Slimefun: complete server guide for Minecraft in 2026

Slimefun4 turns a vanilla Paper server into a small modpack-like sandbox without forcing anyone to install a single client mod. Players join with a regular Mojang client and somehow get GregTech-style crafts, magical altars, cargo networks and dozens of machines. Below is the practical 2026 walkthrough for getting Slimefun4 onto a Paper 1.21 box, surviving the research and cargo systems, and not melting your TPS to single digits in the second week.

What Slimefun4 actually is and why bother

Slimefun4 is the maintained fork of the original Slimefun, run by TheBusyBiscuit and the community since 2019. As of 2026 the active branch is the RC-38 line, with builds on Hangar at hangar.papermc.io and on the GitHub repo Slimefun/Slimefun4. The whole thing is server side. Players need nothing extra in their launcher, the UI is rendered through vanilla items and inventory menus.

Why run Slimefun on a survival or semi-vanilla project:

• Long progression curve. The research tree easily covers a few hundred hours from upgraded pickaxes to quantum teleporters.
• Tech and magic in one plugin. Rune-engraved altars sit next to electric furnaces and auto crafters.
• A real addon ecosystem. ExoticGarden adds farming and cooking, InfinityExpansion gives you endgame, Soul Jars lets you trap mobs in jars.
• Zero client side mods. No Forge, no Fabric, no extra resource pack required for the basics.

The honest downsides up front: Slimefun is heavy on CPU and disk I/O because of BlockStorage, it does not love huge cargo networks, and the data folder grows fast. The performance section below is the most important part of this article, do not skip it.

Installing Slimefun4 on Paper 1.21

The 2026 baseline: Paper 1.21.x on Java 21 LTS. Older 1.20 and 1.19 are still nominally supported, but new addons and optimisations get tested on 1.21 first.

Minimum stack:

1. Paper 1.21.4 or newer.
2. Java 21 (Temurin, Zulu, Liberica, whatever your hosting prefers).
3. Slimefun4 RC-38 or newer from Hangar.
4. No CS-CoreLib. That dependency was killed years ago. If a tutorial tells you to install it, the tutorial is from 2018.

On a typical Linux dedicated box or VPS:

cd ~/server
wget -O server.jar https://api.papermc.io/v2/projects/paper/versions/1.21.4/builds/latest/downloads/paper-1.21.4-latest.jar
mkdir -p plugins
cd plugins
# Grab the latest Slimefun4 jar from Hangar
# https://hangar.papermc.io/Slimefun/Slimefun4
ls -la Slimefun-*.jar

Launch:

java -Xms6G -Xmx6G \
  -XX:+UseG1GC -XX:+ParallelRefProcEnabled \
  -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions \
  -XX:+DisableExplicitGC -XX:+AlwaysPreTouch \
  -jar server.jar nogui

First boot creates plugins/Slimefun/. Stop the server and look at the configs before letting players in.

Base configuration: the keys that actually matter

Main file: plugins/Slimefun/config.yml. The keys I touch on day one:

options:
  auto-update: false
  chat-prefix: "&aSlimefun &7> "
  enable-armor-effects: true
  backwards-compatibility: false
  talismans-actionbar: true

researches:
  enable-researching: true
  free-creative-research: true
  guide-give-on-first-join: true
  use-levels: true

guide:
  default-view-book: false
  show-vanilla-recipes: true

logging:
  vanilla-recipes: false

Why these specifically:

• free-creative-research: true saves an admin from wasting XP just to test a balance question.
• guide-give-on-first-join: true saves you a hundred chat questions per week. New player gets the guide book in slot 9 automatically.
• use-levels: true keeps research economy meaningful. Set to false and the whole progression collapses.
• default-view-book: false switches the guide into inventory mode. Clicking is a lot faster than scrolling pages.
• auto-update: false is non-negotiable on prod. Stage updates on a test box first.

Second file worth knowing: plugins/Slimefun/Researches.yml. Override the cost of any individual research there. Useful for season resets where you want certain endgame items to be more expensive.

Guide book and item categories

The Slimefun Guide is the central UI of the plugin. A player opens it and sees categories: Tools, Weapons, Armor, Food, Magical Items, Cargo Management, Electricity and many more. Every item carries a level requirement and an XP cost.

