diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 025b5533..a2437596 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -65,10 +65,23 @@ jobs:
         run: |
           set -e
           dotnet --version
+          # Core.Net runs SINGLE-THREADED here, on its own, and the split is
+          # measured rather than defensive: on this 6-core container the
+          # assembly FAILS in 40 s with default parallelism and PASSES in 10 s
+          # with one thread. Its sessions do real socket work on background
+          # threads, so contention both breaks and slows them. Windows has 18
+          # cores, passes in ~7 s parallel, and REGRESSED when serialized, so
+          # this stays scoped to Linux.
+          echo '::group::AcDream.Core.Net.Tests (single-threaded)'
+          dotnet test tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj \
+            -c Release --nologo \
+            --filter 'Lane!=InstalledDat&Lane!=PreparedPackage&Lane!=Live&Lane!=Manual&Lane!=Timing&Lane!=Windows&Lane!=SystemFont&Purpose!=Diagnostic&Status!=KnownFailure' \
+            -- xUnit.MaxParallelThreads=1
+          echo '::endgroup::'
+
           for p in \
             tests/AcDream.Platform.Tests \
             tests/AcDream.Core.Tests \
-            tests/AcDream.Core.Net.Tests \
             tests/AcDream.Content.Tests \
             tests/AcDream.Runtime.Tests \
             tests/AcDream.Headless.Tests \