change with your version at all:

Custom items — their models, textures, names, lore, permissions and actions
Custom blocks, including hardness and the right tool
Models you place, sit on, animate and bind to entities
Emotes, including worn sets and held items
Sounds, and the sound categories
Icons in chat, GUI screens, and HUD overlays
Recipes, custom entities and custom liquids
Pairing with Studio, and pushing a pack to a player

What needs a newer version#

Feature	Needs	On an older server
Stacking several packs on a player	1.20.3	One pack at a time; a player due more than one bundle gets only the last. A server using a single bundle is unaffected.
Models that glide when they move	1.20.2	A model that moves — a walking emote, a bound model following its entity — jumps between positions instead of gliding. Models that stay put are identical.
Per-item stack size, food and durability	1.20.5	Those options are reported at load and not applied. The item is otherwise itself.
Any item wearable as armour	1.21.2	Only materials that are already armour can be worn.
Per-item armour artwork	1.21.4	Worn armour draws with vanilla artwork.
Item models addressed by name	1.21.4	Items are addressed by an allocated number instead — see below.
Tinted water and lava	1.21.6	Liquids keep their vanilla colour. Everything else about a custom liquid works.
Reading a driver's movement keys	Paper 1.21.4	Vehicles are driven with the mouse: look where you want to go, right-click to speed up a notch, left-click to slow down or reverse. Steering follows your look on every version, so a vehicle handles the same — only the throttle differs. Needs Paper; a Spigot server gets this on any version.

Below 1.21.4, items are numbered#

This is the one difference worth understanding rather than just reading.

From 1.21.4 an item's model is chosen by its id: mypack:ruby is written straight onto the item, and the pack has a file at that path. Nothing is allocated, so two packs from two different people work side by side with nothing to configure.

Before 1.21.4 the game matches a model by a number. RP Engine allocates those for you and keeps them in plugins/RPEngine/model-numbers.json. Three things follow:

Do not delete or edit that file. Those numbers are on items in your players' inventories and on models placed in your world. Change them and those items become different items, and there is no way to undo it. The file says so in its own contents.
Two packs can clash. On 1.21.4 and up, installing two content packs is never a negotiation. Below it, packs that were numbered separately can disagree, so combining content from two sources needs care.
Your base item's own look can change. Below 1.21.4 a custom model is a predicate inside the base material's own model file, which replaces vanilla's copy of it. RP Engine reproduces vanilla for the ordinary cases, but a few items — a bow, a compass, a crossbow, a shulker box — have models it cannot reconstruct. Build a custom item on one of those and the build warns you: the item works, but plain bows may look wrong for everybody on the server. Use a simpler material like PAPER or STICK, or run 1.21.4.

The pack format#

You do not set it. RP Engine reads your server's version and writes the right pack_format into the built pack.

config.yml still has a pack.format key, commented out. It exists for one case: a Minecraft newer than your copy of RP Engine, whose format number the plugin cannot know yet. The startup log tells you when that has happened. Set it, update the plugin, then remove it again.

What the plugin tells you#

On a current server, nothing — a plugin that prints a paragraph every start teaches you to skip its output. On an older one, it names each reduced feature, the version it would need, and what you get instead:

[RPEngine] Running on Minecraft 1.20.1, so 7 features are reduced:
[RPEngine]   - Multiple resource packs per player (needs 1.20.3). Players hold one
             pack at a time, so a player due more than one bundle gets only the last
             of them and the rest do not arrive. Swapping bundles still works and
             costs a full download each time rather than being instant. A server
             that uses a single bundle is unaffected.
[RPEngine]   - Item models addressed by name (needs 1.21.4). Custom items are
             addressed by an allocated number instead. The engine keeps that
             allocation itself, but two packs built by different people can no
             longer be installed side by side without it.
[RPEngine]   - Models that glide when they move (needs 1.20.2). A model that moves
             jumps between positions about five times a second instead of gliding.
             Models that stay put look exactly the same.

On 1.21.4 it is one line, about tinted liquids. On 1.21.6 and up it says nothing at all.

Individual items say so too, naming the item and the option, so an option that could not be applied is never a mystery:

[RPEngine] mypack:ruby: max-stack needs Minecraft 1.20.5 and this server is
           older. An item's stack size, food values and durability are whatever
           its base material's are.

Everything here is about your server's version#

Not your players'. A plain Paper or Spigot server only accepts clients on its own version, so the two are the same number and there is one answer.

If you run ViaVersion so that clients on other versions can join, that does not change any of the above: the plugin runs on your server's version and builds one pack for it. A client joining through Via is shown a pack built for the server, which is the same thing every other pack on that server does.