diff --git a/src/AcDream.Core/World/Cells/CellPortal.cs b/src/AcDream.Core/World/Cells/CellPortal.cs
index 456c5bd..c82c5f3 100644
--- a/src/AcDream.Core/World/Cells/CellPortal.cs
+++ b/src/AcDream.Core/World/Cells/CellPortal.cs
@@ -5,12 +5,17 @@ using System.Numerics;
 namespace AcDream.Core.World.Cells;
 
 /// <summary>
-/// Unified cell-to-cell portal edge. Superset of the three legacy portal types
-/// (render <c>CellPortalInfo</c>, physics <c>PortalInfo</c>, <c>PortalPlane</c>).
+/// Unified cell-to-cell portal edge for Phase-U purposes. Superset of the three
+/// legacy portal types (render <c>CellPortalInfo</c>, physics <c>PortalInfo</c>,
+/// <c>PortalPlane</c>). <c>ExactMatch</c> (packed bit 0 of <see cref="Flags"/>)
+/// is carried in <see cref="Flags"/> but not yet exposed as a named property.
 /// Retail anchor: CCellPortal (acclient.h:32300).
 /// </summary>
 public readonly struct CellPortal
 {
+    /// <summary>
+    /// Full 32-bit (landblock-prefixed) cell id, unlike physics PortalInfo.OtherCellId which is low-16 only.
+    /// </summary>
     public uint OtherCellId { get; }
     public ushort OtherPortalId { get; }
     public ushort PolygonId { get; }
diff --git a/src/AcDream.Core/World/Cells/EnvCell.cs b/src/AcDream.Core/World/Cells/EnvCell.cs
index 3a256b1..f4dc2e0 100644
--- a/src/AcDream.Core/World/Cells/EnvCell.cs
+++ b/src/AcDream.Core/World/Cells/EnvCell.cs
@@ -1,7 +1,9 @@
+using System;
 using System.Collections.Generic;
 using System.Numerics;
 using AcDream.Core.Physics;          // BSPQuery
-using DatReaderWriter.Types;          // CellBSPTree
+using DatReaderWriter.Enums;          // EnvCellFlags
+using DatReaderWriter.Types;          // CellBSPTree, CellStruct
 
 namespace AcDream.Core.World.Cells;
 
@@ -30,4 +32,55 @@ public sealed class EnvCell : ObjCell
             && local.Y >= LocalBoundsMin.Y && local.Y <= LocalBoundsMax.Y
             && local.Z >= LocalBoundsMin.Z && local.Z <= LocalBoundsMax.Z;
     }
+
+    /// <summary>
+    /// Build an EnvCell from dat data. Mirrors the render derivation in
+    /// BuildLoadedCell (GameWindow.cs:5588-5704) so Core and render stay equivalent.
+    /// <paramref name="worldTransform"/> MUST be the physics-verbatim transform
+    /// (no +2 cm render lift).
+    /// </summary>
+    public static EnvCell FromDat(uint id, DatReaderWriter.DBObjs.EnvCell datCell,
+                                  CellStruct cellStruct, Matrix4x4 worldTransform)
+    {
+        Matrix4x4.Invert(worldTransform, out var inverse);
+
+        var min = new Vector3(float.MaxValue);
+        var max = new Vector3(float.MinValue);
+        foreach (var kvp in cellStruct.VertexArray.Vertices)
+        {
+            var p = new Vector3(kvp.Value.Origin.X, kvp.Value.Origin.Y, kvp.Value.Origin.Z);
+            min = Vector3.Min(min, p);
+            max = Vector3.Max(max, p);
+        }
+        if (min.X == float.MaxValue) { min = Vector3.Zero; max = Vector3.Zero; }
+
+        var portals = new List<CellPortal>(datCell.CellPortals.Count);
+        foreach (var p in datCell.CellPortals)
+        {
+            portals.Add(new CellPortal(
+                otherCellId:   p.OtherCellId,
+                otherPortalId: p.OtherPortalId,
+                polygonId:     p.PolygonId,
+                flags:         (ushort)p.Flags,
+                polygonLocal:  ResolvePortalPolygon(cellStruct, p.PolygonId)));
+        }
+
+        uint lbPrefix = id & 0xFFFF0000u;
+        var stab = new List<uint>(datCell.VisibleCells.Count);
+        foreach (var low in datCell.VisibleCells) stab.Add(lbPrefix | low);
+        bool seenOutside = datCell.Flags.HasFlag(EnvCellFlags.SeenOutside);
+
+        return new EnvCell(id, worldTransform, inverse, min, max, portals, stab,
+                           seenOutside, cellStruct.CellBSP);
+    }
+
+    private static IReadOnlyList<Vector3> ResolvePortalPolygon(CellStruct cellStruct, ushort polygonId)
+    {
+        if (!cellStruct.Polygons.TryGetValue(polygonId, out var poly)) return Array.Empty<Vector3>();
+        var verts = new List<Vector3>(poly.VertexIds.Count);
+        foreach (var vid in poly.VertexIds)
+            if (cellStruct.VertexArray.Vertices.TryGetValue((ushort)vid, out var v))
+                verts.Add(new Vector3(v.Origin.X, v.Origin.Y, v.Origin.Z));
+        return verts;
+    }
 }
diff --git a/src/AcDream.Core/World/Cells/ObjCell.cs b/src/AcDream.Core/World/Cells/ObjCell.cs
index 7fdb619..9c8f0bb 100644
--- a/src/AcDream.Core/World/Cells/ObjCell.cs
+++ b/src/AcDream.Core/World/Cells/ObjCell.cs
@@ -20,7 +20,11 @@ public abstract class ObjCell
     public IReadOnlyList<uint> StabList { get; }
     public bool SeenOutside { get; }
 
