diff --git a/dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AServerServiceCollectionExtensions.cs b/dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AServerServiceCollectionExtensions.cs
index 8b7ff6d28b..deafbb2ced 100644
--- a/dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AServerServiceCollectionExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AServerServiceCollectionExtensions.cs
@@ -145,7 +145,7 @@ public static class A2AServerServiceCollectionExtensions
             if (agentSessionStore?.GetService<IsolationKeyScopedAgentSessionStore>() is null)
             {
                 agentSessionStore ??= new InMemoryAgentSessionStore();
-                agentSessionStore = new IsolationKeyScopedAgentSessionStore(agentSessionStore, isolationKeyProvider, new());
+                agentSessionStore = new IsolationKeyScopedAgentSessionStore(agentSessionStore, isolationKeyProvider, new() { Strict = isolationKeyProvider != null });
             }
 
             var hostAgent = new AIHostAgent(