diff --git a/tests/AcDream.Core.Tests/Rendering/RenderingDiagnosticsTests.cs b/tests/AcDream.Core.Tests/Rendering/RenderingDiagnosticsTests.cs
new file mode 100644
index 0000000..8d020e5
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Rendering/RenderingDiagnosticsTests.cs
@@ -0,0 +1,62 @@
+using AcDream.Core.Rendering;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public sealed class RenderingDiagnosticsTests
+{
+    [Fact]
+    public void IndoorAll_True_TurnsAllFlagsOn()
+    {
+        // Reset all flags off first to make the test deterministic
+        // regardless of env-var state on the test runner.
+        RenderingDiagnostics.ProbeIndoorWalkEnabled   = false;
+        RenderingDiagnostics.ProbeIndoorLookupEnabled = false;
+        RenderingDiagnostics.ProbeIndoorUploadEnabled = false;
+        RenderingDiagnostics.ProbeIndoorXformEnabled  = false;
+        RenderingDiagnostics.ProbeIndoorCullEnabled   = false;
+
+        RenderingDiagnostics.IndoorAll = true;
+
+        Assert.True(RenderingDiagnostics.ProbeIndoorWalkEnabled);
+        Assert.True(RenderingDiagnostics.ProbeIndoorLookupEnabled);
+        Assert.True(RenderingDiagnostics.ProbeIndoorUploadEnabled);
+        Assert.True(RenderingDiagnostics.ProbeIndoorXformEnabled);
+        Assert.True(RenderingDiagnostics.ProbeIndoorCullEnabled);
+        Assert.True(RenderingDiagnostics.IndoorAll);
+    }
+
+    [Fact]
+    public void IndoorAll_False_TurnsAllFlagsOff()
+    {
+        RenderingDiagnostics.IndoorAll = true;  // start from all-on
+        RenderingDiagnostics.IndoorAll = false;
+
+        Assert.False(RenderingDiagnostics.ProbeIndoorWalkEnabled);
+        Assert.False(RenderingDiagnostics.ProbeIndoorLookupEnabled);
+        Assert.False(RenderingDiagnostics.ProbeIndoorUploadEnabled);
+        Assert.False(RenderingDiagnostics.ProbeIndoorXformEnabled);
+        Assert.False(RenderingDiagnostics.ProbeIndoorCullEnabled);
+        Assert.False(RenderingDiagnostics.IndoorAll);
+    }
+
+    [Fact]
+    public void IndoorAll_OneOff_ReadsAsFalse()
+    {
+        RenderingDiagnostics.IndoorAll = true;
+        RenderingDiagnostics.ProbeIndoorCullEnabled = false;  // flip one off
+        Assert.False(RenderingDiagnostics.IndoorAll);
+    }
+
+    [Theory]
+    [InlineData(0x00000029ul, false)]   // outdoor cell 0x29 in 8x8 grid
+    [InlineData(0xA9B40029ul, false)]   // outdoor cell with landblock prefix
+    [InlineData(0x00000100ul, true)]    // indoor cell minimum
+    [InlineData(0x00000105ul, true)]    // typical Holtburg Inn interior
+    [InlineData(0xA9B40105ul, true)]    // indoor with landblock prefix
+    [InlineData(0xA9B401FFul, true)]    // indoor near top of range
+    public void IsEnvCellId_DistinguishesOutdoorVsIndoorByLow16Bits(ulong id, bool expected)
+    {
+        Assert.Equal(expected, RenderingDiagnostics.IsEnvCellId(id));
+    }
+}