feat(physics): retail PvP exemption + viewer/creature/missile gates (Commit C)

The retail-faithful exemption block at the top of
CPhysicsObj::FindObjCollisions
(acclient_2013_pseudo_c.txt:276782-276839,276971), ported line-for-
line as a small static helper.

Behaviour now matches retail:
  - Two non-PK players walk through each other.
  - Two PK players collide.
  - Two PKLite players collide.
  - Mismatched PK status (PK vs non-PK, PK vs PKLite) — exempt.
  - Impenetrable target ("Free" PK status) — always collides.
  - Player vs creature/NPC — always collides (this is what closes the
    user-facing complaint that walking into a Holtburg vendor was
    walking through them).
  - Mover with IGNORE_CREATURES — walks through creature targets.
  - Viewer (camera ray) — walks through creatures.
  - Target with ETHEREAL+IGNORE_COLLISIONS — universally exempt.

CollisionExemption.ShouldSkip(targetState, targetFlags, moverState)
  - new file src/AcDream.Core/Physics/CollisionExemption.cs.
  - 13-test matrix covering every documented case
    (CollisionExemptionTests.cs).
  - Static + pure → cheap to call from the hot path.

Wiring:
  - TransitionTypes.FindObjCollisions: after broadphase distance
    reject, call ShouldSkip on the obj and ObjectInfo.State; on true,
    `continue`. Static landblock entries (State=0, Flags=None) fall
    through cheaply — no behavior change for static collision.
  - PhysicsEngine.ResolveWithTransition: new optional moverFlags
    parameter (default None for back-compat). PlayerMovementController
    passes ObjectInfoState.IsPlayer; remote dead-reckoning leaves it
    None (matches non-player movers, no PvP exemption applies).
  - PK/PKLite/Impenetrable bits for the LOCAL player are not yet
    sourced from PlayerDescription's PlayerKillerStatus property —
    that's a follow-up. Default "non-PK player" matches ACE's
    character-creation default and the user's +Acdream test
    character.

Cross-checked against ACE PhysicsObj.cs:381-405 (line-for-line C# port
of the same retail block). Only intentional divergence: ACE adds
state.HasFlag(IsImpenetrable) (mover-impenetrable) to the collide list;
retail's pseudo-C only checks the target — acdream follows retail.

dotnet build green, dotnet test 1467 passing (+13 new). Live test:
+Acdream walking into Holtburg vendors now stops at their cylinder;
walking through small plants still passes (Commit B's phantom skip).

Closes the live-entity collision arc: A (plumbing) + B (registration)
+ C (exemption).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -413,7 +413,13 @@ public sealed class PlayerMovementController
             stepUpHeight: StepUpHeight,
             stepDownHeight: 0.04f,  // retail default
             isOnGround: _body.OnWalkable,
-            body: _body);           // persist ContactPlane across frames for slope tracking
+            body: _body,           // persist ContactPlane across frames for slope tracking
+            // Commit C 2026-04-29 — local player is always IsPlayer.
+            // The PK/PKLite/Impenetrable bits come from PlayerDescription's
+            // PlayerKillerStatus property; not yet parsed (non-PK pair → walks
+            // through other non-PK players, which is retail's default for
+            // ACE's character creation defaults too).
+            moverFlags: AcDream.Core.Physics.ObjectInfoState.IsPlayer);
 
         // Apply resolved position.
         _body.Position = resolveResult.Position;

src/AcDream.App/Rendering/GameWindow.cs

@@ -2067,6 +2067,18 @@ public sealed class GameWindow : IDisposable
         // UpdateMotion / UpdatePosition events can reseat this entity by guid.
         _entitiesByServerGuid[spawn.Guid] = entity;
 
+        // Commit B 2026-04-29 — live-entity collision registration. The
+        // local player is the simulator (its PhysicsBody is the source of
+        // truth for our own movement); only remotes register as targets.
+        // Phantom-Setup entities (no CylSpheres / no Spheres / no Radius)
+        // are deliberately skipped — retail FUN's `FindObjCollisions`
+        // falls through to OK_TS for any object with no collision
+        // geometry (acclient_2013_pseudo_c.txt:276917,276987).
+        if (spawn.Guid != _playerServerGuid)
+        {
+            RegisterLiveEntityCollision(entity, setup, spawn, origin);
+        }
+
         // Phase B.2: capture the server-sent MotionTableId for our own
         // character so UpdatePlayerAnimation can pass it to GetIdleCycle.
         // The Setup's DefaultMotionTable is often 0 for human characters;
@@ -2284,6 +2296,100 @@ public sealed class GameWindow : IDisposable
         }
     }
 
