diff --git a/libraries/session/src/main/java/androidx/media3/session/MediaLibraryService.java b/libraries/session/src/main/java/androidx/media3/session/MediaLibraryService.java
index 1708aa7488..e3b7a715d7 100644
--- a/libraries/session/src/main/java/androidx/media3/session/MediaLibraryService.java
+++ b/libraries/session/src/main/java/androidx/media3/session/MediaLibraryService.java
@@ -148,10 +148,12 @@ public abstract class MediaLibraryService extends MediaSessionService {
        * MediaItem#mediaId}. The media id is required for the browser to get the children under the
        * root.
        *
-       * <p>Interoperability: This method may be called on the main thread regardless of the
-       * application thread if legacy {@link android.support.v4.media.MediaBrowserCompat} requested
-       * a {@link androidx.media.MediaBrowserServiceCompat.BrowserRoot}. In this case, you must
-       * return a completed future.
+       * <p>Interoperability: If this callback is called because a legacy {@link
+       * android.support.v4.media.MediaBrowserCompat} has requested a {@link
+       * androidx.media.MediaBrowserServiceCompat.BrowserRoot}, then the main thread may be blocked
+       * until the returned future is done. If your service may be queried by a legacy {@link
+       * android.support.v4.media.MediaBrowserCompat}, you should ensure that the future completes
+       * quickly to avoid blocking the main thread for a long period of time.
        *
        * @param session The session for this event.
        * @param browser The browser information.
diff --git a/libraries/session/src/main/java/androidx/media3/session/MediaLibraryServiceLegacyStub.java b/libraries/session/src/main/java/androidx/media3/session/MediaLibraryServiceLegacyStub.java
index 8f874b1459..2687c75cca 100644
--- a/libraries/session/src/main/java/androidx/media3/session/MediaLibraryServiceLegacyStub.java
+++ b/libraries/session/src/main/java/androidx/media3/session/MediaLibraryServiceLegacyStub.java
@@ -35,6 +35,7 @@ import androidx.core.util.ObjectsCompat;
 import androidx.media.MediaBrowserServiceCompat;
 import androidx.media.MediaSessionManager.RemoteUserInfo;
 import androidx.media3.common.MediaItem;
+import androidx.media3.common.util.ConditionVariable;
 import androidx.media3.common.util.Log;
 import androidx.media3.common.util.Util;
 import androidx.media3.session.MediaLibraryService.LibraryParams;
@@ -49,6 +50,7 @@ import java.util.List;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicReference;
 
 /**
  * Implementation of {@link MediaBrowserServiceCompat} for interoperability between {@link
@@ -89,22 +91,24 @@ import java.util.concurrent.Future;
     @Nullable
     LibraryParams params =
         MediaUtils.convertToLibraryParams(librarySessionImpl.getContext(), rootHints);
-    // Call onGetLibraryRoot() directly instead of posting to the application thread not to block
-    // the main thread as MediaBrowserServiceCompat requires to return browser root here.
-    // onGetLibraryRoot() has documentation that it may be called on the main thread.
-    ListenableFuture<LibraryResult<MediaItem>> future =
-        checkNotNull(
-            librarySessionImpl
-                .getCallback()
-                .onGetLibraryRoot(librarySessionImpl.getInstance(), controller, params),
-            "onGetLibraryRoot must return non-null future");
-    if (!future.isDone()) {
-      throw new RuntimeException(
-          "onGetLibraryRoot must return a completed future " + "for legacy MediaBrowserCompat");
-    }
+    AtomicReference<ListenableFuture<LibraryResult<MediaItem>>> futureReference =
+        new AtomicReference<>();
+    ConditionVariable haveFuture = new ConditionVariable();
+    postOrRun(
+        librarySessionImpl.getApplicationHandler(),
+        () -> {
+          futureReference.set(
+              checkNotNull(
+                  librarySessionImpl
+                      .getCallback()
+                      .onGetLibraryRoot(librarySessionImpl.getInstance(), controller, params),
+                  "onGetLibraryRoot must return non-null future"));
+          haveFuture.open();
+        });
     @Nullable LibraryResult<MediaItem> result = null;
     try {
-      result = checkNotNull(future.get(), "LibraryResult must not be null");
+      haveFuture.block();
+      result = checkNotNull(futureReference.get().get(), "LibraryResult must not be null");
     } catch (CancellationException | ExecutionException | InterruptedException e) {
       Log.e(TAG, "Couldn't get a result from onGetLibraryRoot", e);
     }
diff --git a/libraries/session/src/main/java/androidx/media3/session/MediaSession.java b/libraries/session/src/main/java/androidx/media3/session/MediaSession.java
index fdf08ace0c..b145d60c78 100644
--- a/libraries/session/src/main/java/androidx/media3/session/MediaSession.java
+++ b/libraries/session/src/main/java/androidx/media3/session/MediaSession.java
@@ -792,8 +792,10 @@ public class MediaSession {
      * #sendCustomCommand}, {@link #setCustomLayout}) will be ignored. Use {@link #onPostConnect}
      * for custom initialization of the controller instead.
      *
-     * <p>Interoperability: this callback may be called on the main thread, regardless of the
-     * application thread.
+     * <p>Interoperability: If a legacy controller is connecting to the session then this callback
+     * may block the main thread, even if it's called on a different application thread. If it's
+     * possible that legacy controllers will connect to the session, you should ensure that the
+     * callback returns quickly to avoid blocking the main thread for a long period of time.
      *
      * @param session The session for this event.
      * @param controller The controller information.
diff --git a/libraries/session/src/main/java/androidx/media3/session/MediaSessionServiceLegacyStub.java b/libraries/session/src/main/java/androidx/media3/session/MediaSessionServiceLegacyStub.java
index eaea4a5d93..0e77087224 100644
--- a/libraries/session/src/main/java/androidx/media3/session/MediaSessionServiceLegacyStub.java
+++ b/libraries/session/src/main/java/androidx/media3/session/MediaSessionServiceLegacyStub.java
@@ -15,6 +15,9 @@
  */
 package androidx.media3.session;
 
