diff --git a/tests/AcDream.App.Tests/Rendering/Walk/WalkBuildingJoinDiagnosticTests.cs b/tests/AcDream.App.Tests/Rendering/Walk/WalkBuildingJoinDiagnosticTests.cs
new file mode 100644
index 00000000..98c00214
--- /dev/null
+++ b/tests/AcDream.App.Tests/Rendering/Walk/WalkBuildingJoinDiagnosticTests.cs
@@ -0,0 +1,78 @@
+using AcDream.App.Tests.Rendering;
+using AcDream.App.Rendering.Walk;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using System.Text;
+
+namespace AcDream.App.Tests.Rendering.Walk;
+
+/// <summary>
+/// FW1 look-in adjudication instrument: retail's street-outdoor trace
+/// punched look-ins ONLY at buildings a9b4001a (cells 16e,16a,16c) and
+/// a9b40022 (164,162,167,169). This pins the data-side join: those cells
+/// must be reachable through the respective building's portal table, and
+/// every drawing-BSP PortalRef must index INTO that table. A failure
+/// message carries the full dump for adjudication.
+/// </summary>
+[Trait("Lane", "InstalledDat")]
+public sealed class WalkBuildingJoinDiagnosticTests
+{
+    [Theory]
+    [InlineData(0xA9B4001Au, new uint[] { 0xA9B4016Eu, 0xA9B4016Au, 0xA9B4016Cu })]
+    [InlineData(0xA9B40022u, new uint[] { 0xA9B40164u, 0xA9B40162u, 0xA9B40167u, 0xA9B40169u })]
+    public void Traced_lookin_cells_are_reachable_through_the_building_portal_table(
+        uint buildingCellId, uint[] tracedLookInCells)
+    {
+        string? datDir = CornerFloodReplayTests.ResolveDatDir();
+        if (datDir is null)
+        {
+            Assert.Fail("Lane=InstalledDat requires an installed retail DAT directory.");
+        }
+        using var dats = new DatCollection(datDir!, DatAccessType.Read);
+        List<WalkWorldDatAdapter.BuildingEntry> entries =
+            WalkWorldDatAdapter.BuildBuildings(dats, 0xA9B40000u);
+        WalkBuilding building = Assert.Single(
+            entries, e => e.Building.PositionCellId == buildingCellId).Building;
+
+        var dump = new StringBuilder();
+        dump.AppendLine($"building {buildingCellId:x8}: {building.Portals.Length} portals");
+        var directCells = new HashSet<uint>();
+        for (int i = 0; i < building.Portals.Length; i++)
+        {
+            ref WalkBldPortal p = ref building.Portals[i];
+            directCells.Add(p.OtherCellId);
+            dump.AppendLine(
+                $"  portal[{i}]: side={p.PortalSide} exact={p.ExactMatch} "
+                + $"other={p.OtherCellId:x8} otherPortal={p.OtherPortalId} "
+                + $"stabs=[{string.Join(',', p.StabList.Select(s => s.ToString("x8")))}]");
+        }
+        var bspJoins = new List<int>();
+        CollectPortalIndices(building.DrawingBsp, bspJoins);
+        dump.AppendLine($"  BSP PortalRef indices: [{string.Join(',', bspJoins)}]");
+
+        // Every BSP portal ref must join into the portal table.
+        Assert.All(bspJoins, i => Assert.InRange(i, 0, building.Portals.Length - 1));
+
+        // Every traced look-in cell must be directly behind a portal or in a
+        // portal's stab list (the flood reaches deeper cells through those).
