diff --git a/Projects/Server.Tests/Tests/PropertyList/ObjectPropertyListReentrancyTests.cs b/Projects/Server.Tests/Tests/PropertyList/ObjectPropertyListReentrancyTests.cs
index 8da5f748a..b7816ae96 100644
--- a/Projects/Server.Tests/Tests/PropertyList/ObjectPropertyListReentrancyTests.cs
+++ b/Projects/Server.Tests/Tests/PropertyList/ObjectPropertyListReentrancyTests.cs
@@ -8,6 +8,9 @@ namespace Server.Tests;
 /// hole is evaluated while it is live. A Reset()/Dispose() landing in that window used to leave the
 /// next Append* spanning a null array: ArgumentNullException, parameter "array".
 /// </summary>
+// Sequential: building a list rents and returns through STArrayPool, whose bucket cache is a plain static
+// with a check-then-act initialize. Two test classes doing this on different threads race it.
+[Collection("Sequential Server Tests")]
 public class ObjectPropertyListReentrancyTests
 {
     // Stands in for a property getter that invalidates while its own tooltip is being built.
@@ -52,6 +55,7 @@ public class ObjectPropertyListReentrancyTests
 /// The guard is per-list, so nested builds (a GetProperties override that reads another entity's
 /// PropertyList) cannot unguard the outer one the way a single shared slot would.
 /// </summary>
+[Collection("Sequential Server Tests")] // same STArrayPool exposure as above
 public class ObjectPropertyListNestedBuildTests
 {
     [Fact]
diff --git a/Projects/UOContent.Tests/Tests/Network/AutoDenylist/AutoDenylistTests.cs b/Projects/UOContent.Tests/Tests/Network/AutoDenylist/AutoDenylistTests.cs
index 71aa03aeb..ef5788ee4 100644
--- a/Projects/UOContent.Tests/Tests/Network/AutoDenylist/AutoDenylistTests.cs
+++ b/Projects/UOContent.Tests/Tests/Network/AutoDenylist/AutoDenylistTests.cs
@@ -22,6 +22,11 @@ namespace Server.Tests.Network.AutoDenylists;
 
 // Static store, so every test resets it first. Addresses come from TEST-NET-2 (198.51.100.0/24) to stay clear
 // of the other ban tests if xUnit runs these classes concurrently.
+//
+// Sequential because the cap tests reach AutoDenylist.Sweep, which rents through STArrayPool. That pool is
+// single-threaded by design and its bucket cache is a plain static, so touching it from two test threads
+// races. The blocklist tests need no such marking because BlocklistSnapshot.Build asks for the mt pool.
+[Collection("Sequential UOContent Tests")]
 public class AutoDenylistTests
 {
     private const long DurationMs = 900_000; // 15 minutes, the shipped default