An "incompatible version" or "invalid version" error means the client build does not match the server build. Since the V3.1/V3.2 stable releases the single biggest cause is a version mismatch: one side is still on an older build (V2.6, V3.0 or V3.1) and the other is on V3.2.0. Both the client and the server must be on the same Steam branch, and Steam's build IDs differ between branches even when both say they are "updated."
V3.2.0 has been the current stable build on the default install (Default Public Version) since 26 August 2026. V3.1, V3.0 and V2.6 are older lines. A client on one line cannot join a server on another, in either direction. There is no cross-branch compatibility, so "we are both updated" is not enough: both sides have to be on the same branch.
The fix is to pick one branch and put everyone on it. Which branch depends on what you are running.
If you are a player who opted into a branch and can no longer join your usual server, match the server rather than the other way round.
Steam only offers the branches The Fun Pimps keep published, and old point releases do get removed. Read the live list yourself rather than trusting a guide, including this one: in Steam, right-click the game, Properties, Betas. For a dedicated server, steamcmd +login anonymous +app_info_update 1 +app_info_print 294420 +quit prints them under branches.
Checked on 17 September 2026 for the dedicated server (app 294420):
| Branch | Last updated | Use |
|---|---|---|
public | 31 Aug 2026 | Default. V3.2.0. Use this unless an overhaul says otherwise. |
v3.2.0, v3.1.0, v3.0.1 | Aug to Jul 2026 | Pinned 3.x point releases. |
v2.6 | 1 Apr 2026 | The only surviving 2.x branch. Earlier 2.x point releases have been removed, so a server pinned to one of those has nowhere to return to and must move to v2.6 or forward. |
v1.4 | Apr 2025 | The 1.0 line. |
alpha21.2, alpha20.7, and older | 2024 and earlier | Where the long-running overhauls pin. |
| Cause | What it looks like | Fix |
|---|---|---|
| Server behind on build | Server is on an older build than up-to-date clients; everyone gets kicked. | Restart the server (managed hosts update on start) or force a fresh Steam update server-side. See the server-side fix below. |
| Client behind | One or two players cannot join while others can. | Update the game in Steam, restart Steam, and verify the install. Confirm the client is on the same branch as the server. |
| Branch mismatch (stable vs experimental) | Client and server are both "fully updated" but still incompatible. | Put both sides on the same branch. This is the dominant cause during the 3.0 launch. |
On managed hosts with auto-update enabled, the server runs its Steam update automatically on every start or restart, so there is generally no separate "update game files" button - the update happens before the server boots. That means the fix for a server that is behind is almost always: restart it, and make sure the branch is set correctly first. Restarting a server that is configured for an experimental branch will keep it on that branch, so check the branch setting before you blame the restart. Once the server is on its intended branch, every player simply matches their client to that branch in Steam.
Sometimes a server flips branches (for example off an experimental branch back to default) but stays stuck on the old build, so players keep getting a version mismatch even after a restart. This is a stale Steam PICS (product info) cache: SteamCMD can report "Success! App already up to date" while silently keeping the old build. Force a fresh product-info refresh by adding +app_info_update 1:
steamcmd +login anonymous +app_info_update 1 +app_update 294420 validate +quit
A few important notes:
+app_info_update 1, SteamCMD can say "Success, No Error" while keeping the old build, which is exactly why players still see a version mismatch after what looked like a successful update.-beta flag (or -beta ""). Only if you deliberately want to test a newer experimental build, add the beta flag: +app_update 294420 -beta latest_experimental validate.On top of the branch and build checks, every mod version must match on the server and the client. A mismatched or missing mod produces the same incompatible-version result. Update or remove mismatched mods so both sides load an identical mod set. Note that most V2.6 mods do not load on V3.0, so plan mod updates around your branch move.
+app_info_update 1