diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorApplyFailureTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorApplyFailureTests.cs
index af17654b..f113ea06 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorApplyFailureTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorApplyFailureTests.cs
@@ -23,6 +23,16 @@ namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.OpcUa;
 ///     <c>otopcua.opcua.apply.failed</c> meter and log at Error — instead of the optimistic Info line. The
 ///     happy path must stay Info-only (zero increments).
 /// </summary>
+/// <remarks>
+///     Shares the <c>opcua-publish-meters</c> collection with <c>OpcUaPublishActorRebuildTests</c> so the two
+///     classes NEVER run in parallel. The zero-increment assertions below (<c>recorder.Total.ShouldBe(0)</c>)
+///     read a process-wide <see cref="MeterListener"/> filtered only on the instrument NAME, and
+///     <c>otopcua.opcua.apply.failed</c> carries no tag identifying the emitting actor instance (only
+///     <c>kind=rebuild|nodes</c>) — so the assertion cannot be scoped to this test's own actor. The sibling
+///     class's degraded-apply tests deliberately increment the same counter, which intermittently made these
+///     zero assertions fail when the two classes ran concurrently in different xUnit collections.
+/// </remarks>
+[Collection("opcua-publish-meters")]
 public sealed class OpcUaPublishActorApplyFailureTests : RuntimeActorTestBase
 {
     /// <summary>A rebuild whose sink throws increments otopcua.opcua.apply.failed (kind=rebuild). R2-07 — a
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorRebuildTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorRebuildTests.cs
index 63a2cd17..5851527b 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorRebuildTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorRebuildTests.cs
@@ -16,6 +16,18 @@ using ZB.MOM.WW.OtOpcUa.Runtime.Tests.Harness;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.OpcUa;
 
+/// <summary>
+///     Shares the <c>opcua-publish-meters</c> collection with <see cref="OpcUaPublishActorApplyFailureTests"/>
+///     so the two classes NEVER run in parallel. <c>MeterRecorder</c> is a process-wide
+///     <see cref="System.Diagnostics.Metrics.MeterListener"/> filtered only on the instrument NAME, and
+///     <c>otopcua.opcua.apply.failed</c> carries no tag identifying which actor instance emitted it (only
+///     <c>kind=rebuild|nodes</c>) — so it cannot be scoped to one test. The degraded-apply tests here
+///     (<see cref="Degraded_non_rebuild_apply_does_not_advance_last_applied"/> and
+///     <see cref="Rebuild_with_failures_still_advances_last_applied"/>) deliberately increment that counter,
+///     which intermittently broke the sibling class's <c>Total.ShouldBe(0)</c> happy-path assertions when the
+///     two classes ran concurrently in different collections. Serialising them is the fix.
+/// </summary>
+[Collection("opcua-publish-meters")]
 public sealed class OpcUaPublishActorRebuildTests : RuntimeActorTestBase
 {
     /// <summary>R2-07 — RebuildAddressSpace with dbFactory loads the artifact, composes, and applies. Two