+        var reachable = new HashSet<uint>(directCells);
+        foreach (WalkBldPortal p in building.Portals)
+            foreach (uint stab in p.StabList)
+                reachable.Add(stab);
+        foreach (uint traced in tracedLookInCells)
+        {
+            Assert.True(
+                reachable.Contains(traced),
+                $"traced look-in cell {traced:x8} is not reachable\n{dump}");
+        }
+    }
+
+    private static void CollectPortalIndices(WalkBspNode? node, List<int> into)
+    {
+        if (node is null) return;
+        if (node.InPortals is not null)
+            into.AddRange(node.InPortals.Select(p => p.PortalIndex));
+        CollectPortalIndices(node.PosNode, into);
+        CollectPortalIndices(node.NegNode, into);
+    }
+}
diff --git a/tests/AcDream.App.Tests/Rendering/Walk/WalkLookInGateSweepTests.cs b/tests/AcDream.App.Tests/Rendering/Walk/WalkLookInGateSweepTests.cs
new file mode 100644
index 00000000..fa393784
--- /dev/null
+++ b/tests/AcDream.App.Tests/Rendering/Walk/WalkLookInGateSweepTests.cs
@@ -0,0 +1,115 @@
+using AcDream.App.Tests.Rendering;
+using AcDream.App.Rendering.Walk;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+using System.Text;
+
+namespace AcDream.App.Tests.Rendering.Walk;
+
+/// <summary>
+/// FW1 look-in gate adjudication sweep: with data joins proven correct,
+/// the wrong-building punches must come from the sidedness gate's two
+/// unvalidated inputs — the GfxObj portal-poly plane sign and the
+/// BuildingPortal side-flag decode. This sweeps all combinations against
+/// the street-outdoor fixture and reports which (if any) reproduces
+/// retail's punch set {001a, 0022}. Diagnostic: always passes; the result
+/// lands in the campaign record via the assert message when no arm matches.
+/// </summary>
+[Trait("Lane", "InstalledDat")]
+public sealed class WalkLookInGateSweepTests
+{
+    private sealed class Recorder : IWalkEventSink
+    {
+        public readonly List<WalkEvent> Events = new();
+        public void Emit(in WalkEvent walkEvent) => Events.Add(walkEvent);
+    }
+
+    [Fact(Skip = "Adjudicated 2026-08-30: NO gate decode reproduces retail, and "
+        + "the join diagnostic proves the data is right — the missing mechanism "
+        + "is CEnvCell::GetVisible's LOADED-interior-cell gate (retail punched "
+        + "only the two buildings nearest the player; their interiors were "
+        + "loaded, farther ones were not; the replay loads everything). Port "
+        + "the interior load radius (landcell stab-list pull around the "
+        + "player) and re-run this sweep to pin the plane/side decode.")]
+    public void Sweep_the_lookin_gate_decodes_against_the_street_fixture()
+    {
+        string? datDir = CornerFloodReplayTests.ResolveDatDir();
+        if (datDir is null)
+        {
+            Assert.Fail("Lane=InstalledDat requires an installed retail DAT directory.");
+        }
+        IReadOnlyList<WalkOracleFrame> frames =
+            WalkOracleTrace.Load("posed/holtburg-street-outdoor");
+        WalkOracleFrame frame = frames[1];
+        Assert.NotNull(frame.Pose);
+
+        // Retail's ground truth for this frame: which buildings punched, and
+        // which cells each punch listed.
+        string expectedPunches = PunchSignature(
+            frame.Events.Select(e => e.Kind == WalkOracleEventKind.Building
+                ? ("BLD", e.CellId!.Value, Array.Empty<uint>())
+                : e.Kind == WalkOracleEventKind.DrawCells
+                    ? ("DC", 0u, e.Cells.ToArray())
+                    : ("", 0u, Array.Empty<uint>())));
+
+        var report = new StringBuilder();
+        report.AppendLine($"RETAIL: {expectedPunches}");
+        string? winner = null;
+
+        foreach (bool flipPlanes in new[] { false, true })
+        {
+            foreach (int sideMode in new[] { 0, 1, 2, 3 })
+            {
+                // sideMode: 0 = (Flags & 0x2), 1 = inverted 0x2,
+                //           2 = (Flags & 0x1), 3 = inverted 0x1.
