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Race Day Patch 1.21.1-f4 - State of Mods and Patch 101

Cities: Skylines · Community Announcements · March 09, 2026

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On March 10 at 10 AM CET, the latest patch for Cities: Skylines will go live. Along with the new Expansion, there is also the free patch, given to and released to EVERYONE who owns the game.

There are significant changes in the code for this patch, so this update will probably break many mods. If you use mods, this is good information to have for tomorrow.

While some modders have been able to update their mods before this update goes live, they might not be able to update their mods as soon as it goes live.

We understand that picking up an old project from many years ago isn’t always the easiest, and with all the changes that are included in this patch, we ask that you give your favorite modders time to update their mods. Please don’t bug them to update their mods; they’re probably already working on it. 🙏

With this in mind, we have dusted off the cobwebs of this old Patch 101, updated it, and adjusted the text that needed adjusting. We think that it should help with sharing the basic knowledge of what a Patch can do to your mods, and what you can do to start playing faster. BUT FIRST;

! Don't Panic !

If you play without mods, you can enjoy everything the Expansion, Content Creator Packs, and Free Update provide at the time of release.

If you use mods, there may be issues when you try to launch the game at the time of release. This is because some mods you rely on may be broken, and if you try to play without them, the game will be altered/glitched/broken.


Table of Contents:

  1. Patch Day 101

  2. Back up your save files

  3. In-game warnings about compatibility

  4. Dealing with broken mods


1. Patch Day 101


2. Back up your save files

Windows: %LOCALAPPDATA%\\Colossal Order\\Cities_Skylines\\Saves

Mac: /Users/\/Library/Application Support/Colossal Order/Cities_Skylines/Saves

Linux: $XDG_DATA_HOME/Colossal Order/Cities_Skylines/Saves


3. In-game warnings about compatibility


4. Dealing with broken mods

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