-    /// <summary>Retail magnitude dispatch (CObjCell::GetVisible, pseudo_c:308215).</summary>
+    /// <summary>
+    /// Retail magnitude dispatch (CObjCell::GetVisible, pseudo_c:308215).
+    /// Note: retail's CObjCell::GetVisible tests the full id; every real prefixed EnvCell id is >= 0x100.
+    /// This masks the low-16 so it works for both bare-16 and landblock-prefixed ids.
+    /// </summary>
     public bool IsEnv => (Id & 0xFFFFu) >= 0x100u;
 
     protected ObjCell(uint id, Matrix4x4 worldTransform, Matrix4x4 inverseWorldTransform,
diff --git a/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs b/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs
new file mode 100644
index 0000000..b13d5c7
--- /dev/null
+++ b/tests/AcDream.Core.Tests/World/Cells/EnvCellFromDatTests.cs
@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.World.Cells;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using Xunit;
+using DatEnvCell = DatReaderWriter.DBObjs.EnvCell;
+using DatCellPortal = DatReaderWriter.Types.CellPortal;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+public class EnvCellFromDatTests
+{
+    [Fact]
+    public void FromDat_DerivesSeenOutside_OtherPortalId_PrefixedStab_AndBoundsFallback()
+    {
+        var cellStruct = new CellStruct
+        {
+            VertexArray = new VertexArray { Vertices = new Dictionary<ushort, SWVertex>() }, // empty -> bounds fallback
+            Polygons = new Dictionary<ushort, Polygon>(),
+        }; // CellBSP defaults to null → ContainmentBsp will be null
+        var dat = new DatEnvCell
+        {
+            Flags = EnvCellFlags.SeenOutside,
+            CellPortals = new List<DatCellPortal>
+            {
+                new() { OtherCellId = 0x0105, PolygonId = 0, OtherPortalId = 7, Flags = (PortalFlags)0 },
+            },
+            VisibleCells = new List<ushort> { 0x0105, 0x0106 },
+        };
+
+        var env = EnvCell.FromDat(0xA9B40104u, dat, cellStruct, Matrix4x4.Identity);
+
+        Assert.True(env.SeenOutside);
+        Assert.Single(env.Portals);
+        Assert.Equal(0x0105u, env.Portals[0].OtherCellId);
+        Assert.Equal((ushort)7, env.Portals[0].OtherPortalId);
+        Assert.Equal(new[] { 0xA9B40105u, 0xA9B40106u }, env.StabList);
+        Assert.Equal(Vector3.Zero, env.LocalBoundsMin);
+        Assert.Equal(Vector3.Zero, env.LocalBoundsMax);
+        Assert.Null(env.ContainmentBsp);
+    }
+}