diff --git a/src/AcDream.Core/Physics/ShadowObjectRegistry.cs b/src/AcDream.Core/Physics/ShadowObjectRegistry.cs
index 6b4ea11..fd3673e 100644
--- a/src/AcDream.Core/Physics/ShadowObjectRegistry.cs
+++ b/src/AcDream.Core/Physics/ShadowObjectRegistry.cs
@@ -126,6 +126,47 @@ public sealed class ShadowObjectRegistry
                  t.State, t.Flags);
     }
 
+    /// <summary>
+    /// Update the cached <see cref="ShadowEntry.State"/> bits for an
+    /// already-registered entity. Called by the inbound
+    /// <c>SetState (0xF74B)</c> dispatcher when the server broadcasts a
+    /// post-spawn <c>PhysicsState</c> change — chiefly doors flipping
+    /// <c>ETHEREAL_PS = 0x4</c> on Use, so the
+    /// <see cref="CollisionExemption.ShouldSkip"/> short-circuit can honor
+    /// the new state on the next resolve.
+    ///
+    /// <para>
+    /// Retail equivalent: <c>CPhysicsObj::set_state</c> at
+    /// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:283044</c>
+    /// — direct write `this->state = arg2`. Retail also fires side-effect
+    /// handlers for the 0x800 (lighting), 0x20 (nodraw), 0x4000 (hidden)
+    /// changed bits; ETHEREAL (0x4) doesn't trigger any of them, so slice 1
+    /// scopes to the bare state-write.
+    /// </para>
+    ///
+    /// <para>
+    /// Implementation: <see cref="ShadowEntry"/> is a value-type record
+    /// copied into per-cell lists, so we rewrite the copy in each cell the
+    /// entity occupies. Unregistered entities are a no-op (callers don't
+    /// have to gate).
+    /// </para>
+    /// </summary>
+    public void UpdatePhysicsState(uint entityId, uint newState)
+    {
+        if (!_entityToCells.TryGetValue(entityId, out var cellIds))
+            return; // not registered — no-op
+
+        foreach (var cellId in cellIds)
+        {
+            if (!_cells.TryGetValue(cellId, out var list)) continue;
+            for (int i = 0; i < list.Count; i++)
+            {
+                if (list[i].EntityId == entityId)
+                    list[i] = list[i] with { State = newState };
+            }
+        }
+    }
+
     /// <summary>Remove an entity from all cells it was registered in.</summary>
     public void Deregister(uint entityId)
     {
diff --git a/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs b/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs
index 73143d8..f3d7b08 100644
--- a/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs
+++ b/tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using System.Numerics;
 using AcDream.Core.Physics;
 using Xunit;
@@ -252,4 +253,52 @@ public class ShadowObjectRegistryTests
         Assert.Equal(0u, entry.State);
         Assert.Equal(EntityCollisionFlags.None, entry.Flags);
     }
+
+    // -----------------------------------------------------------------------
+    // UpdatePhysicsState — L.2g slice 1 (doors flip ETHEREAL post-spawn)
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void UpdatePhysicsState_FlipsEthereal_NextLookupSeesNewBits()
+    {
+        var reg = new ShadowObjectRegistry();
+        const uint doorId = 0x000F4244u;
+        reg.Register(doorId, 0x020019FFu, new Vector3(12f, 12f, 50f),
+                     Quaternion.Identity, 1f, OffX, OffY, LbId,
+                     state: 0u, flags: EntityCollisionFlags.None);
+
+        var before = reg.AllEntriesForDebug().Single(e => e.EntityId == doorId);
+        Assert.Equal(0u, before.State);
+
+        reg.UpdatePhysicsState(doorId, 0x00000004u);
+
+        var after = reg.AllEntriesForDebug().Single(e => e.EntityId == doorId);
+        Assert.Equal(0x00000004u, after.State);
+    }
+
+    [Fact]
+    public void UpdatePhysicsState_UnregisteredEntity_IsNoOp()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.UpdatePhysicsState(0xDEADBEEFu, 0x00000004u);
+        Assert.Equal(0, reg.TotalRegistered);
+    }
+
+    [Fact]
+    public void UpdatePhysicsState_EntitySpanningMultipleCells_AllCellsUpdated()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(99u, 0x01000099u, new Vector3(24f, 12f, 50f),
+                     Quaternion.Identity, 2f, OffX, OffY, LbId,
+                     state: 0u);
+
+        reg.UpdatePhysicsState(99u, 0x00000004u);
+
+        uint cellA = LbId | 1u;
+        uint cellB = LbId | (1u*8 + 0 + 1);
+        var inA = reg.GetObjectsInCell(cellA).Single(e => e.EntityId == 99u);
+        var inB = reg.GetObjectsInCell(cellB).Single(e => e.EntityId == 99u);
+        Assert.Equal(0x00000004u, inA.State);
+        Assert.Equal(0x00000004u, inB.State);
+    }
 }