diff --git a/tests/AcDream.Core.Tests/World/Cells/CellGraphFixtureTests.cs b/tests/AcDream.Core.Tests/World/Cells/CellGraphFixtureTests.cs
new file mode 100644
index 0000000..aa78c12
--- /dev/null
+++ b/tests/AcDream.Core.Tests/World/Cells/CellGraphFixtureTests.cs
@@ -0,0 +1,65 @@
+using System;
+using System.IO;
+using System.Numerics;
+using AcDream.Core.Physics;
+using AcDream.Core.World.Cells;
+using Xunit;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+/// <summary>
+/// UCG Stage 1 grounding test: exercises <see cref="CellGraph"/> against a REAL
+/// Holtburg cottage-cellar cell loaded from the #98 fixture set. No production
+/// code is added — this test validates that the existing types work correctly
+/// with real dat-derived geometry.
+///
+/// Vertex-space note (Step 1 finding): <see cref="CellPhysics.Resolved"/> vertices
+/// are CELL-LOCAL (sourced from VertexArray.Origin in PhysicsDataCache.ResolvePolygons
+/// without applying WorldTransform). The bounds accumulation below is correct as-is
+/// and the non-degenerate assertions are valid.
+/// </summary>
+public class CellGraphFixtureTests
+{
+    private const uint CellarCellId = 0xA9B40147u;   // #98 cottage cellar fixture
+
+    private static string FixtureDir()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !File.Exists(Path.Combine(dir, "AcDream.slnx")))
+            dir = Directory.GetParent(dir)?.FullName;
+        Assert.NotNull(dir);
+        return Path.Combine(dir!, "tests", "AcDream.Core.Tests", "Fixtures", "issue98");
+    }
+
+    private static EnvCell EnvFromFixture(uint cellId)
+    {
+        var path = Path.Combine(FixtureDir(), $"0x{cellId:X8}.json");
+        Assert.True(File.Exists(path), $"missing fixture {path}");
+        var cp = CellDumpSerializer.Hydrate(CellDumpSerializer.Read(path));
+
+        // Accumulate cell-local bounds from resolved polygon vertices.
+        // Vertices are cell-local (VertexArray.Origin, no WorldTransform applied).
+        var min = new Vector3(float.MaxValue);
+        var max = new Vector3(float.MinValue);
+        foreach (var poly in cp.Resolved.Values)
+            foreach (var v in poly.Vertices) { min = Vector3.Min(min, v); max = Vector3.Max(max, v); }
+        if (min.X == float.MaxValue) { min = Vector3.Zero; max = Vector3.Zero; }
+
+        return new EnvCell(cellId, cp.WorldTransform, cp.InverseWorldTransform, min, max,
+            Array.Empty<CellPortal>(), Array.Empty<uint>(), false, cp.CellBSP);
+    }
+
+    [Fact]
+    public void RealCottageCell_ResolvesViaGetVisible_AndIsEnv()
+    {
+        var g = new CellGraph();
+        var env = EnvFromFixture(CellarCellId);
+        g.Add(env);
+
+        var resolved = g.GetVisible(CellarCellId);
+        Assert.Same(env, resolved);
+        Assert.True(resolved!.IsEnv);
+        Assert.True(env.LocalBoundsMax.X > env.LocalBoundsMin.X, "non-degenerate bounds X");
+        Assert.True(env.LocalBoundsMax.Y > env.LocalBoundsMin.Y, "non-degenerate bounds Y");
+    }
+}