Fix: Package version mismatch / Failed launching URL (Killing Floor)

If you tried to join a server and got an error like:

Failed launching URL Package 'Flares' version mismatch
Failed launching URL Package 'BlackCat' version mismatch

…this is usually a cache conflict. Your client has an old version of a package (map/mutator/resource), and the server expects a different one.

In plain words: you already downloaded something with the same name, but a different version.

The fix (works in most cases)

1. Completely close Killing Floor.
2. Open your game folder and find the Cache folder.
3. Rename or move the folder (this clears the cache).
4. Start the game and join the server again. The needed files will download again.

That’s it. It’s the same fix we recommend in our main FAQ, just pulled into a dedicated page so it’s easy to share.

Where is the Cache folder?

It depends on how your client is installed, but for Steam it’s typically inside the game directory:

...steamapps\common\killingfloor\Cache

If you cannot find it, open the game folder from Steam and search for “Cache”.

Extra tips

• If the server uses custom content, first join may take a few minutes — let it finish downloading.
• If you’re reconnecting a lot, consider increasing cache retention so files don’t disappear too quickly:
  • Optimization guide (INI, cache)

Why this happens (so it makes sense)

Community servers often update custom maps and packages. Your client keeps cached copies. When the server has a newer (or different) build with the same package name, KF detects the mismatch and refuses to load it.

The fix is not “download harder”. The fix is to remove the old cached copy so the client can download the correct one.

What not to do (common mistakes)

• Do not reinstall the whole game first. Clearing cache is faster and usually enough.
• Do not delete random .u packages in other folders unless you know exactly what they are.
• Do not ignore the error and keep reconnecting for 30 minutes. Fix cache once, then join.

If it still doesn’t work

• Try reconnecting several times using the console command:

reconnect

• Verify that you are joining the same server (and not a different one with a similar name). Different servers can ship different versions of the same packages.

• If you recently joined many different servers with lots of custom content, doing one clean cache reset is often faster than trying to delete single packages.

• Check the general FAQ and common pitfalls:

  • F.A.Q.

If you’re still stuck, ask in the community:

• Discord