+    /// <summary>
+    /// Commit B 2026-04-29 — register a live (server-spawned) entity into
+    /// the <see cref="ShadowObjectRegistry"/> as a single collision body.
+    /// One entry per entity (in contrast to static scenery's per-CylSphere
+    /// registration) so <c>RemoveLiveEntityByServerGuid</c>'s single
+    /// <c>Deregister(entity.Id)</c> cleans it up without leaks.
+    ///
+    /// <para>
+    /// Geometry-priority order matches retail
+    /// (<c>acclient_2013_pseudo_c.txt:276858-276987</c>): CylSpheres &gt;
+    /// Sphere fallback &gt; Setup.Radius. Phantom Setups (no shape) are
+    /// rejected — retail's <c>FindObjCollisions</c> falls through to
+    /// OK_TS in that case.
+    /// </para>
+    ///
+    /// <para>
+    /// Carries <see cref="EntityCollisionFlags"/> derived from the PWD
+    /// bitfield (<c>acclient.h:6431-6463</c>) plus <c>IsCreature</c>
+    /// derived from the inbound ItemType. Commit C consumes these in
+    /// the PvP exemption block.
+    /// </para>
+    /// </summary>
+    private void RegisterLiveEntityCollision(
+        AcDream.Core.World.WorldEntity entity,
+        DatReaderWriter.DBObjs.Setup setup,
+        AcDream.Core.Net.WorldSession.EntitySpawn spawn,
+        System.Numerics.Vector3 origin)
+    {
+        if (spawn.Position is null) return;
+
+        bool hasCyl    = setup.CylSpheres.Count > 0;
+        bool hasSphere = setup.Spheres.Count > 0;
+        bool hasRadius = setup.Radius > 0.0001f;
+
+        // Retail-faithful phantom skip (acclient_2013_pseudo_c.txt:276917).
+        if (!hasCyl && !hasSphere && !hasRadius)
+            return;
+
+        float entScale = spawn.ObjScale ?? 1.0f;
+        float radius;
+        float height;
+
+        if (hasCyl)
+        {
+            // Pick the largest CylSphere as the body cylinder. Retail
+            // tests every CylSphere in turn (276891) but for collision
+            // BLOCKING the largest is sufficient — the player will stop
+            // at the body's outer radius.
+            var sph = setup.CylSpheres[0];
+            for (int i = 1; i < setup.CylSpheres.Count; i++)
+            {
+                if (setup.CylSpheres[i].Radius > sph.Radius) sph = setup.CylSpheres[i];
+            }
+            radius = sph.Radius * entScale;
+            height = (sph.Height > 0 ? sph.Height : sph.Radius * 4f) * entScale;
+        }
+        else if (hasRadius)
+        {
+            radius = setup.Radius * entScale;
+            height = (setup.Height > 0 ? setup.Height : setup.Radius * 2f) * entScale;
+        }
+        else
+        {
+            // Sphere-only: largest sphere as a Cylinder approximation.
+            var sph = setup.Spheres[0];
+            for (int i = 1; i < setup.Spheres.Count; i++)
+            {
+                if (setup.Spheres[i].Radius > sph.Radius) sph = setup.Spheres[i];
+            }
+            radius = sph.Radius * entScale;
+            height = sph.Radius * 2f * entScale;
+        }
+
+        if (radius <= 0f) return;
+
+        // Decode PvP / Player / Impenetrable from PWD._bitfield.
+        // IsCreature comes from the spawn's ItemType (server-known type).
+        var flags = AcDream.Core.Physics.EntityCollisionFlags.None;
+        if (spawn.ObjectDescriptionFlags is { } odf)
+            flags = AcDream.Core.Physics.EntityCollisionFlagsExt.FromPwdBitfield(odf);
+        if (spawn.ItemType == (uint)AcDream.Core.Items.ItemType.Creature)
+            flags |= AcDream.Core.Physics.EntityCollisionFlags.IsCreature;
+
+        uint state = spawn.PhysicsState ?? 0u;
+
+        _physicsEngine.ShadowObjects.Register(
+            entity.Id, entity.SourceGfxObjOrSetupId,
+            entity.Position, entity.Rotation, radius,
+            origin.X, origin.Y, spawn.Position.Value.LandblockId,
+            AcDream.Core.Physics.ShadowCollisionType.Cylinder,
+            cylHeight: height, scale: 1.0f,
+            state: state, flags: flags);
+    }
+
     private bool RemoveLiveEntityByServerGuid(uint serverGuid, bool logDelete)
     {
         if (!_entitiesByServerGuid.TryGetValue(serverGuid, out var existingEntity))
@@ -2980,6 +3086,18 @@ public sealed class GameWindow : IDisposable
         entity.Position = worldPos;
         entity.Rotation = rot;
 
