diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index faf1a558..359b2f8e 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -5967,7 +5967,12 @@ public sealed class GameWindow : IDisposable
                     {
                         bldPortals.Add(new AcDream.Core.Physics.BldPortalInfo(
                             otherCellId:   lbPrefix | (uint)bp.OtherCellId,
-                            otherPortalId: bp.OtherPortalId,
+                            // DatReaderWriter parses the dat's 16-bit field as
+                            // ushort; retail sign-extends it to a SIGNED int
+                            // (CBldPortal.other_portal_id, acclient.h:32098) and
+                            // check_building_transit gates on `>= 0`
+                            // (Ghidra 0x0052c5dc). 0xFFFF → -1 = no reciprocal.
+                            otherPortalId: unchecked((short)bp.OtherPortalId),
                             flags:         (ushort)bp.Flags));
                     }
 
diff --git a/src/AcDream.Core/Physics/BuildingPhysics.cs b/src/AcDream.Core/Physics/BuildingPhysics.cs
index f717a58c..30081f8a 100644
--- a/src/AcDream.Core/Physics/BuildingPhysics.cs
+++ b/src/AcDream.Core/Physics/BuildingPhysics.cs
@@ -24,7 +24,7 @@ public sealed class BuildingPhysics
 /// </summary>
 public readonly struct BldPortalInfo
 {
-    public BldPortalInfo(uint otherCellId, ushort otherPortalId, ushort flags)
+    public BldPortalInfo(uint otherCellId, short otherPortalId, ushort flags)
     {
         OtherCellId   = otherCellId;
         OtherPortalId = otherPortalId;
@@ -33,8 +33,20 @@ public readonly struct BldPortalInfo
 
     /// <summary>Full id of the interior EnvCell this portal connects to.</summary>
     public uint   OtherCellId   { get; }
-    /// <summary>The portal id within the destination EnvCell.</summary>
-    public ushort OtherPortalId { get; }
+    /// <summary>
+    /// The portal id within the destination EnvCell — SIGNED, like retail's
+    /// <c>CBldPortal.other_portal_id</c> (<c>int</c>, acclient.h:32098,
+    /// sign-extended from the dat's 16-bit field). <c>-1</c> (wire
+    /// <c>0xFFFF</c>) means "no reciprocal portal"; retail's
+    /// <c>CEnvCell::check_building_transit</c> (Ghidra 0x0052c5d0) rejects
+    /// the whole transit when this is negative — the <c>arg2 &gt;= 0</c>
+    /// gate is the first instruction. BN's pseudo-C renders the comparison
+    /// unsigned (wrong); the sign-extension is Ghidra-proven
+    /// (wf1-interior-collision.md, BR-7 verified corrections).
+    /// DatReaderWriter parses the field as <c>ushort</c>; construction
+    /// sites reinterpret via <c>unchecked((short)value)</c>.
+    /// </summary>
+    public short  OtherPortalId { get; }
     public ushort Flags         { get; }
 