+import static androidx.media3.common.util.Assertions.checkNotNull;
+import static androidx.media3.common.util.Util.postOrRun;
+
 import android.os.Bundle;
 import android.support.v4.media.MediaBrowserCompat.MediaItem;
 import android.support.v4.media.session.MediaSessionCompat;
@@ -22,8 +25,11 @@ import androidx.annotation.Nullable;
 import androidx.media.MediaBrowserServiceCompat;
 import androidx.media.MediaSessionManager;
 import androidx.media.MediaSessionManager.RemoteUserInfo;
+import androidx.media3.common.util.ConditionVariable;
+import androidx.media3.common.util.Log;
 import androidx.media3.session.MediaSession.ControllerInfo;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicReference;
 
 /**
  * Implementation of {@link MediaBrowserServiceCompat} for interoperability between {@link
@@ -31,6 +37,8 @@ import java.util.List;
  */
 /* package */ class MediaSessionServiceLegacyStub extends MediaBrowserServiceCompat {
 
+  private static final String TAG = "MSSLegacyStub";
+
   private final MediaSessionManager manager;
   private final MediaSession.MediaSessionImpl sessionImpl;
   private final ConnectedControllersManager<RemoteUserInfo> connectedControllersManager;
@@ -55,19 +63,30 @@ import java.util.List;
       String clientPackageName, int clientUid, @Nullable Bundle rootHints) {
     RemoteUserInfo info = getCurrentBrowserInfo();
     MediaSession.ControllerInfo controller = createControllerInfo(info);
-    // Call onConnect() directly instead of posting to the application thread not to block the main
-    // thread as MediaBrowserServiceCompat requires to return browser root here.
-    // onConnect() has documentation that it may be called on the main thread.
-    MediaSession.ConnectionResult connectionResult =
-        sessionImpl.getCallback().onConnect(sessionImpl.getInstance(), controller);
-    if (!connectionResult.isAccepted) {
+
+    AtomicReference<MediaSession.ConnectionResult> resultReference = new AtomicReference<>();
+    ConditionVariable haveResult = new ConditionVariable();
+    postOrRun(
+        sessionImpl.getApplicationHandler(),
+        () -> {
+          resultReference.set(
+              checkNotNull(
+                  sessionImpl.getCallback().onConnect(sessionImpl.getInstance(), controller),
+                  "onConnect must return non-null future"));
+          haveResult.open();
+        });
+    try {
+      haveResult.block();
+    } catch (InterruptedException e) {
+      Log.e(TAG, "Couldn't get a result from onConnect", e);
+      return null;
+    }
+    MediaSession.ConnectionResult result = resultReference.get();
+    if (!result.isAccepted) {
       return null;
     }
     connectedControllersManager.addController(
-        info,
-        controller,
-        connectionResult.availableSessionCommands,
-        connectionResult.availablePlayerCommands);
+        info, controller, result.availableSessionCommands, result.availablePlayerCommands);
     // No library root, but keep browser compat connected to allow getting session.
     return MediaUtils.defaultBrowserRoot;
   }