diff --git a/SVSim.Database/Repositories/Card/ICardInventoryRepository.cs b/SVSim.Database/Repositories/Card/ICardInventoryRepository.cs
index 90bb475..bedb6f4 100644
--- a/SVSim.Database/Repositories/Card/ICardInventoryRepository.cs
+++ b/SVSim.Database/Repositories/Card/ICardInventoryRepository.cs
@@ -26,6 +26,12 @@ public interface ICardInventoryRepository
     /// </summary>
     /// <param name="createCounts">cardId → num_to_create. Empty dict is rejected by the caller.</param>
     Task<CreateOutcome> CreateCards(long viewerId, IReadOnlyDictionary<long, int> createCounts);
+
+    /// <summary>
+    /// Toggle the <see cref="OwnedCardEntry.IsProtected"/> flag for a single card. Idempotent.
+    /// Accepts cards with Count=0 (preserves the destruct→re-craft round-trip invariant).
+    /// </summary>
+    Task<ProtectOutcome> SetProtected(long viewerId, long cardId, bool isProtected);
 }
 
 /// <summary>
@@ -75,3 +81,14 @@ public enum CreateError
     WouldExceedMaxCopies,
     InsufficientVials,
 }
+
+public sealed record ProtectOutcome(bool IsSuccess, ProtectError? Error)
+{
+    public static ProtectOutcome Ok() => new(true, null);
+    public static ProtectOutcome Fail(ProtectError e) => new(false, e);
+}
+
+public enum ProtectError
+{
+    UnknownCard,
+}