     /// <summary>
diff --git a/src/AcDream.Core/Physics/CellTransit.cs b/src/AcDream.Core/Physics/CellTransit.cs
index 60dac924..0b307182 100644
--- a/src/AcDream.Core/Physics/CellTransit.cs
+++ b/src/AcDream.Core/Physics/CellTransit.cs
@@ -356,9 +356,44 @@ public static class CellTransit
         Vector3 worldSphereCenter,
         float sphereRadius,
         ICollection<uint> candidates)
+        => CheckBuildingTransit(
+            cache, building,
+            new[] { new Sphere { Origin = worldSphereCenter, Radius = sphereRadius } },
+            1, candidates, out _);
+
+    /// <summary>
+    /// Multi-sphere form matching retail's call shape: every path/flood
+    /// sphere is tested and the FIRST one intersecting the interior cell's
+    /// BSP admits the cell (<c>CEnvCell::check_building_transit</c>,
+    /// Ghidra 0x0052c5d0 — per-sphere loop at 0052c5fe, break-on-hit).
+    /// </summary>
+    /// <param name="hitsInteriorCell">True when at least one interior cell
+    /// was admitted — retail writes <c>SPHEREPATH.hits_interior_cell = 1</c>
+    /// at 0052c650 the moment a sphere lands a building-transit cell. Feeds
+    /// the building-shell <c>bldg_check</c> weakening in
+    /// <c>BSPTREE::find_collisions</c> (0x0053a440).</param>
+    public static void CheckBuildingTransit(
+        PhysicsDataCache cache,
+        BuildingPhysics building,
+        IReadOnlyList<Sphere> worldSpheres,
+        int numSpheres,
+        ICollection<uint> candidates,
+        out bool hitsInteriorCell)
     {
+        hitsInteriorCell = false;
+        int sphereCount = EffectiveSphereCount(worldSpheres, numSpheres);
+        if (sphereCount == 0) return;
+
         foreach (var portal in building.Portals)
         {
+            // BR-7 / A6.P4 (2026-06-11): retail's first gate — the whole
+            // transit is rejected when other_portal_id is negative
+            // (`if (arg2 >= 0)` at 0x0052c5dc; arg2 is the SIGNED
+            // sign-extended CBldPortal.other_portal_id, acclient.h:32098).
+            // Wire 0xFFFF = -1 = "no reciprocal portal".
+            if (portal.OtherPortalId < 0)
+                continue;
+
             var otherCell = cache.GetCellStruct(portal.OtherCellId);
             if (otherCell?.CellBSP?.Root is null)
             {
@@ -381,17 +416,26 @@ public static class CellTransit
             // it, login-inside-the-inn keeps the player classified outdoor
             // until they walk further in (sphere center crosses), letting
             // them run through exterior walls on the way out.
-            var localCenter = Vector3.Transform(worldSphereCenter, otherCell.InverseWorldTransform);
-            bool inside = BSPQuery.SphereIntersectsCellBsp(otherCell.CellBSP.Root, localCenter, sphereRadius);
-
-            if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+            bool inside = false;
+            for (int i = 0; i < sphereCount && !inside; i++)
             {
-                Console.WriteLine(System.FormattableString.Invariant(
-                    $"[check-bldg] portal->0x{portal.OtherCellId:X8} wpos=({worldSphereCenter.X:F3},{worldSphereCenter.Y:F3},{worldSphereCenter.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) r={sphereRadius:F3} inside={inside}"));
+                var sphere = worldSpheres[i];
+                var localCenter = Vector3.Transform(sphere.Origin, otherCell.InverseWorldTransform);
+                inside = BSPQuery.SphereIntersectsCellBsp(
+                    otherCell.CellBSP.Root, localCenter, sphere.Radius);
+
+                if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+                {
+                    Console.WriteLine(System.FormattableString.Invariant(
+                        $"[check-bldg] portal->0x{portal.OtherCellId:X8} sphere#{i} wpos=({sphere.Origin.X:F3},{sphere.Origin.Y:F3},{sphere.Origin.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) r={sphere.Radius:F3} inside={inside}"));
+                }
             }
 
             if (inside)
             {
+                // Retail sets SPHEREPATH.hits_interior_cell the moment a
+                // building-transit sphere lands an interior cell (0052c650).
+                hitsInteriorCell = true;
                 candidates.Add(portal.OtherCellId);
             }
         }
diff --git a/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs b/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
index 6ea51dc9..722e94fa 100644
--- a/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/CellTransitCheckBuildingTransitTests.cs
@@ -58,8 +58,125 @@ public class CellTransitCheckBuildingTransitTests
         Assert.Empty(candidates);
     }
 
