diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index 43a1a8d..06a3180 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -512,6 +512,56 @@ public static class PhysicsDiagnostics
 
     public static bool ProbeDumpGfxObjsEnabled => ProbeDumpGfxObjIds.Count > 0;
 
+    /// <summary>
+    /// Test-only reset: set every probe flag to <c>false</c> and clear any
+    /// side-channel fields.  Does NOT re-read environment variables — tests
+    /// run in environments where the env vars are all absent, so false is
+    /// the correct default.
+    ///
+    /// <para>
+    /// Call from test constructors and <c>IDisposable.Dispose()</c> to
+    /// prevent one test class from leaving enabled probes that corrupt
+    /// timing-sensitive tests in another class (the static-leak root cause
+    /// documented in T0).
+    /// </para>
+    ///
+    /// <para>
+    /// This method is intentionally <c>public</c> so test projects can call
+    /// it without reflection, but it must NEVER be called from production
+    /// code paths.
+    /// </para>
+    /// </summary>
+    public static void ResetForTest()
+    {
+        ProbeResolveEnabled           = false;
+        ProbeCellEnabled              = false;
+        ProbeBuildingEnabled          = false;
+        ProbeCellSetEnabled           = false;
+        ProbeAutoWalkEnabled          = false;
+        ProbeUseabilityFallbackEnabled= false;
+        DumpSteepRoofEnabled          = false;
+        ProbeIndoorBspEnabled         = false;
+        ProbeCellCacheEnabled         = false;
+        ProbeContactPlaneEnabled      = false;
+        ProbeWalkMissEnabled          = false;
+        ProbePushBackEnabled          = false;
+        ProbePolyDumpEnabled          = false;
+        ProbePlacementFailEnabled     = false;
+        ProbeSweptEnabled             = false;
+        ProbeStepWalkEnabled          = false;
+
+        // Side-channel fields
+        LastBspHitPoly                = null;
+        LastPlacementFailPolyId       = 0;
+        LastPlacementFailPolyNormal   = default;
+        LastPlacementFailPolyD        = 0f;
+        LastPlacementFailSolidLeaf    = false;
+
+        // Dump-trigger sets
+        ProbeDumpCellIds              = new System.Collections.Generic.HashSet<uint>();
+        ProbeDumpGfxObjIds            = new System.Collections.Generic.HashSet<uint>();
+    }
+
     private static IReadOnlySet<uint> ParseHexIdList(string? raw)
     {
         if (string.IsNullOrWhiteSpace(raw))
diff --git a/src/AcDream.Core/Physics/PhysicsResolveCapture.cs b/src/AcDream.Core/Physics/PhysicsResolveCapture.cs
index ad514cd..69f508d 100644
--- a/src/AcDream.Core/Physics/PhysicsResolveCapture.cs
+++ b/src/AcDream.Core/Physics/PhysicsResolveCapture.cs
@@ -148,6 +148,24 @@ public static class PhysicsResolveCapture
     public static void ResetTickCounter() =>
         Interlocked.Exchange(ref _tickCounter, 0);
 
+    /// <summary>
+    /// Test-only reset: close any open writer, clear <see cref="CapturePath"/>,
+    /// and reset the tick counter to 0.  Call from test constructors and
+    /// <c>IDisposable.Dispose()</c> to prevent static state from leaking
+    /// across test-class boundaries.
+    ///
+    /// <para>
+    /// This method is intentionally <c>public</c> so test projects can call it
+    /// without reflection, but it must NEVER be called from production code paths.
+    /// </para>
+    /// </summary>
+    public static void ResetForTest()
+    {
+        Close();
+        CapturePath = null;
+        Interlocked.Exchange(ref _tickCounter, 0);
+    }
+
     private static void EnsureWriter_NoLock()
     {
         if (_writer is not null)
diff --git a/tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj b/tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj
index 1c83fc3..f848e8a 100644
--- a/tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj
+++ b/tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj
@@ -18,6 +18,15 @@
     <Using Include="Xunit" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- T0: force serial test execution to prevent PhysicsResolveCapture /
+         PhysicsDiagnostics static state from leaking between test classes.
+         These statics are mutated by diagnostic-harness tests; xUnit's
+         default parallel execution races them against victim tests like
+         MotionInterpreterTests and PlayerMovementControllerTests. -->
+    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\src\AcDream.Core\AcDream.Core.csproj" />
     <ProjectReference Include="..\..\src\AcDream.App\AcDream.App.csproj" />
diff --git a/tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs b/tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs
index a409bb1..0054a82 100644
--- a/tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs
@@ -80,8 +80,25 @@ namespace AcDream.Core.Tests.Physics;
 /// trajectory shape.
 /// </para>
 /// </summary>
-public class CellarUpTrajectoryReplayTests
+public class CellarUpTrajectoryReplayTests : IDisposable
 {
+    // ── T0: constructor + Dispose reset PhysicsResolveCapture static ────────
+    // xUnit creates a fresh instance per test, so the constructor runs before
+    // each test and Dispose runs after. Resetting here guarantees that no
+    // prior test in this class can leave CapturePath set (which would cause
+    // Capture_SkipsNonPlayerCalls to find an unexpected file).
+    public CellarUpTrajectoryReplayTests()
+    {
+        PhysicsResolveCapture.ResetForTest();
+        PhysicsDiagnostics.ResetForTest();
+    }
+
+    public void Dispose()
+    {
+        PhysicsResolveCapture.ResetForTest();
+        PhysicsDiagnostics.ResetForTest();
+    }
+
     // ── Cellar / cottage geometry constants ────────────────────────
     private const uint CellarId         = 0xA9B40147u;
     private const uint CottageNeighborA = 0xA9B40143u;
diff --git a/tests/AcDream.Core.Tests/xunit.runner.json b/tests/AcDream.Core.Tests/xunit.runner.json
new file mode 100644
index 0000000..c315589
--- /dev/null
+++ b/tests/AcDream.Core.Tests/xunit.runner.json
@@ -0,0 +1,6 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "parallelizeAssembly": false,
+  "parallelizeTestCollections": false,
+  "maxParallelThreads": 1
+}