ID Mismatch Fabric 1.21.1 when adding mods to pre-existing world

wondering if there's a solution to this problem or would it be better just to restore the backup

Never dealt with this personally but I consulted AI and it seemed surprisingly helpful 🤣 (I don’t recommend AI in almost any circumstance)

what did it say?

on my phone, i’ll try to copy/paste here as it doesn’t appear to have a share button

Yes, an ID mismatch in a modded Minecraft world usually occurs when the block or item IDs (or registry names, in newer Minecraft versions) have changed due to:

- Mods being added/removed/updated
- Load order or config changes
- Inconsistent mod versions between client and server

Here are some ways to fix or mitigate the mismatch, depending on the situation:

### 1. Confirm Mod Version and Load Order Are Identical
Make absolutely sure that:
- The client and server have the exact same mods in the same versions.
- The config folder (especially files like blockIDs.cfg, forge.cfg, etc., in older versions) is the same between client and server.
- The mod loading order hasn't changed (more critical in older versions).

### 2. Use a Tool Like Forge’s MissingMappings
In Minecraft 1.13+, Forge supports datafixers and registry remapping:
- You can remap missing or mismatched blocks/items by editing the datafixer JSONs or letting Forge do some automatic remapping.
- If Forge detects missing registry entries, it may prompt you to accept a remap. Be careful—this can result in lost blocks/entities.

### 3. Use a Mod Like FML Missing Registry Remapper (1.12.x)
In Minecraft 1.12.2 and below, try:
- The Forge level.dat registry fix tool or NBT editors (like NBTExplorer) to edit level.dat and *.dat region files to correct or remove problematic IDs.
- Alternatively, use WorldEdit or similar mods to replace broken blocks.

### 4. Restore From Backup and Add Mods Carefully
If the mismatch broke the world:
- Restore from backup.
- Add one mod at a time, then load into the world and ensure everything works.
- This lets Minecraft safely register new blocks/items without displacing old ones.

### 5. Use Tools Like NBTExplorer or Amidst
If blocks have been replaced (e.g., a custom block becomes air or another block), you can:
- Use NBTExplorer to manually inspect and edit affected chunks.
- Use MCEdit or Amidst to visually replace blocks en masse.

Take this with a grain of salt please as it’s directly from ChatGPT. Also, to reiterate, I don’t recommend consulting AI, but being that it’s 5:30am for me and I’ve never had to deal with this issue, i’ll happily share this if it can help you

i did already try to match server and client configs alongside making sure mods match as well

but that doesnt seem to fix it

so i might just try to add things one by one and see how it goes

How many mods did you add to the modpack after the world was already created?

i added 3 and deleted 1

this definitely breaks our discord rules so I don’t recommend doing it but uhhhh @AeonRemnant the resident king of all things modding. you know what to do in this situation? 😭😭😭

Wait wot? Against rules?

rule #4

Smh.

Do this.

Find what's broken, hone on that.

Thank you Aeon 👑