+        // Commit B 2026-04-29 — keep the shadow registry in sync with
+        // server-authoritative position so the player's collision broadphase
+        // tests against the up-to-date target body. Skip the local player
+        // (its body is the simulator, not a target). Retail does the
+        // equivalent via SetPosition → change_cell → AddShadowObject
+        // (acclient_2013_pseudo_c.txt:284276 / 281200 / 282862).
+        if (update.Guid != _playerServerGuid)
+        {
+            _physicsEngine.ShadowObjects.UpdatePosition(
+                entity.Id, worldPos, rot, origin.X, origin.Y, p.LandblockId);
+        }
+
 // Track remote-entity motion for stop detection. Only record the
         // timestamp when position moved MEANINGFULLY (> 0.05m). Updates
         // that report the same position keep the old Time, so the

src/AcDream.Core.Net/Messages/CreateObject.cs

@@ -84,6 +84,24 @@ public static class CreateObject
     /// SetupTableId are nullable because their corresponding
     /// physics-description-flag bits may not be set on every CreateObject.
     /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <see cref="PhysicsState"/> (<c>acclient.h:2815</c>) carries flag
+    /// bits like <c>ETHEREAL_PS=0x4</c>, <c>IGNORE_COLLISIONS_PS=0x10</c>,
+    /// <c>HAS_PHYSICS_BSP_PS=0x10000</c> — the bits retail's
+    /// <c>FindObjCollisions</c> reads to short-circuit ethereal /
+    /// no-collision entities. Pre-2026-04-29 (Commit A of the live-entity
+    /// collision port) the parser silently dropped this field.
+    /// </para>
+    /// <para>
+    /// <see cref="ObjectDescriptionFlags"/> is the <c>PWD._bitfield</c>
+    /// trailer (<c>acclient.h:6431-6463</c>) — bits include <c>BF_PLAYER (0x8)</c>,
+    /// <c>BF_PLAYER_KILLER (0x20)</c>, <c>BF_FREE_PKSTATUS (0x200000)</c>,
+    /// <c>BF_PKLITE_PKSTATUS (0x2000000)</c>. Decoded into
+    /// <c>EntityCollisionFlags</c> at registration time for the PvP
+    /// exemption gate.
+    /// </para>
+    /// </remarks>
     public readonly record struct Parsed(
         uint Guid,
         ServerPosition? Position,
@@ -100,7 +118,9 @@ public static class CreateObject
         ushort InstanceSequence = 0,
         ushort TeleportSequence = 0,
         ushort ServerControlSequence = 0,
-        ushort ForcePositionSequence = 0);
+        ushort ForcePositionSequence = 0,
+        uint? PhysicsState = null,
+        uint? ObjectDescriptionFlags = null);
 
     /// <summary>
     /// The relevant subset of the server-sent <c>MovementData</c> /
@@ -260,6 +280,12 @@ public static class CreateObject
         float? objScale = null;
         ServerMotionState? motionState = null;
         uint? motionTableId = null;
+        // Commit A 2026-04-29 — live-entity collision plumbing. PhysicsState
+        // (acclient.h:2815) was previously skipped at line ~337; the PWD
+        // _bitfield (acclient.h:6431-6463) was previously discarded as
+        // "ObjectDescriptionFlags" at the WeenieHeader trailer.
+        uint? physicsState = null;
+        uint? objectDescriptionFlags = null;
 
         try
         {
@@ -334,7 +360,10 @@ public static class CreateObject
             if (body.Length - pos < 8) return null;
             var physicsFlags = (PhysicsDescriptionFlag)BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(pos));
             pos += 4;
-            pos += 4;  // PhysicsState (skip)
+            // PhysicsState (acclient.h:2815). Previously skipped, now
+            // surfaced for live-entity collision (Commit A 2026-04-29).
+            physicsState = BinaryPrimitives.ReadUInt32LittleEndian(body.Slice(pos));
+            pos += 4;
 
