diff --git a/tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj b/tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj
index a77f941d..dc186f13 100644
--- a/tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj
+++ b/tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj
@@ -22,4 +22,15 @@
     <ProjectReference Include="..\..\src\AcDream.Core.Net\AcDream.Core.Net.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- Several transport tests drive a virtual clock but assert on REAL-time
+         waits (2 s SpinUntil windows). Under default parallelism on a small CI
+         container PausedSelector_SeededDroppedServerReady_RecoversOnIdleSweep
+         took 37-42 s and missed its window, while passing 5/5 in ~350 ms in
+         isolation on the same machine. Serialize the assembly rather than
+         loosen a Campaign N transport assertion. Same settings
+         AcDream.Core.Tests and AcDream.Launcher.Tests already use. -->
+    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
 </Project>
diff --git a/tests/AcDream.Core.Net.Tests/xunit.runner.json b/tests/AcDream.Core.Net.Tests/xunit.runner.json
new file mode 100644
index 00000000..c3155894
--- /dev/null
+++ b/tests/AcDream.Core.Net.Tests/xunit.runner.json
@@ -0,0 +1,6 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "parallelizeAssembly": false,
+  "parallelizeTestCollections": false,
+  "maxParallelThreads": 1
+}