diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index 897c35d..5930c2d 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -243,6 +243,39 @@ public static class PhysicsDiagnostics
     public static bool ProbeContactPlaneEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_CONTACT_PLANE") == "1";
 
+    /// <summary>
+    /// Indoor walking ISSUES #83 H-disambiguation spike (2026-05-21).
+    /// When true, two diagnostic emissions activate:
+    /// <list type="bullet">
+    ///   <item><description>One <c>[walk-miss]</c> line per
+    ///     <see cref="Transition.TryFindIndoorWalkablePlane"/> MISS
+    ///     event, dumping foot world/local position, the nearest
+    ///     walkable polygon in the cell (with XY-containment flag and
+    ///     vertical gap), and whether the LandCell terrain at the same
+    ///     XY would have grounded the player.</description></item>
+    ///   <item><description>One <c>[floor-polys]</c> line per indoor
+    ///     cell cached, enumerating each walkable-eligible polygon's
+    ///     id, normal Z, local-XY bounding box, and plane Z at the
+    ///     bbox center.</description></item>
+    /// </list>
+    /// Together these answer H1 (multi-cell iteration missing) vs H2
+    /// (probe distance too short) vs H3 (poly absent /
+    /// <c>walkable_hits_sphere</c> rejection) for the ISSUES #83
+    /// stuck-falling bug. Spike-only — remove once the root cause is
+    /// identified and the fix lands.
+    ///
+    /// <para>
+    /// Initial state from <c>ACDREAM_PROBE_WALK_MISS=1</c>.
+    /// No DebugPanel mirror — one-shot diagnostic.
+    /// </para>
+    ///
+    /// <para>
+    /// Spec: <c>docs/superpowers/specs/2026-05-21-indoor-walk-miss-probe-design.md</c>.
+    /// </para>
+    /// </summary>
+    public static bool ProbeWalkMissEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_WALK_MISS") == "1";
+
     public static void LogCpBoolWrite(string field, bool oldValue, bool newValue)
     {
         var caller = GetCpCallerName();
diff --git a/tests/AcDream.Core.Tests/Physics/WalkMissDiagnosticTests.cs b/tests/AcDream.Core.Tests/Physics/WalkMissDiagnosticTests.cs
new file mode 100644
index 0000000..91de8c4
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/WalkMissDiagnosticTests.cs
@@ -0,0 +1,41 @@
+using AcDream.Core.Physics;
+using DatReaderWriter.Enums;
+using System.Collections.Generic;
+using System.Numerics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Tests for the ISSUES #83 H-disambiguation probe spike (spec
+/// 2026-05-21-indoor-walk-miss-probe-design.md).
+///
+/// Covers:
+/// 1. PhysicsDiagnostics.ProbeWalkMissEnabled flag get/set roundtrip.
+/// 2. WalkMissDiagnostic.AggregateNearestWalkable selects the nearest
+///    walkable polygon by |dz| when the foot XY lies inside a poly's
+///    local XY bounding box.
+/// 3. WalkMissDiagnostic.AggregateNearestWalkable falls back to the
+///    nearest poly by |dz| when no walkable poly XY-contains the foot,
+///    reporting ContainsFootXY=false.
+/// </summary>
+public class WalkMissDiagnosticTests
+{
+    [Fact]
+    public void ProbeWalkMiss_StaticApi_Roundtrip()
+    {
+        bool initial = PhysicsDiagnostics.ProbeWalkMissEnabled;
+        try
+        {
+            PhysicsDiagnostics.ProbeWalkMissEnabled = true;
+            Assert.True(PhysicsDiagnostics.ProbeWalkMissEnabled);
+
+            PhysicsDiagnostics.ProbeWalkMissEnabled = false;
+            Assert.False(PhysicsDiagnostics.ProbeWalkMissEnabled);
+        }
+        finally
+        {
+            PhysicsDiagnostics.ProbeWalkMissEnabled = initial;
+        }
+    }
+}