             if ((physicsFlags & PhysicsDescriptionFlag.Movement) != 0)
             {
@@ -463,7 +492,16 @@ public static class CreateObject
                     if (body.Length - pos >= 4)
                         itemType = ReadU32(body, ref pos);
                     if (body.Length - pos >= 4)
-                        _ = ReadU32(body, ref pos);  // ObjectDescriptionFlags
+                    {
+                        // ObjectDescriptionFlags = retail PWD._bitfield
+                        // (acclient.h:6431-6463). Carries BF_PLAYER (0x8),
+                        // BF_PLAYER_KILLER (0x20), BF_FREE_PKSTATUS (0x200000),
+                        // BF_PKLITE_PKSTATUS (0x2000000) — the bits that
+                        // acclient_2013_pseudo_c.txt:406898-406918 read for
+                        // IsPK() / IsPKLite() / IsImpenetrable(). Previously
+                        // discarded; now surfaced for the PvP collision rule.
+                        objectDescriptionFlags = ReadU32(body, ref pos);
+                    }
                     AlignTo4(ref pos);
                 }
                 catch { /* truncated name — partial result is still useful */ }
@@ -471,13 +509,15 @@ public static class CreateObject
 
             return new Parsed(guid, position, setupTableId, animParts,
                 textureChanges, subPalettes, basePaletteId, objScale, name, itemType, motionState, motionTableId,
-                instanceSeq, teleportSeq, serverControlSeq, forcePositionSeq);
+                instanceSeq, teleportSeq, serverControlSeq, forcePositionSeq,
+                physicsState, objectDescriptionFlags);
 
             // Local helper: if we ran out of fields past PhysicsData, still
             // return the useful prefix (guid/position/setup/animParts/textures/palettes/scale/motion).
             Parsed PartialResult() => new(
                 guid, position, setupTableId, animParts,
-                textureChanges, subPalettes, basePaletteId, objScale, null, null, motionState, motionTableId);
+                textureChanges, subPalettes, basePaletteId, objScale, null, null, motionState, motionTableId,
+                PhysicsState: physicsState, ObjectDescriptionFlags: objectDescriptionFlags);
         }
         catch
         {

src/AcDream.Core.Net/WorldSession.cs

@@ -56,7 +56,14 @@ public sealed class WorldSession : IDisposable
         string? Name,
         uint? ItemType,
         CreateObject.ServerMotionState? MotionState,
-        uint? MotionTableId);
+        uint? MotionTableId,
+        // Commit A 2026-04-29 — live-entity collision plumbing.
+        // PhysicsState: retail acclient.h:2815 (ETHEREAL_PS=0x4,
+        // IGNORE_COLLISIONS_PS=0x10, HAS_PHYSICS_BSP_PS=0x10000, ...).
+        // ObjectDescriptionFlags: retail PWD._bitfield (acclient.h:6431-6463)
+        // — drives IsPlayer/IsPK/IsPKLite/IsImpenetrable for PvP gating.
+        uint? PhysicsState = null,
+        uint? ObjectDescriptionFlags = null);
 
     /// <summary>Fires when the session finishes parsing a CreateObject.</summary>
     public event Action<EntitySpawn>? EntitySpawned;
@@ -648,7 +655,9 @@ public sealed class WorldSession : IDisposable
                         parsed.Value.Name,
                         parsed.Value.ItemType,
                         parsed.Value.MotionState,
-                        parsed.Value.MotionTableId));
+                        parsed.Value.MotionTableId,
+                        parsed.Value.PhysicsState,
+                        parsed.Value.ObjectDescriptionFlags));
                 }
             }
             else if (op == DeleteObject.Opcode)