-    // A second test that uses a synthetic CellBSP whose Root.Type == BSPNodeType.Leaf
-    // (which PointInsideCellBsp short-circuits as "inside") would verify the
-    // happy path. Constructing a CellBSPTree by hand from DatReaderWriter
-    // types is awkward; deferred to integration testing at visual-verify time.
+    /// <summary>
+    /// BR-7 / A6.P4 C1 (2026-06-11): retail's first gate in
+    /// <c>CEnvCell::check_building_transit</c> (Ghidra 0x0052c5dc) is
+    /// <c>if (other_portal_id &gt;= 0)</c> on the SIGNED sign-extended
+    /// portal id — a portal whose dat value is 0xFFFF (-1, "no reciprocal
+    /// portal") never admits its interior cell, even when a sphere
+    /// overlaps it. A leaf-root CellBSP makes the sphere test
+    /// unconditionally "inside", so the only thing separating the two
+    /// portals below is the sign of OtherPortalId.
+    /// </summary>
+    [Fact]
+    public void NegativeOtherPortalId_RejectsTransit_PositiveAdmits()
+    {
+        var leafBsp = new DatReaderWriter.Types.CellBSPTree
+        {
+            Root = new DatReaderWriter.Types.CellBSPNode
+            {
+                Type = DatReaderWriter.Enums.BSPNodeType.Leaf,
+            },
+        };
+
+        CellPhysics MakeLeafCell() => new CellPhysics
+        {
+            WorldTransform = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved = new Dictionary<ushort, ResolvedPolygon>(),
+            CellBSP = leafBsp,
+        };
+
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40101u, MakeLeafCell());
+        cache.RegisterCellStructForTest(0xA9B40102u, MakeLeafCell());
+
+        var building = new BuildingPhysics
+        {
+            WorldTransform = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Portals = new[]
+            {
+                // Wire 0xFFFF reinterpreted signed = -1 → must be skipped.
+                new BldPortalInfo(
+                    otherCellId: 0xA9B40101u,
+                    otherPortalId: unchecked((short)0xFFFF),
+                    flags: 0),
+                // Non-negative id → admitted (leaf BSP ⇒ sphere inside).
+                new BldPortalInfo(
+                    otherCellId: 0xA9B40102u,
+                    otherPortalId: 0,
+                    flags: 0),
+            },
+        };
+
+        var candidates = new HashSet<uint>();
+        CellTransit.CheckBuildingTransit(
+            cache, building,
+            worldSphereCenter: new Vector3(0, 0, 0),
+            sphereRadius: 0.5f,
+            candidates);
+
+        Assert.DoesNotContain(0xA9B40101u, candidates);
+        Assert.Contains(0xA9B40102u, candidates);
+    }
+
+    /// <summary>
+    /// Multi-sphere form (retail per-sphere loop at 0052c5fe): the first
+    /// intersecting sphere admits the cell and sets
+    /// <c>hitsInteriorCell</c> (retail SPHEREPATH.hits_interior_cell write
+    /// at 0052c650). All-spheres-outside ⇒ no admit, flag false.
+    /// </summary>
+    [Fact]
+    public void MultiSphere_AnySphereAdmits_SetsHitsInteriorCell()
+    {
+        var leafBsp = new DatReaderWriter.Types.CellBSPTree
+        {
+            Root = new DatReaderWriter.Types.CellBSPNode
+            {
+                Type = DatReaderWriter.Enums.BSPNodeType.Leaf,
+            },
+        };
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40103u, new CellPhysics
+        {
+            WorldTransform = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved = new Dictionary<ushort, ResolvedPolygon>(),
+            CellBSP = leafBsp,
+        });
+
+        var building = new BuildingPhysics
+        {
+            WorldTransform = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Portals = new[]
+            {
+                new BldPortalInfo(0xA9B40103u, otherPortalId: 1, flags: 0),
+            },
+        };
+
+        var spheres = new[]
+        {
+            new DatReaderWriter.Types.Sphere { Origin = new Vector3(500f, 0f, 0f), Radius = 0.5f },
+            new DatReaderWriter.Types.Sphere { Origin = Vector3.Zero,              Radius = 0.5f },
+        };
+
+        var candidates = new HashSet<uint>();
+        CellTransit.CheckBuildingTransit(
+            cache, building, spheres, spheres.Length, candidates, out bool hits);
+
+        // Leaf-root BSP treats every sphere as inside, so even the far
+        // sphere admits; the point of this test is the plumbing: candidates
+        // populated + hitsInteriorCell set through the multi-sphere form.
+        Assert.Contains(0xA9B40103u, candidates);
+        Assert.True(hits);
+
+        // Zero spheres → nothing admitted, flag stays false.
+        var empty = new HashSet<uint>();
+        CellTransit.CheckBuildingTransit(
+            cache, building, spheres, 0, empty, out bool hitsNone);
+        Assert.Empty(empty);
+        Assert.False(hitsNone);
+    }
 }