+                using var dats = new DatCollection(datDir!, DatAccessType.Read);
+                WalkWorldDatAdapter.FlipGfxPolygonPlanes = flipPlanes;
+                WalkWorldDatAdapter.BuildingSideMode = sideMode;
+                try
+                {
+                    WalkLandscapeDatBuilder.BuiltWorld world =
+                        WalkLandscapeDatBuilder.Build(dats, frame.Pose!.CellId);
+                    var ctx = new WalkTraceReplayContext(frame.Pose, world.Cells)
+                    {
+                        Buildings = world.Buildings,
+                    };
+                    var walk = new RetailFrameWalk();
+                    var recorder = new Recorder();
+                    walk.WalkFrame(frame.Pose.CellId, null, world.Landscape, ctx, recorder);
+                    string actual = PunchSignature(
+                        recorder.Events.Select(e => e.Kind == WalkEventKind.Building
+                            ? ("BLD", e.CellId, Array.Empty<uint>())
+                            : e.Kind == WalkEventKind.DrawCells
+                                ? ("DC", 0u, e.Cells.ToArray())
+                                : ("", 0u, Array.Empty<uint>())));
+                    string arm = $"flip={flipPlanes} side={sideMode}";
+                    report.AppendLine($"{arm}: {actual}");
+                    if (actual == expectedPunches)
+                        winner ??= arm;
+                }
+                finally
+                {
+                    WalkWorldDatAdapter.FlipGfxPolygonPlanes = false;
+                    WalkWorldDatAdapter.BuildingSideMode = 0;
+                }
+            }
+        }
+
+        Assert.True(
+            winner is not null,
+            $"no gate decode reproduces retail's punch pattern\n{report}");
+    }
+
+    private static string PunchSignature(
+        IEnumerable<(string Kind, uint Id, uint[] Cells)> events)
+    {
+        var parts = new List<string>();
+        foreach ((string kind, uint id, uint[] cells) in events)
+        {
+            if (kind == "BLD") parts.Add($"B{id:x8}");
+            else if (kind == "DC")
+                parts.Add($"D[{string.Join(',', cells.Select(c => (c & 0xFFFF).ToString("x3")))}]");
+        }
+        return string.Join("|", parts);
+    }
+}
diff --git a/tests/AcDream.App.Tests/Rendering/Walk/WalkWorldDatAdapter.cs b/tests/AcDream.App.Tests/Rendering/Walk/WalkWorldDatAdapter.cs
index dc7f0fcc..907e31bc 100644
--- a/tests/AcDream.App.Tests/Rendering/Walk/WalkWorldDatAdapter.cs
+++ b/tests/AcDream.App.Tests/Rendering/Walk/WalkWorldDatAdapter.cs
@@ -137,7 +137,7 @@ public static class WalkWorldDatAdapter
                 BuildingPortal portal = buildingInfo.Portals[i];
                 portals[i] = new WalkBldPortal
                 {
-                    PortalSide = ((ushort)portal.Flags & 0x2) != 0 ? 1 : 0,
+                    PortalSide = DecodeBuildingSide((ushort)portal.Flags),
                     ExactMatch = ((ushort)portal.Flags & 0x1) != 0,
                     OtherCellId = portal.OtherCellId == 0xFFFF
                         ? 0xFFFFFFFFu
@@ -217,6 +217,19 @@ public static class WalkWorldDatAdapter
     /// cross convention admits.</summary>
     public static bool FlipGfxPolygonPlanes;
 
+    /// <summary>Adjudication toggle: the BuildingPortal side-flag decode.
+    /// 0 = (Flags &amp; 0x2), 1 = inverted 0x2, 2 = (Flags &amp; 0x1),
+    /// 3 = inverted 0x1.</summary>
+    public static int BuildingSideMode;
+
+    private static int DecodeBuildingSide(ushort flags) => BuildingSideMode switch
+    {
+        0 => (flags & 0x2) != 0 ? 1 : 0,
+        1 => (flags & 0x2) != 0 ? 0 : 1,
+        2 => (flags & 0x1) != 0 ? 1 : 0,
+        _ => (flags & 0x1) != 0 ? 0 : 1,
+    };
+
     private static WalkPolygon? BuildGfxPolygon(GfxObj gfxObj, ushort polygonId)
     {
         if (!gfxObj.Polygons.TryGetValue(polygonId, out Polygon? poly)