src/AcDream.Core/Physics/CollisionExemption.cs

@@ -0,0 +1,123 @@
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// The retail-faithful exemption gate at the top of
+/// <c>CPhysicsObj::FindObjCollisions</c>. Decides — based on the moving
+/// object's <see cref="ObjectInfoState"/> bits and the target's raw
+/// <c>PhysicsState</c> + decoded <see cref="EntityCollisionFlags"/> —
+/// whether collision against the target should be skipped entirely
+/// (return <c>OK_TS</c>) or proceed to broad-phase / shape dispatch.
+///
+/// <para>
+/// Ported from the named retail decompilation:
+/// </para>
+/// <list type="bullet">
+///   <item><c>acclient_2013_pseudo_c.txt:276782</c> — target
+///     <c>ETHEREAL_PS=0x4 &amp; IGNORE_COLLISIONS_PS=0x10</c>: walk through.</item>
+///   <item><c>acclient_2013_pseudo_c.txt:276787</c> — viewer mover vs
+///     creature target: walk through (camera ray ignores creatures).</item>
+///   <item><c>acclient_2013_pseudo_c.txt:276971</c> — mover with
+///     <c>IGNORE_CREATURES (state &amp; 0x400)</c> vs creature target:
+///     walk through.</item>
+///   <item><c>acclient_2013_pseudo_c.txt:276807-276839</c> — PvP rule:
+///     <para>
+///     If both are players: skip <em>unless</em> target is Impenetrable,
+///     or both are PK, or both are PKLite. Mismatched PK status (PK vs
+///     non-PK, PK vs PKLite) is exempted — players in different pools
+///     pass through each other, matching retail muscle memory.
+///     </para>
+///   </item>
+/// </list>
+///
+/// <para>
+/// Cross-checked against ACE
+/// <c>references/ACE/Source/ACE.Server/Physics/PhysicsObj.cs:381-405</c>
+/// (line-for-line C# port of the same logic). Note: ACE adds
+/// <c>state.HasFlag(IsImpenetrable)</c> (mover-impenetrable) to the
+/// collide list; retail's pseudo-C only checks the target's
+/// <c>IsImpenetrable()</c>. acdream follows retail.
+/// </para>
+/// </summary>
+public static class CollisionExemption
+{
+    private const uint ETHEREAL_PS         = 0x4u;   // acclient.h:2819
+    private const uint IGNORE_COLLISIONS_PS = 0x10u; // acclient.h:2821
+
+    /// <summary>
+    /// Should the moving object skip collision testing against this
+    /// target entirely? Returns <c>true</c> if exempt (no further
+    /// shape dispatch).
+    /// </summary>
+    /// <param name="targetState">Raw retail <c>PhysicsState</c> bits
+    ///   captured at <c>CreateObject</c> time (ETHEREAL/IGNORE/etc.).</param>
+    /// <param name="targetFlags">Decoded
+    ///   <see cref="EntityCollisionFlags"/> from the target's PWD bitfield
+    ///   plus its <c>ItemType</c>-derived <c>IsCreature</c> bit.</param>
+    /// <param name="moverState">The moving object's
+    ///   <see cref="ObjectInfoState"/> — typically the local player's
+    ///   IsPlayer + (PK/PKLite/Impenetrable bits if known) flags.</param>
+    public static bool ShouldSkip(uint targetState, EntityCollisionFlags targetFlags,
+                                   ObjectInfoState moverState)
+    {
+        // 1. Target ETHEREAL + IGNORE_COLLISIONS → walk through.
+        //    acclient_2013_pseudo_c.txt:276782 — wraps the entire body of
+        //    FindObjCollisions; we hoist it as the first early-out.
+        if ((targetState & ETHEREAL_PS) != 0
+            && (targetState & IGNORE_COLLISIONS_PS) != 0)
+        {
+            return true;
+        }
+
+        // 2. Viewer mover + creature target → walk through.
+        //    acclient_2013_pseudo_c.txt:276787-276790.
+        bool moverIsViewer = (moverState & ObjectInfoState.IsViewer) != 0;
+        bool targetIsCreature = (targetFlags & EntityCollisionFlags.IsCreature) != 0;
+        if (moverIsViewer && targetIsCreature)
+            return true;
+
+        // 3. IGNORE_CREATURES mover + creature target → walk through.
+        //    acclient_2013_pseudo_c.txt:276971.
+        bool moverIgnoresCreatures = (moverState & ObjectInfoState.IgnoreCreatures) != 0;
+        if (moverIgnoresCreatures && targetIsCreature)
+            return true;
+
+        // 4. PvP exemption block.
+        //    acclient_2013_pseudo_c.txt:276807-276839.
+        bool moverIsPlayer = (moverState & ObjectInfoState.IsPlayer) != 0;
+        bool targetIsPlayer = (targetFlags & EntityCollisionFlags.IsPlayer) != 0;
+        if (moverIsPlayer && targetIsPlayer)
+        {
+            // Tentatively exempt (retail `ebp_1 = 1`). Then disqualify
+            // if any of the COLLIDE conditions hold.
+            bool collide = false;
+
+            // 4a. Impenetrable target → collide.
+            //     acclient_2013_pseudo_c.txt:276826.
+            if ((targetFlags & EntityCollisionFlags.IsImpenetrable) != 0)
+                collide = true;
+
+            // 4b. Both PK → collide.
+            //     acclient_2013_pseudo_c.txt:276832-276836.
+            if (!collide
+                && (moverState & ObjectInfoState.IsPK) != 0
+                && (targetFlags & EntityCollisionFlags.IsPK) != 0)
+            {
+                collide = true;
+            }
+
+            // 4c. Both PKLite → collide.
+            //     acclient_2013_pseudo_c.txt:276837.
+            if (!collide
+                && (moverState & ObjectInfoState.IsPKLite) != 0
+                && (targetFlags & EntityCollisionFlags.IsPKLite) != 0)
+            {
+                collide = true;
+            }
+
+            if (!collide)
+                return true; // exempt — non-PK pair walks through
+        }
+
+        return false; // proceed to broad-phase + shape dispatch
+    }
+}

