diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 977cd8a..11dbd7c 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -1387,6 +1387,121 @@ public sealed class Transition
     /// Ported from pseudocode section 4 (LandCell.FindEnvCollisions + ValidateWalkable).
     /// ACE: LandCell.FindEnvCollisions / ObjectInfo.ValidateWalkable.
     /// </summary>
+
+    /// <summary>
+    /// Phase A4 (2026-05-20). Port of retail's
+    /// <c>CTransition::check_other_cells</c> at
+    /// <c>acclient_2013_pseudo_c.txt:272717-272798</c>.
+    ///
+    /// <para>
+    /// After the primary cell's BSP collision returns OK, iterate every
+    /// other cell in the sphere's overlap set and run BSP collision
+    /// against each. Halt on the first Collided/Adjusted/Slid; OK
+    /// continues. Mirrors retail's behaviour exactly — no save/restore
+    /// of <see cref="Transition"/> state between cells.
+    /// </para>
+    /// </summary>
+    internal TransitionState CheckOtherCells(
+        PhysicsEngine engine,
+        Vector3 footCenter,
+        float sphereRadius,
+        System.Collections.Generic.IReadOnlyCollection<uint> cellSet)
+    {
+        if (engine.DataCache is null) return TransitionState.OK;
+        var sp = SpherePath;
+
+        // Deterministic order for greppable probe logs. Skip the primary
+        // cell — caller has already run its BSP.
+        var ordered = new System.Collections.Generic.List<uint>(cellSet);
+        ordered.Sort();
+
+        foreach (uint cellId in ordered)
+        {
+            if (cellId == sp.CheckCellId) continue;
+
+            var cell = engine.DataCache.GetCellStruct(cellId);
+            // R2 guard: stale CellPhysics loaded for render but not physics.
+            if (cell?.BSP?.Root is null) continue;
+
+            // Transform sphere into THIS cell's local space. Mirrors the
+            // primary-cell pattern at TransitionTypes.cs (FindEnvCollisions,
+            // ~line 1413) AND the Bug B world-origin fix that decomposes
+            // WorldTransform per cell so BSP Path-3 + Path-4 land write
+            // world-space ContactPlanes.
+            var localCenter     = Vector3.Transform(footCenter, cell.InverseWorldTransform);
+            var localCurrCenter = Vector3.Transform(sp.GlobalCurrCenter[0].Origin, cell.InverseWorldTransform);
+
+            var localSphere = new DatReaderWriter.Types.Sphere
+            {
+                Origin = localCenter,
+                Radius = sphereRadius,
+            };
+            DatReaderWriter.Types.Sphere? localSphere1 = null;
+            if (sp.NumSphere > 1)
+            {
+                localSphere1 = new DatReaderWriter.Types.Sphere
+                {
+                    Origin = Vector3.Transform(sp.GlobalSphere[1].Origin, cell.InverseWorldTransform),
+                    Radius = sp.GlobalSphere[1].Radius,
+                };
+            }
+
+            System.Numerics.Quaternion cellRotation;
+            Vector3 cellOrigin;
+            if (!System.Numerics.Matrix4x4.Decompose(cell.WorldTransform, out _,
+                    out cellRotation, out cellOrigin))
+            {
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[other-cells] WARN cell 0x{cellId:X8} WorldTransform did not decompose — falling back to identity rotation"));
+                cellRotation = System.Numerics.Quaternion.Identity;
+                cellOrigin   = cell.WorldTransform.Translation;
+            }
+
+            var result = BSPQuery.FindCollisions(
+                cell.BSP.Root, cell.Resolved, this,
+                localSphere, localSphere1, localCurrCenter,
+                Vector3.UnitZ, 1.0f, cellRotation, engine,
+                worldOrigin: cellOrigin);
+
+            if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+            {
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[other-cells] primary=0x{sp.CheckCellId:X8} iter=0x{cellId:X8} result={result}"));
+            }
+
+            if (ApplyOtherCellResult(result, out var halted))
+                return halted;
+        }
+
+        return TransitionState.OK;
+    }
+
+    /// <summary>
+    /// Phase A4 (2026-05-20). Combine helper for
+    /// <see cref="CheckOtherCells"/>. Mirrors retail's switch at
+    /// <c>acclient_2013_pseudo_c.txt:272739-272752</c>:
+    /// Collided/Adjusted halt with <c>CollidedWithEnvironment</c>; Slid
+    /// halts AND clears the contact-plane fields; OK continues.