Daily admin commands:

/sf guide                       # give yourself a fresh guide
/sf research <player> all       # unlock everything for a player (balance testing)
/sf research <player> reset     # wipe progress, clean up cheaters
/sf give <player> <item> <n>    # spawn any Slimefun item
/sf timings                     # plugin load profiler
/sf debug                       # toggle debug mode for bug reports

Item IDs are written in UPPER_SNAKE_CASE. Examples: ELECTRIC_SMELTERY, CARGO_NODE_INPUT, ENERGY_REGULATOR. You can grab the full list from the GitHub wiki or by enabling cheat mode in the config.

Cargo network and energy: how it works

These two systems are the heaviest in the plugin, so they are also the ones you must understand before opening the gates.

Energy network. Three roles:

1. Generators: Coal Generator, Solar Generator, Bio Reactor, Nuclear Reactor.
2. Regulator: the Energy Regulator. One per network, physically wired to generators and consumers through Energy Connectors.
3. Consumers: Electric Furnace, Grind Stone, Auto Crafter, and so on.

Energy does not buffer in wires. The regulator gathers per-tick output from generators and distributes it to consumers within the same tick. If you need a buffer, drop in a Capacitor. They scale from Small Capacitor up to Carbonado Capacitor with millions of J of storage.

Cargo network. Automated item movement between Slimefun machines and vanilla chests. Building blocks:

• Cargo Manager: the brain, one per network.
• Cargo Node (Input): pulls from a container.
• Cargo Node (Output): pushes into a container.
• Cargo Connector Node: pure transit pipes.
• Advanced Cargo Output Node: filters by NBT and Slimefun ID.

Every node has a channel number assigned through its GUI. The single most common newbie mistake is putting two Input nodes on the same chest with mismatched channels and wondering why nothing flows.

A typical small factory: a tree farm fills a chest, an Input node on channel 1 pulls oak logs into an Electric Smeltery, channel 2 carries coal in from a separate chest, an Output node on channel 3 pulls finished charcoal into a storage chest. Once you draw the channels on paper before placing nodes, the whole system clicks.

The advice that actually saves debugging time: sketch channels on paper or in Excalidraw before placing the first node. A 30-node network is fine to keep in your head, a 200-node one is not. Players who skip this step usually show up at the end of the season asking for their factory to be wiped so they can start over.

One more catch: a Cargo Manager only operates inside loaded chunks. If your auto factory sits outside the per-chunk loading radius, it simply halts. Fix either by adding a chunk loader through an addon like ChrislieChunkLoader, or by placing factories near spawn inside chunks force-loaded with /forceload add.

Popular addons in 2026

Hangar and Modrinth list a few dozen addons of varying quality. The list I would actually install on a fresh survival server today:

Addon	What it adds	1.21 stability	When to install
ExoticGarden	Fruits, berries, cooking, wine	good	Any survival with farming focus
InfinityExpansion	Endgame machines and materials	good	Long seasons, late-game players
Soul Jars	Mob capture and spawner crafting	mixed	When vanilla spawners feel bland
Networks (Slimefun Networks)	Wireless cargo replacement	good	When normal cargo gets laggy
FoxyMachines	Extra automation machines	good	Wider mid-game crafting
HotBricks	Decorative blocks	stable	Builders and creative tasks
RykenSlimefunCustomizer	Custom items and recipes	mixed	Want your own mechanics
GalactiFun	Space and planets	beta	Themed servers

Each addon is a jar in plugins/. Slimefun picks them up at startup automatically. Before any Paper minor version bump, double check that every addon already has a build for the target version, otherwise expect NoSuchMethodError on boot.

Performance: BlockStorage and AsyncBlockTicker

The ugly part. Slimefun keeps custom per-block data through BlockStorage. Every Slimefun block on the map is a row in a YAML file under plugins/Slimefun/data-storage/Slimefun/stored-blocks/. With 200k Cargo Connector nodes scattered around four worlds you get a 200k row data set spread across many files weighing hundreds of megabytes.

The knobs that genuinely help:

# config.yml
worlds:
  enable-async-block-storage: true

performance:
  type: balanced  # low | balanced | high
  show-hidden-blocks-in-inventory: false
  show-vanilla-recipes-in-guide: false
  show-stats-on-startup: false

performance.type is a preset for AsyncBlockTicker. Slimefun decides per tick how many blocks to process and adapts to the current TPS. When TPS drops below 18, the low preset trims machine ticks instead of locking the whole plugin. Adaptive throttling beats hard freezes every time.

Tool number two: /sf timings. It lists the slowest blocks and addons. If Energy Regulator is sitting at 80% of the budget, you have one giant power grid that needs to be split into several independent ones.