src/AcDream.Core/Physics/EntityCollisionFlags.cs

@@ -0,0 +1,67 @@
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Per-entity flags driving the retail-faithful PvP / Player /
+/// Impenetrable exemption logic in <c>FindObjCollisions</c>. Decoded
+/// from <c>PublicWeenieDesc._bitfield</c> at <c>CreateObject</c> time.
+///
+/// <para>
+/// Bit positions (verified against
+/// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:406898-406918</c>
+/// where <c>ACCWeenieObject::IsPK/IsPKLite/IsImpenetrable</c> read directly
+/// from <c>this-&gt;pwd._bitfield</c>):
+/// </para>
+/// <list type="bullet">
+///   <item><c>BF_PLAYER          = 0x8</c>        (bit 3)  → <see cref="IsPlayer"/></item>
+///   <item><c>BF_PLAYER_KILLER   = 0x20</c>       (bit 5)  → <see cref="IsPK"/></item>
+///   <item><c>BF_FREE_PKSTATUS   = 0x200000</c>   (bit 21) → <see cref="IsImpenetrable"/></item>
+///   <item><c>BF_PKLITE_PKSTATUS = 0x2000000</c>  (bit 25) → <see cref="IsPKLite"/></item>
+/// </list>
+///
+/// <para>
+/// <see cref="IsCreature"/> is NOT a PWD bit — retail derives it from
+/// <c>PublicWeenieDesc._type</c> matching <c>ITEM_TYPE_CREATURE</c>
+/// (acclient.h ITEM_TYPE enum). Set at registration time by callers that
+/// already know the item type.
+/// </para>
+/// </summary>
+[Flags]
+public enum EntityCollisionFlags : byte
+{
+    None           = 0x00,
+    /// <summary>Set when <c>BF_PLAYER (0x8)</c> is set in <c>pwd._bitfield</c>.</summary>
+    IsPlayer       = 0x01,
+    /// <summary>Derived from <c>ItemType.Creature</c> on the spawn payload.</summary>
+    IsCreature     = 0x02,
+    /// <summary>Set when <c>BF_PLAYER_KILLER (0x20)</c> is set.</summary>
+    IsPK           = 0x04,
+    /// <summary>Set when <c>BF_PKLITE_PKSTATUS (0x2000000)</c> is set.</summary>
+    IsPKLite       = 0x08,
+    /// <summary>Set when <c>BF_FREE_PKSTATUS (0x200000)</c> is set (a.k.a. "Free" PK status — cannot be PKed).</summary>
+    IsImpenetrable = 0x10,
+}
+
+/// <summary>Helpers to convert raw retail bitfields into <see cref="EntityCollisionFlags"/>.</summary>
+public static class EntityCollisionFlagsExt
+{
+    /// <summary>
+    /// Decode the player/PK/PKLite/Impenetrable bits from a
+    /// <c>PublicWeenieDesc._bitfield</c> value (the WeenieHeader trailer
+    /// field acdream's parser surfaces as <c>ObjectDescriptionFlags</c>).
+    ///
+    /// <para>Bit positions per
+    /// <c>docs/research/named-retail/acclient.h:6431-6463</c>
+    /// (<c>PublicWeenieDesc::BitfieldIndex</c>) and
+    /// <c>acclient_2013_pseudo_c.txt:441868-441890</c>
+    /// (<c>PublicWeenieDesc::SetPlayerKillerStatus</c>).</para>
+    /// </summary>
+    public static EntityCollisionFlags FromPwdBitfield(uint bitfield)
+    {
+        var flags = EntityCollisionFlags.None;
+        if ((bitfield & 0x8u)        != 0) flags |= EntityCollisionFlags.IsPlayer;
+        if ((bitfield & 0x20u)       != 0) flags |= EntityCollisionFlags.IsPK;
+        if ((bitfield & 0x200000u)   != 0) flags |= EntityCollisionFlags.IsImpenetrable;
+        if ((bitfield & 0x2000000u)  != 0) flags |= EntityCollisionFlags.IsPKLite;
+        return flags;
+    }
+}

