When CK3 mods break

Mods you installed by hand, and the .mod files that describe them, live in your CK3 user directory:

Windows   Documents\Paradox Interactive\Crusader Kings III\mod\
macOS     ~/Documents/Paradox Interactive/Crusader Kings III/mod/
Linux     ~/.local/share/Paradox Interactive/Crusader Kings III/mod/

Mods you subscribed to on the Steam Workshop do not go there. They land in Steam's own folder, under CK3's app id:

Steam\steamapps\workshop\content\1158310\

Each subfolder in there is named after the Workshop item's id, which is the long number at the end of its Workshop URL. That is how you work out which downloaded folder is which mod.

The third folder is the one almost nobody opens, and it is where the answers are:

Documents\Paradox Interactive\Crusader Kings III\logs\

The launcher never mentions it. It holds error.log, and more on that further down.

This is the single most useful thing to understand about CK3 mods, and it is one sentence. When two mods contain the same file, the one lower in the playset overwrites the one above it. That is the whole mechanic. The load order is not a suggestion or a loading sequence; it is a stack of overwrites, resolved top to bottom, file by file.

Two consequences follow, and between them they explain most "why is this mod not doing anything" reports.

The first: a mod that seems to have no effect is usually not broken. It is being overwritten by something below it that touches the same files. Move it down.

The second, which catches people out more: sorting a mod below another one is not the same as disabling it. If two mods ship the same file and you push one to the bottom, the one at the bottom wins that file — but every file the loser has that the winner does not is still loaded. You end up with a mix of both. When two mods are genuinely incompatible, the fix is to untick one, not to reorder it.

Compatibility patches exist because of this rule. A patch is a mod that contains the merged version of the files two other mods both changed, which is why a patch always goes last: it has to be the one doing the final overwrite.

CK3 updates. Your mods do not update at the same moment, because a human has to go and fix each one. This is the most common cause of a save that will not load and a main menu full of red.

What actually happens: a mod replaces whole files. When Paradox changes one of those files in a patch, the mod is still handing the game its older copy, which now refers to things that moved or no longer exist. The mod is not corrupt. It is out of date, which looks identical from the outside.

The supported_version line in a mod's descriptor is the author saying which game version they last checked it against. The launcher shows a warning when that does not match your game. Take the warning seriously but not literally — plenty of mods keep working across a patch that changed nothing they touch, and an author who has not updated the line yet is common. It is a hint, not a verdict.

If a patch just landed and everything is on fire, the honest answer is usually to wait a few days. Big mods get fixed. Starting a fresh campaign on a broken setup is how you lose thirty hours of it later.

With forty mods enabled and a crash on load, the instinct is to guess which one it is. Do not guess. Halving finds it in about five or six launches no matter how many mods you have, and guessing can take all evening.

Turn off the bottom half of your playset. Launch. If it works, the problem is in the half you turned off; if it still breaks, it is in the half you kept. Either way you have just eliminated half your list. Take the half that contains the problem, cut that in half, and go again. Forty mods becomes twenty, ten, five, three, two, one.

Two things make this go wrong. Keep any mod that others depend on switched on the whole time — for a Game of Thrones setup that means AGOT itself, since every submod fails without it and you will get a false result on every round. And change one thing per launch. Toggling a mod and reordering another in the same pass means you no longer know which one did it.

When you are down to a single mod, check its Workshop comments or its Discord before reporting anything. Whatever you found, someone found it four days ago.

Open Documents\Paradox Interactive\Crusader Kings III\logs\error.log in any text editor. It is plain text and it is written fresh each time the game runs, so the quickest thing you can do is delete it, launch, reproduce the problem, and read what is in the new one. Now everything in the file belongs to the run that just failed.

It will be long, and most of it does not matter. Vanilla CK3 logs errors about itself on a completely clean install, so a file with a few hundred lines is not evidence of anything. You are not trying to get it to zero.

What you are looking for is a filename you recognise and a reason next to it. Lines naming a specific file in a specific mod are worth reading. Lines about a missing localization key are usually cosmetic — that is the cause of text showing up as a raw key like my_trait_desc instead of a sentence. Lines about a trait, event or decision that could not be found are the ones that break behaviour, and they usually mean a mod is referring to something another mod removed or renamed.

You do not have to diagnose it. Finding the mod name in the log and pasting the line into that mod's Discord is enough, and it is dramatically more useful to the author than "it crashes".

The received wisdom is that mods disable achievements and that you need Ironman for them. That was true once and it is worth knowing it changed, because a lot of guides still say it.

Since CK3 version 1.9, neither Ironman mode nor an unmodified checksum is required for achievements to stay enabled. A modded game can still earn them.

What does still switch them off is certain game rules — broadly the ones that make the game easier. That is a per-rule thing rather than a mod thing, and the game tells you when a rule you have picked will do it. So if achievements are the reason you have been playing unmodded, that reason has expired.

Sometimes the right answer is to not take the patch. A campaign you are forty years into, on a mod list that works, is worth more than the newest version number.

Steam keeps older CK3 builds available as beta branches. In your Steam library, right-click Crusader Kings III, open Properties, and look under Betas — the previous major versions are listed there, and picking one downloads that build. The mods you already have keep working because the game stopped moving.

Two warnings. Saves made on a newer version will generally not open on an older one, so roll back before you play rather than after. And multiplayer requires everyone on the same version, so agree it with whoever you play with first.

There is no equivalent on Game Pass, and GOG and Epic handle old builds differently. If you are not on Steam, keeping your own copy of a mod version that works is the closest thing to insurance you have.

Keep a playset you do not touch. Make a second playset with the handful of mods you know work together and leave it alone. When something breaks, launching that one tells you in thirty seconds whether the problem is a mod or the game itself.

Delete old versions properly. Unsubscribing on the Workshop usually removes the files; a hand-installed mod never does. Both the folder and its .mod file have to go, and a leftover .mod file pointing at a folder that is gone is its own flavour of confusing.

Add mods one at a time when a campaign is running. Mid-save changes are not automatically dangerous, but six at once means you cannot tell which one caused the thing you noticed two hours later.

Back up the save folder before a big change. It is in the same user directory, under save games. Copy it somewhere. It costs nothing and it has saved more campaigns than any of the advice above.

A mod is in my folder but the launcher does not list it. Hand-installed mods need a .mod file sitting next to the folder, not the descriptor.mod inside it. The launcher only reads the outer one.

The launcher says "descriptor file error". The path= line in that .mod file is wrong. Use forward slashes even on Windows, and match the folder name exactly, capital letters included.

My save will not load after an update. Check whether a mod in the playset updated or was removed. A save remembers the mods it was made with, and one going missing is more likely to break it than one going out of date.

Everything is in English except a few bits of nonsense text. Missing localization. Cosmetic, usually a mod overwriting a file whose keys moved in a patch. It is safe to keep playing.

Do I need to start a new game after adding a mod? Depends entirely on the mod. Ones that add events, traits or decisions are usually fine mid-save. Ones that change the map, the starting setup or existing characters are usually not. The mod's own page should say, and if it does not, ask.

Everything above applies to any CK3 mod. If you got here from one of mine, The Long Night & Azor Ahai is an AGOT submod that plays out the Long Night as a real endgame, and it has a full wiki covering every mechanic in it. Supernatural puts vampires, werewolves, witches and hunters into the base game. Both are free, both have a Discord where load-order questions get answered, and both can be installed without Steam.