Fixes & Performance

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

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?

...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:

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

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 the mismatch is on a stock game package (after patches 1054-1056 in 2013 this happened with KFMapEndTextures.utx and KFMapEndTextures.rus_utx), the server and client are on different builds. The server owner has to update to the same build as Steam clients, or take that file from a current Steam install (from the "Patches 1054-1055-1056: problems and fixes" thread on the killingfloor.ru forum).

  • If the client is the outdated side, update the whole client, not a single file. In the "Не могу зайти на серверы" (can't join servers) thread on the killingfloor.ru forum, a player with client 1046 joining a 1060 server had the download (file kf_gore_trip_sm) drop at 20%, later at 91%, with the connection lost on map change. The answer: the versions do not match; either find and install the patches or download a full new 1060 client.

  • 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.

Additions

You can also cause this error yourself by replacing a stock package with a modified one. The "Как заткнуть торговку?" (how to shut the trader up) thread shared a stripped KF_Trader.uax that mutes the trader's voice without touching other sounds: it went into the Sounds folder, overwriting the original (in the Russian version under the name KF_Trader.rus_uax). ЛОХМАТЫЙ tested it online: when joining a server the game first downloads the "correct" KF_Trader.uax (7 MB) and then reports Package 'KF_Trader.uax' version mismatch. So with such a replacement you cannot join servers; to play online again, restore the original file from your backup (the thread advised making one before replacing).

The same goes for the "old film" noise you see in spectator mode (the "Как удалить шум у спектатора?" thread). According to Proton, this effect lives in Textures\InterfaceArt2_tex.utx, and it can be removed from that file, but then joining servers gives a package version mismatch error for InterfaceArt2_tex.utx: solo play works, servers don't. F4TAL1ST added that a player with the edited file should be able to join a server that uses the same edited texture, but then players with the normal InterfaceArt2_tex.utx can't join that server.