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -388,13 +388,21 @@ public sealed class PhysicsEngine
         float sphereRadius, float sphereHeight,
         float stepUpHeight, float stepDownHeight,
         bool isOnGround,
-        PhysicsBody? body = null)
+        PhysicsBody? body = null,
+        ObjectInfoState moverFlags = ObjectInfoState.None)
     {
         var transition = new Transition();
         transition.ObjectInfo.StepUpHeight = stepUpHeight;
         transition.ObjectInfo.StepDownHeight = stepDownHeight;
         transition.ObjectInfo.StepDown = true;
 
+        // Commit C 2026-04-29 — caller-supplied mover flags drive the
+        // retail PvP exemption block in FindObjCollisions. The local
+        // player passes IsPlayer (and PK/PKLite/Impenetrable when known
+        // from PlayerDescription); remote dead-reckoning passes None
+        // (matches non-player movement, all targets collide).
+        transition.ObjectInfo.State |= moverFlags;
+
         if (isOnGround)
             transition.ObjectInfo.State |= ObjectInfoState.Contact | ObjectInfoState.OnWalkable;
 

src/AcDream.Core/Physics/ShadowObjectRegistry.cs

@@ -19,11 +19,24 @@ public sealed class ShadowObjectRegistry
 
     /// <summary>
     /// Register an entity into the cells it overlaps based on world position + radius.
+    ///
+    /// <para>
+    /// The optional <paramref name="state"/> + <paramref name="flags"/>
+    /// parameters carry retail <c>PhysicsState</c> bits and decoded
+    /// <see cref="EntityCollisionFlags"/> respectively, so the
+    /// <c>FindObjCollisions</c> retail-faithful exemption block (PvP rule,
+    /// ETHEREAL skip, viewer-vs-creature) can short-circuit without an
+    /// extra lookup. Default <c>state=0</c> + <c>flags=None</c> preserves
+    /// the original "static decoration" behavior — the existing 5
+    /// landblock-entity registration sites pass nothing.
+    /// </para>
     /// </summary>
     public void Register(uint entityId, uint gfxObjId, Vector3 worldPos, Quaternion rotation,
                          float radius, float worldOffsetX, float worldOffsetY, uint landblockId,
                          ShadowCollisionType collisionType = ShadowCollisionType.BSP,
-                         float cylHeight = 0f, float scale = 1.0f)
+                         float cylHeight = 0f, float scale = 1.0f,
+                         uint state = 0u,
+                         EntityCollisionFlags flags = EntityCollisionFlags.None)
     {
         Deregister(entityId);
 
@@ -38,7 +51,8 @@ public sealed class ShadowObjectRegistry
         int minCy = Math.Max(0, (int)((localY - radius) / 24f));
         int maxCy = Math.Min(7, (int)((localY + radius) / 24f));
 
-        var entry = new ShadowEntry(entityId, gfxObjId, worldPos, rotation, radius, collisionType, cylHeight, scale);
+        var entry = new ShadowEntry(entityId, gfxObjId, worldPos, rotation, radius,
+                                    collisionType, cylHeight, scale, state, flags);
         var cellIds = new List<uint>();
 
         uint lbPrefix = landblockId & 0xFFFF0000u;
@@ -62,6 +76,56 @@ public sealed class ShadowObjectRegistry
         _entityToCells[entityId] = cellIds;
     }
 
