diff --git a/src/AcDream.Core/Rendering/RenderingDiagnostics.cs b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
new file mode 100644
index 0000000..ff0a32f
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+++ b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
@@ -0,0 +1,109 @@
+using System;
+
+namespace AcDream.Core.Rendering;
+
+/// <summary>
+/// 2026-05-19 — runtime-toggleable diagnostic flags for the indoor cell
+/// rendering pipeline. Initialized from env vars at process start;
+/// flippable at runtime via the DebugPanel mirror. Log call sites read
+/// these statics so a checkbox toggle takes effect on the next frame
+/// without relaunching.
+///
+/// <para>
+/// Mirrors the L.2a <see cref="AcDream.Core.Physics.PhysicsDiagnostics"/>
+/// pattern. The master <see cref="IndoorAll"/> toggle is the user's
+/// common case — flipping it cascades to all five probe flags.
+/// </para>
+///
+/// <para>
+/// Spec: <c>docs/superpowers/specs/2026-05-19-indoor-cell-rendering-fix-design.md</c>.
+/// </para>
+/// </summary>
+public static class RenderingDiagnostics
+{
+    /// <summary>
+    /// When true, <c>WbDrawDispatcher.WalkVisibleEntities</c> emits one
+    /// <c>[indoor-walk]</c> line per visible cell entity per second:
+    /// entity id, world position, parent cell id, landblock visible flag,
+    /// AABB-visible flag, "in visible cells" flag, drew flag.
+    /// Initial state from <c>ACDREAM_PROBE_INDOOR_WALK=1</c>.
+    /// </summary>
+    public static bool ProbeIndoorWalkEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_WALK") == "1"
+        || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
+
+    /// <summary>
+    /// When true, <c>WbDrawDispatcher</c> emits one <c>[indoor-lookup]</c>
+    /// line per visible cell entity per second: render-data hit/miss,
+    /// IsSetup flag, SetupParts count, parts-hit / parts-miss tallies.
+    /// Initial state from <c>ACDREAM_PROBE_INDOOR_LOOKUP=1</c>.
+    /// </summary>
+    public static bool ProbeIndoorLookupEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_LOOKUP") == "1"
+        || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
+
+    /// <summary>
+    /// When true, <c>WbMeshAdapter</c> emits two lines per EnvCell id:
+    /// <c>[indoor-upload] requested</c> on first IncrementRefCount and
+    /// <c>[indoor-upload] completed</c> when WB's staged drain produces
+    /// its <c>ObjectMeshData</c>. Missing "completed" lines indicate WB
+    /// silently returned null (hypothesis H1).
+    /// Initial state from <c>ACDREAM_PROBE_INDOOR_UPLOAD=1</c>.
+    /// </summary>
+    public static bool ProbeIndoorUploadEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_UPLOAD") == "1"
+        || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
+
+    /// <summary>
+    /// When true, <c>WbDrawDispatcher</c> emits one <c>[indoor-xform]</c>
+    /// line per visible cell entity per second: cell-geometry SetupPart's
+    /// composed world matrix translation. Disambiguates transform
+    /// double-apply (hypothesis H5).
+    /// Initial state from <c>ACDREAM_PROBE_INDOOR_XFORM=1</c>.
+    /// </summary>
+    public static bool ProbeIndoorXformEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_XFORM") == "1"
+        || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
+
+    /// <summary>
+    /// When true, <c>WbDrawDispatcher.WalkVisibleEntities</c> emits one
+    /// <c>[indoor-cull]</c> line per cell entity that gets culled, with
+    /// the reason (visibleCellIds-miss, frustum, landblock). Disambiguates
+    /// cull bugs (hypothesis H3).
+    /// Initial state from <c>ACDREAM_PROBE_INDOOR_CULL=1</c>.
+    /// </summary>
+    public static bool ProbeIndoorCullEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_CULL") == "1"
+        || Environment.GetEnvironmentVariable("ACDREAM_PROBE_INDOOR_ALL") == "1";
+
+    /// <summary>
+    /// Master toggle. Reading reflects the AND of all five flags
+    /// (true only when every probe is on). Writing cascades — setting
+    /// to <see langword="true"/> turns ALL five flags on; setting to
+    /// <see langword="false"/> turns ALL five off.
+    /// </summary>
+    public static bool IndoorAll
+    {
+        get => ProbeIndoorWalkEnabled
+            && ProbeIndoorLookupEnabled
+            && ProbeIndoorUploadEnabled
+            && ProbeIndoorXformEnabled
+            && ProbeIndoorCullEnabled;
+        set
+        {
+            ProbeIndoorWalkEnabled   = value;
+            ProbeIndoorLookupEnabled = value;
+            ProbeIndoorUploadEnabled = value;
+            ProbeIndoorXformEnabled  = value;
+            ProbeIndoorCullEnabled   = value;
+        }
+    }
+
+    /// <summary>
+    /// Helper for probe call sites. Returns <see langword="true"/> when
+    /// the low 16 bits of <paramref name="id"/> are ≥ 0x0100 — the AC
+    /// convention for EnvCell (indoor) cells, as opposed to outdoor cells
+    /// in the 8×8 landblock grid (0x0001–0x0040).
+    /// </summary>
+    public static bool IsEnvCellId(ulong id) => (id & 0xFFFFu) >= 0x0100u;
+}