Mod compatibility issues are common when running modded servers. This guide helps identify and resolve conflicts between mods, missing dependencies, and version mismatches.

Understand Common Compatibility Issues

Learn to recognize different types of mod conflicts:

Types of Conflicts:

ID conflicts: Two mods using same block/item IDs
Version mismatches: Mod requires different MC version
Missing dependencies: Required library mods not installed
API incompatibility: Forge/Fabric/Quilt conflicts
Core mod conflicts: Mods modifying same game mechanics

Common Error Messages:

"Missing mods" or "Missing dependencies"
"Mod X requires version Y"
"ID conflict detected"
"Mixin conflict" or "ASM conflict"
"Incompatible FML modded server"

Check Crash Reports and Logs

Analyze error messages to identify the problem:

Locate crash report: Check crash-reports/ folder
Find latest log: Open logs/latest.log
Search for errors: Look for "ERROR", "FATAL", or mod names
Identify problematic mods: Note which mods appear in error messages
Check stack traces: Find the root cause of crashes

Key log indicators:

[ERROR] Missing or unsupported mandatory dependencies
[FATAL] Mixin apply failed
[WARN] Skipping incompatible mod
java.lang.NoClassDefFoundError: Missing library

Verify Mod Versions

Ensure all mods are compatible with your server version:

Version Checking Steps:

Check your Minecraft server version
Verify Forge/Fabric/Quilt version
Review each mod's compatible versions
Update outdated mods
Remove incompatible mods

Version Compatibility Matrix:

Minecraft 1.20.1 → Forge 47.2.0 → Mods for 1.20.1
Minecraft 1.19.2 → Fabric 0.14.22 → Mods for 1.19.2
Minecraft 1.18.2 → Forge 40.2.0 → Mods for 1.18.2

Never mix Forge and Fabric mods. They use different mod loading systems and are incompatible.

Resolve Missing Dependencies

Install required library and core mods:

Read mod descriptions: Check for required dependencies
Common libraries: Architectury, Cloth Config, Kotlin for Forge
Download dependencies: Get exact versions specified
Load order: Ensure libraries load before dependent mods
Check nested dependencies: Some libraries require other libraries

Popular dependency mods:

• Architectury API - Cross-platform mod library
• Cloth Config API - Configuration screens
• GeckoLib - Animation engine
• Patchouli - Documentation books
• JEI/REI - Recipe viewing

Fix ID Conflicts

Resolve conflicts when mods use the same IDs:

ID Conflict Resolution:

Identify conflicting IDs in crash report
Open mod configuration files
Change block/item IDs to unused values
Use ID conflict resolver mods if available
Test after each change

Example ID change in config:

# Before (conflicting)
blockID=500
itemID=5000

# After (resolved)
blockID=5500
itemID=15000

Handle Mixin Conflicts

Address advanced conflicts between core mods:

Identify mixin conflicts: Look for "mixin" in error logs
Check mod compatibility: Some mods cannot work together
Update mods: Newer versions may fix conflicts
Disable conflicting features: Turn off overlapping functionality
Report to developers: Some conflicts need mod author fixes

Mixin conflicts occur when multiple mods try to modify the same game code. These often require choosing between conflicting mods.

Test Mods Systematically

Use binary search method to find problematic mods:

Backup your mods folder
Remove half of the mods
Test if server starts
If it works: Problem is in removed mods
If it fails: Problem is in remaining mods
Repeat with smaller groups until you find the conflict
Test mod combinations to confirm compatibility

Testing strategy:

Start: 100 mods → Server crashes
Test 1: 50 mods → Server works (problem in other 50)
Test 2: 25 of problematic mods → Server crashes
Test 3: 12 mods → Server works
Continue until problematic mod found

Use Compatibility Tools

Leverage tools to prevent and fix conflicts:

Helpful Tools and Mods:

Mod Menu: Easily manage and configure mods
Configured: In-game configuration editing
CraftTweaker: Modify recipes and resolve conflicts
KubeJS: Script custom compatibility fixes
Roughly Enough Items: Check for recipe conflicts

Modpack Solutions:

Use pre-made modpacks with tested compatibility
ATLauncher, CurseForge, and FTB handle dependencies
Custom modpacks often include compatibility patches

Prevent Future Conflicts

Best practices for maintaining mod compatibility:

Read mod requirements: Always check dependencies first
Update gradually: Don't update all mods at once
Keep backups: Save working mod configurations
Test locally: Try new mods in single-player first
Document changes: Track which mods you add/remove
Join mod communities: Learn about known conflicts
Use mod managers: Tools like MultiMC help manage versions

Most mod compatibility issues can be resolved by ensuring correct versions, installing dependencies, and removing conflicting mods. When in doubt, start with fewer mods and add them gradually.

How to Fix Mod Compatibility Issues