Fix: Unreal Online Subsystem Session Create Fails

A multiplayer prototype calls SessionInterface->CreateSession(0, NAME_GameSession, Settings). The callback fires with bWasSuccessful = false. No specific error returned. The Steam overlay is running; the build is on the Steam version; settings look fine.

OnlineSubsystem Quick Audit

Before CreateSession can succeed:

1. The OnlineSubsystem is loaded and active.
2. The local user is signed in (identity layer).
3. The session settings are valid.
4. The backend (Steam, Epic) is reachable.

Failure in any layer aborts session creation.

Step 1: Verify Subsystem Config

DefaultEngine.ini should contain:

[OnlineSubsystem]
DefaultPlatformService=Steam

[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=480
GameServerQueryPort=27015

[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="OnlineSubsystemSteam.SteamNetConnection"

[/Script/Engine.GameEngine]
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="OnlineSubsystemSteam.SteamNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")

For Steam specifically. For Epic, replace with EpicOnlineServices. SteamDevAppId 480 is the Spacewar test app — works for development; replace with your real app ID before shipping.

Step 2: Identity Check

IOnlineSubsystem* OSS = IOnlineSubsystem::Get();
if (!OSS) {
    UE_LOG(LogTemp, Error, TEXT("OSS not loaded"));
    return;
}
IOnlineIdentityPtr Identity = OSS->GetIdentityInterface();
if (Identity->GetLoginStatus(0) != ELoginStatus::LoggedIn) {
    UE_LOG(LogTemp, Error, TEXT("Not logged in to OSS identity"));
    return;
}

If LoginStatus is NotLoggedIn or UsingLocalProfile, CreateSession will fail. Trigger login via the platform’s flow before any session API.

Step 3: Session Settings Sanity

FOnlineSessionSettings Settings;
Settings.NumPublicConnections = 4;   // at least 1
Settings.bShouldAdvertise = true;
Settings.bAllowJoinInProgress = true;
Settings.bIsLANMatch = false;
Settings.bUsesPresence = true;
Settings.bAllowJoinViaPresence = true;

NumPublicConnections = 0 silently fails on Steam. Set to your intended player count.

Step 4: Read the OSS Logs

Set LogOnline and LogOnlineSession to Verbose in DefaultEngine.ini:

[Core.Log]
LogOnline=Verbose
LogOnlineSession=Verbose

Rerun. Output Log shows the specific failure reason: “identity not logged in”, “subsystem disabled”, “invalid settings”. Read and act accordingly.

Steam-Specific Trap: Wrong Process

If you launch the editor from explorer instead of through Steam, the Steam SDK initialization may fail. Either:

• Launch through Steam (using a steam_appid.txt file for development).
• Or place a steam_appid.txt in the editor binary directory with your app ID.

Verifying

Add comprehensive logging around session calls. The OnCreateSessionComplete should fire with bWasSuccessful = true within a few seconds. If still failing, the verbose log will pinpoint the layer.

“OnlineSubsystem failures cascade through layers. Identity, settings, subsystem, backend — check each one.”

Wire a debug UI showing OSS status (subsystem name, identity status, session count) — saves hours diagnosing in builds.