Cities: Skylines II Patch 1.5.8f1 Notes (May 2026)

If your Cities: Skylines II startup times ballooned after the last patch, this one's for you. Patch 1.5.8f1 is out today, and the headline fix is the mod loading slowdown that hit players hard, particularly anyone running a large mod list.

Colossal Order have actually been transparent about what caused it, which I appreciate. A change in the PDX Mods SDK meant that every mod adding new localization strings was triggering a full reinitialisation of the PDX Mods platform on startup. Multiply that by dozens or hundreds of mods and your CPU was grinding through a completely unnecessary workload every time you launched the game. Their internal test set only covered around 100 mods, which clearly wasn't enough, and they've said they'll expand that significantly going forward. A save game crash is also fixed in this build.

It's a small patch, but a necessary one, and the post-mortem explanation is exactly what good developer communication looks like. Here's the full breakdown.

Full Patch Notes

Hello Mayors!

This patch should not affect mods, and loading of old savegames is supported.

After our latest patch, we received a lot of reports about the long loading times that occurred when starting the game. Thank you again for your comments and patience while we were working on a solution! We have now prepared a hotfix that addresses this issue, which you can read more about below.

Paradox Mods:

Fixed long loading times when mods are loaded at startup.

Misc:

Info and Warning level logging no longer print callstacks to log files.
Fixed a crash when loading a save game.

Post-mortem

When you started the game after the recent patch, you might have noticed that the startup loading times were a lot longer than in previous versions, especially if you had large amounts of mods active. Due to changes in the PDX Mods SDK, the way mods were loaded by the game added an unintentional side effect.

To be more specific, the issue causing long loading times on startup was related to mods adding new entries to localization. When new entries were added, it caused reinitialisation of the whole PDX Mods platform within the game, which is a heavy operation. This caused CPU usage to spike on loading, and it was multiplied by the amount of mods that added new localization strings. Any mod that added new localization strings was affected, whether it was a code mod or an asset mod.

We should have caught this in our testing, but our test set contained only around 100 mods. To make sure this does not happen in the future, we will increase this test set significantly and improve our mod testing in general. In the long run, we are also preparing steps to reduce the issues where new patches cause issues with mods. We will discuss this in more detail in our future City Corners.