Tool number three: cargo network size limits in Slimefun.yml. By default Cargo Manager walks up to 5000 nodes, which is brutal on large factories.

# Slimefun.yml
networks:
  cargo:
    max-nodes: 256
  energy:
    max-nodes: 1024

And the hardware reminder: Slimefun writes YAML, and YAML is disk I/O. On a spinning HDD you will feel save lag every five minutes. NVMe or at least a real SSD is non-negotiable.

Memory side. On large worlds with more than 50k Slimefun blocks, BlockStorage keeps the full index in RAM. On a season with InfinityExpansion and ExoticGarden installed, the plugin alone took 2.4 GB of Java heap on my box, so 6 GB of -Xmx was tight. Plan for at least 8 GB and watch /sf debug for slow leaks.

A handy diagnostic command: /sf timings paste. It uploads the timings report to the Slimefun pastebin and prints a URL. The developers and community can actually help debug your bottleneck from there. Just do not paste those links publicly without redaction, the report often contains your server name and IP info from sister plugins.

Typical admin mistakes

The rake collection. Everyone steps on these.

• Skipping a backup of plugins/Slimefun/data-storage/. If a stored-blocks file corrupts, every Slimefun block in the world becomes a ghost. Back up that folder separately from world data, on a separate cron.
• Putting old Slimefun on new Paper. RC-37 and earlier do not boot on Paper 1.21 because of API changes. If the plugin refuses to load, check Hangar for a fresh build before blaming Java.
• Removing Slimefun blocks through WorldEdit. The region clears physically, but BlockStorage still has the records. They become invisible ghost machines. Use /sf clear or the Slimefun WorldEdit Support addon.
• No region protection. A Cargo Manager on an unclaimed plot is a target. One pickaxe swing from a passing griefer kills a whole factory. WorldGuard or GriefPrevention is mandatory.
• Ignoring /sf timings after the first month. A server accumulates machines exponentially. Run timings weekly, especially after installing InfinityExpansion.
• Auto-update on production. New RC builds occasionally regress. Keep auto-update: false and stage updates on a copy first.
• Sharing XP levels with another levelling plugin. If you run CombatLevels, MyXp or similar, research costs eat into the same pool. Either set use-levels: false or move to an addon-based currency.

FAQ

Do I need CS-CoreLib for Slimefun4?

No. CS-CoreLib was a Slimefun 3 dependency, removed many releases ago. If a 2026 walkthrough tells you to install it, the walkthrough is outdated. Slimefun4 RC-38 ships as a single jar.

Can I run Slimefun on Spigot instead of Paper?

Technically yes, in practice nobody does. Paper provides async chunk loading and assorted optimisations that Slimefun depends on under heavy load. Every modern tutorial and the Slimefun maintainers themselves target Paper, with a separate fork for Folia where the status is tracked in the repo issue tracker.

Slimefun is eating 100% of my CPU. What now?

Start with /sf timings and /timings paste from Paper. Find the top five expensive blocks. Most of the time it is bloated cargo networks or a Nuclear Reactor without proper coolant. Reduce max-nodes, switch performance.type to low, and clean up bases of long inactive players.

Does Slimefun conflict with ItemsAdder or Oraxen?

Slimefun lives next to ItemsAdder and Oraxen without item ID clashes, because Slimefun stores its identity through PersistentDataContainer rather than CustomModelData. The only friction shows up when both plugins try to handle the same block-click event. Adjust event priorities or split content across separate worlds.

How do I wipe player progress for a new season?

/sf research <player> reset

For mass reset, drop the YAML files in plugins/Slimefun/data-storage/Slimefun/Players/ after a backup: find . -name '*.yml' -delete. Players keep their inventories but lose research progress.

Does Slimefun protect a server from DDoS attacks?

Obviously not, it is a gameplay plugin. Network protection happens at the perimeter. Slimefun-heavy servers tend to grow fast and attract attention, including the wrong kind, so a filtering layer like MineGuard sits in front of the box. Slimefun does not replace DDoS protection, it makes the case for it stronger.

Where do I find docs and item IDs?

GitHub Slimefun/Slimefun4, the Wiki tab. Items, machines and the addon API are all documented there. Modrinth and Hangar only host the jar and a short description; the full reference lives in the wiki.

That is the foundation. Slimefun4 boots in half an hour, but tuning it for a real player base eats a week of experimentation. The two rules I keep coming back to: back up data-storage separately from the world, and read /sf timings weekly. Apply both and the plugin will hold up for a season, two seasons, ten seasons.