diff --git a/ErsatzTV.Tests/Controllers/PlexMediaSourcesControllerTests.cs b/ErsatzTV.Tests/Controllers/PlexMediaSourcesControllerTests.cs
index 6b7431d15..4c66dacd1 100644
--- a/ErsatzTV.Tests/Controllers/PlexMediaSourcesControllerTests.cs
+++ b/ErsatzTV.Tests/Controllers/PlexMediaSourcesControllerTests.cs
@@ -259,6 +259,27 @@ public class PlexMediaSourcesControllerTests
         await _mediator.DidNotReceive().Send(Arg.Any<UpdatePlexLibraryPreferences>(), Arg.Any<CancellationToken>());
     }
 
+    [Test]
+    public async Task ReplaceLibraries_Should_Return_422_When_An_Owned_Library_Is_Missing()
+    {
+        SourceExists(true);
+        _mediator.Send(Arg.Any<GetPlexLibrariesBySourceId>(), Arg.Any<CancellationToken>())
+            .Returns(new List<PlexLibraryViewModel>
+            {
+                new(5, "Movies", LibraryMediaKind.Movies, true),
+                new(6, "Shows", LibraryMediaKind.Shows, true)
+            });
+
+        // request omits owned id 6 -> 422: the PUT is a complete flag document (§C4a), matching Jellyfin/Emby
+        IActionResult result = await _controller.ReplaceLibraryPreferences(
+            3,
+            new ReplaceRemoteLibraryPreferencesRequest([new RemoteLibraryPreferenceRequest(5, true)]),
+            CancellationToken.None);
+
+        result.ShouldBeOfType<UnprocessableEntityObjectResult>();
+        await _mediator.DidNotReceive().Send(Arg.Any<UpdatePlexLibraryPreferences>(), Arg.Any<CancellationToken>());
+    }
+
     [Test]
     public async Task ReplaceLibraries_Should_Save_Enqueue_Ordered_Pair_And_Return_Reloaded()
     {
diff --git a/ErsatzTV/Controllers/Api/PlexMediaSourcesController.cs b/ErsatzTV/Controllers/Api/PlexMediaSourcesController.cs
index c47d61e6f..e1cc451d6 100644
--- a/ErsatzTV/Controllers/Api/PlexMediaSourcesController.cs
+++ b/ErsatzTV/Controllers/Api/PlexMediaSourcesController.cs
@@ -164,6 +164,18 @@ public class PlexMediaSourcesController(
                 .ToErrorResult();
         }
 
+        // Require the complete set of this source's libraries (§C4a; matches Jellyfin/Emby): the PUT is a
+        // full flag document, so an omitted owned id would silently keep its old sync state.
+        var incomingIds = libraries.Map(l => l.Id).ToList();
+        var missingIds = ownedIds.Where(ownedId => !incomingIds.Contains(ownedId)).ToList();
+        if (missingIds.Count > 0)
+        {
+            return BaseError.New(
+                    $"The request must include every library for Plex media source {id} " +
+                    $"(missing id(s) {string.Join(", ", missingIds)})")
+                .ToErrorResult();
+        }
+
         Either<BaseError, Unit> result =
             await mediator.Send(request.ToPlexCommand(), cancellationToken);
         if (result.IsLeft)
diff --git a/web/src/screens/RemoteLibrariesEditScreen.tsx b/web/src/screens/RemoteLibrariesEditScreen.tsx
index 8e304f1df..ead8afbbc 100644
Binary files a/web/src/screens/RemoteLibrariesEditScreen.tsx and b/web/src/screens/RemoteLibrariesEditScreen.tsx differ