launcher/MainWindow.axaml.cs

@@ -797,6 +797,32 @@ public partial class MainWindow : Window
         try
         {
             SetBusy(true);
+
+            // ─── Refresh the Microsoft session before launch. ─────────────────
+            // _session was set at sign-in; the access token inside is short-lived
+            // (~1 hour). If the launcher's been open longer than that, MC will
+            // send stale credentials to the server and Mojang's session check
+            // rejects the join ("Invalid session"). XboxAuthNet's silent flow
+            // uses the cached refresh token (~14 day TTL) to mint a fresh
+            // access token transparently.
+            UpdateStatus("Refreshing session...", "");
+            var freshSession = await _auth.TryAuthenticateSilentlyAsync();
+            if (freshSession != null)
+            {
+                ApplySession(freshSession);
+                AppendLog("[auth] Session refreshed before launch.");
+            }
+            else
+            {
+                // Refresh token also stale -- need full interactive re-login.
+                // Don't try inline (player would lose context); prompt them to
+                // hit Sign In again.
+                AppendLog("[auth] Silent refresh returned no session; sign-in required.");
+                UpdateStatus("Session expired", "Please sign in again to continue.");
+                ClearSession();
+                return;
+            }
+
             var progress = new Progress<ProgressReport>(OnProgress);
             var installDir = GetInstallDir();
             _launch ??= new LaunchService(installDir);