+    /// </summary>
+    internal bool ApplyOtherCellResult(TransitionState result, out TransitionState finalState)
+    {
+        finalState = result;
+        switch (result)
+        {
+            case TransitionState.Collided:
+            case TransitionState.Adjusted:
+                if (!ObjectInfo.State.HasFlag(ObjectInfoState.Contact))
+                    CollisionInfo.CollidedWithEnvironment = true;
+                return true;
+            case TransitionState.Slid:
+                CollisionInfo.ContactPlaneValid   = false;
+                CollisionInfo.ContactPlaneIsWater = false;
+                return true;
+            default:
+                return false;
+        }
+    }
+
     private TransitionState FindEnvCollisions(PhysicsEngine engine)
     {
         var sp = SpherePath;
diff --git a/tests/AcDream.Core.Tests/Physics/TransitionCheckOtherCellsTests.cs b/tests/AcDream.Core.Tests/Physics/TransitionCheckOtherCellsTests.cs
new file mode 100644
index 0000000..fe69e15
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Physics/TransitionCheckOtherCellsTests.cs
@@ -0,0 +1,142 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Unit tests for the result-combine helper used by
+/// <see cref="Transition.CheckOtherCells"/>. The iteration / per-cell
+/// BSP-query parts are covered end-to-end by
+/// <see cref="FindEnvCollisionsMultiCellTests"/>; this file pins the
+/// retail-faithful halt semantics that
+/// <c>acclient_2013_pseudo_c.txt:272739-272752</c> spells out.
+/// </summary>
+public class TransitionCheckOtherCellsTests
+{
+    private static Transition MakeTransition(bool contactFlag = false)
+    {
+        var t = new Transition();
+        t.SpherePath.InitPath(Vector3.Zero, Vector3.Zero, cellId: 0xA9B40100u, sphereRadius: 0.48f);
+        t.ObjectInfo.State = contactFlag ? ObjectInfoState.Contact : ObjectInfoState.None;
+        // Pre-set CP fields to non-default so the Slid-clears-CP assertion
+        // can detect the clear.
+        t.CollisionInfo.ContactPlaneValid   = true;
+        t.CollisionInfo.ContactPlaneIsWater = true;
+        return t;
+    }
+
+    [Fact]
+    public void OK_ContinuesIteration_DoesNotMutate()
+    {
+        var t = MakeTransition();
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.OK, out var finalState);
+
+        Assert.False(halt);
+        Assert.Equal(TransitionState.OK, finalState);
+        Assert.True(t.CollisionInfo.ContactPlaneValid);
+        Assert.True(t.CollisionInfo.ContactPlaneIsWater);
+        Assert.False(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Collided_HaltsAndSetsCollidedWithEnvironment_WhenNotInContact()
+    {
+        var t = MakeTransition(contactFlag: false);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Collided, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Collided, finalState);
+        Assert.True(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Collided_DoesNotSetCollidedWithEnvironment_WhenInContact()
+    {
+        // Retail oracle gating: the CollidedWithEnvironment flip mirrors
+        // the existing primary-cell behavior in FindEnvCollisions —
+        // skipped when ObjectInfo.State has Contact bit set.
+        var t = MakeTransition(contactFlag: true);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Collided, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Collided, finalState);
+        Assert.False(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Adjusted_HaltsAndSetsCollidedWithEnvironment_WhenNotInContact()
+    {
+        var t = MakeTransition(contactFlag: false);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Adjusted, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Adjusted, finalState);
+        Assert.True(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Slid_HaltsAndClearsContactPlaneFields()
+    {
+        // Retail oracle: acclient_2013_pseudo_c.txt:272746-272750
+        //   case 4:
+        //     this->collision_info.contact_plane_valid = 0;
+        //     this->collision_info.contact_plane_is_water = 0;
+        //     return result;
+        var t = MakeTransition();
+        Assert.True(t.CollisionInfo.ContactPlaneValid);    // pre-condition
+        Assert.True(t.CollisionInfo.ContactPlaneIsWater);  // pre-condition
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Slid, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Slid, finalState);
+        Assert.False(t.CollisionInfo.ContactPlaneValid);
+        Assert.False(t.CollisionInfo.ContactPlaneIsWater);
+    }
+
+    [Fact]
+    public void CheckOtherCells_CellWithNullBspRoot_IsSkippedNoCrash()
+    {
+        // Iteration safety: a CellPhysics in the candidate set with
+        // `BSP = null` (loaded for render but not physics) must be skipped,
+        // not crash. Matches the spec's R2 guard at design §Edge cases E2.
+        var cell = new CellPhysics
+        {
+            BSP                   = null,  // <-- the guard target
+            WorldTransform        = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
+        };
+
+        var engine = new PhysicsEngine();
+        engine.DataCache = new PhysicsDataCache();   // PhysicsEngine has nullable DataCache
+        // FindEnvCollisions has terrain probes downstream; populate a
+        // minimal landblock so the cache + engine are coherent. The cell
+        // we test against doesn't need a real landblock entry.
+        var heights = new byte[81];
+        Array.Fill(heights, (byte)0);
+        var ht = new float[256];
+        for (int i = 0; i < 256; i++) ht[i] = i * 1.0f;
+        engine.AddLandblock(0xA9B4FFFFu, new TerrainSurface(heights, ht),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(),
+            worldOffsetX: 0f, worldOffsetY: 0f);
+        engine.DataCache.RegisterCellStructForTest(0xA9B40157u, cell);
+
+        var t = MakeTransition();
+        var cellSet = new HashSet<uint> { 0xA9B40157u };
+
+        // Call CheckOtherCells directly via the internal seam.
+        var result = t.CheckOtherCells(engine, Vector3.Zero, 0.48f, cellSet);
+
+        Assert.Equal(TransitionState.OK, result);
+    }
+}