diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/EnrollmentOrchestratorTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/EnrollmentOrchestratorTests.cs
index 57f6fe8c..396e4573 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/EnrollmentOrchestratorTests.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/EnrollmentOrchestratorTests.cs
@@ -18,6 +18,10 @@ using Xunit;
 
 namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
 {
+    // Persistence-IO + Task-coordination tests are timing-sensitive enough that the
+    // parallel xUnit pool starves the worker threads on a busy CI box. Serialised
+    // against the other threading-sensitive classes via SerialThreading.
+    [Collection("SerialThreading")]
     public sealed class EnrollmentOrchestratorTests
     {
         private static DateTime At => new DateTime(2026, 4, 20, 10, 0, 0, DateTimeKind.Utc);
diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/RecoveryPathTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/RecoveryPathTests.cs
index c89d0fe6..995139f8 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/RecoveryPathTests.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/RecoveryPathTests.cs
@@ -20,6 +20,10 @@ using Xunit;
 
 namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
 {
+    // Snapshot/journal file IO + ThreadPool-driven persistence races make this class
+    // intermittently flaky under the parallel xUnit pool. Serialised against other
+    // threading-sensitive classes via the SerialThreading collection.
+    [Collection("SerialThreading")]
     public sealed class RecoveryPathTests
     {
         private static DateTime At => new DateTime(2026, 4, 20, 10, 0, 0, DateTimeKind.Utc);
diff --git a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/CollectorBaseReentrancyTests.cs b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/CollectorBaseReentrancyTests.cs
index e8733eec..f327dfda 100644
--- a/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/CollectorBaseReentrancyTests.cs
+++ b/src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/CollectorBaseReentrancyTests.cs
@@ -28,7 +28,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
     /// gate behaviour so a future "let's just remove the guard, it never fires" refactor
     /// has a hard signal.
     /// </para>
+    /// <para>
+    /// Threading-sensitive: the test exercises the <see cref="Interlocked"/> gate via
+    /// real Tasks + SpinUntil. Under the parallel xUnit pool the ThreadPool gets
+    /// contended by unrelated tests and the SpinUntil times out before the gate
+    /// closes. Serialised against the other threading-sensitive classes via
+    /// SerialThreading.
+    /// </para>
     /// </summary>
+    [Collection("SerialThreading")]
     public sealed class CollectorBaseReentrancyTests
     {
         private static AgentLogger NewLogger(TempDirectory tmp)