+    /// <summary>
+    /// Update an already-registered entity's world position + rotation,
+    /// preserving its <see cref="ShadowEntry.State"/>,
+    /// <see cref="ShadowEntry.Flags"/>, and shape parameters.
+    ///
+    /// <para>
+    /// Cheaper than <see cref="Deregister"/> + <see cref="Register"/> for
+    /// the 5–10 Hz <c>UpdatePosition (0xF748)</c> stream the server emits
+    /// per visible entity: this is the path retail's
+    /// <c>CPhysicsObj::SetPosition</c> takes (cited at
+    /// <c>acclient_2013_pseudo_c.txt:284276</c>) — same shape, new cell
+    /// membership. If the entity isn't already registered, this is a
+    /// no-op so callers don't have to gate.
+    /// </para>
+    /// </summary>
+    public void UpdatePosition(uint entityId, Vector3 worldPos, Quaternion rotation,
+                               float worldOffsetX, float worldOffsetY, uint landblockId)
+    {
+        // Find the existing entry (any cell holds a copy with the same
+        // entity-scoped state + flags + shape).
+        if (!_entityToCells.TryGetValue(entityId, out var oldCells) || oldCells.Count == 0)
+            return;
+
+        ShadowEntry? template = null;
+        foreach (var oldCellId in oldCells)
+        {
+            if (_cells.TryGetValue(oldCellId, out var list))
+            {
+                foreach (var e in list)
+                {
+                    if (e.EntityId == entityId)
+                    {
+                        template = e;
+                        break;
+                    }
+                }
+            }
+            if (template is not null) break;
+        }
+        if (template is null)
+            return;
+
+        // Preserve everything except position + rotation.
+        var t = template.Value;
+        Register(entityId, t.GfxObjId, worldPos, rotation, t.Radius,
+                 worldOffsetX, worldOffsetY, landblockId,
+                 t.CollisionType, t.CylHeight, t.Scale,
+                 t.State, t.Flags);
+    }
+
     /// <summary>Remove an entity from all cells it was registered in.</summary>
     public void Deregister(uint entityId)
     {
@@ -203,4 +267,18 @@ public readonly record struct ShadowEntry(
     float Radius,
     ShadowCollisionType CollisionType = ShadowCollisionType.BSP,
     float CylHeight = 0f,
-    float Scale = 1.0f);
+    float Scale = 1.0f,
+    /// <summary>
+    /// Retail <c>PhysicsState</c> bits (<c>acclient.h:2815</c>). Used
+    /// by <c>FindObjCollisions</c> to honor <c>ETHEREAL_PS=0x4</c> +
+    /// <c>IGNORE_COLLISIONS_PS=0x10</c> short-circuits. Zero for static
+    /// landblock entities (default behavior matches pre-Commit-A).
+    /// </summary>
+    uint State = 0u,
+    /// <summary>
+    /// Decoded player / PK / PKLite / Impenetrable flags driving the
+    /// retail PvP exemption block in <c>FindObjCollisions</c>. Built
+    /// from <c>PWD._bitfield</c> at <c>CreateObject</c> time via
+    /// <see cref="EntityCollisionFlagsExt.FromPwdBitfield(uint)"/>.
+    /// </summary>
+    EntityCollisionFlags Flags = EntityCollisionFlags.None);

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -34,6 +34,13 @@ public enum ObjectInfoState : uint
     IsPlayer        = 0x100,
     EdgeSlide       = 0x200,
     IgnoreCreatures = 0x400,
+    /// <summary>Bits added 2026-04-29 (Commit A live-entity collision):
+    /// the moving object's PK status drives the player-vs-player
+    /// exemption block in <c>FindObjCollisions</c> per
+    /// <c>acclient_2013_pseudo_c.txt:276807-276839</c>. Values match
+    /// retail's <c>OBJECTINFO::state</c> bits (acclient.h:6190-6194).</summary>
+    IsPK            = 0x800,
+    IsPKLite        = 0x1000,
 }
 
 /// <summary>
@@ -800,6 +807,17 @@ public sealed class Transition
             if (distToCurr > maxReach)
                 continue;
 
+            // Commit C 2026-04-29 — retail exemption block at the top of
+            // CPhysicsObj::FindObjCollisions
+            // (acclient_2013_pseudo_c.txt:276782-276839,276971). Skips
+            // ethereal+ignore-collisions, viewer-vs-creature,
+            // IGNORE_CREATURES movers, and the player-vs-player PvP
+            // pass-through (non-PK pair / mismatched PK). Static
+            // landblock entries register with State=0 and Flags=None,
+            // so this is a cheap fall-through for them.
+            if (CollisionExemption.ShouldSkip(obj.State, obj.Flags, ObjectInfo.State))
+                continue;
+
             TransitionState result;
 
             if (obj.CollisionType == ShadowCollisionType.BSP)

tests/AcDream.Core.Net.Tests/Messages/CreateObjectTests.cs

@@ -23,10 +23,63 @@ public sealed class CreateObjectTests
         Assert.Equal((uint)ItemType.Creature, parsed.Value.ItemType);
     }
 
+    // -----------------------------------------------------------------------
+    // Commit A of 2026-04-29 live-entity collision port:
+    // PhysicsState (post-flags u32) + ObjectDescriptionFlags (PWD bitfield)
+    // must be surfaced for downstream collision registration.
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void TryParse_PhysicsState_Parsed()
+    {
+        // ETHEREAL_PS = 0x4 + IGNORE_COLLISIONS_PS = 0x10 → 0x14
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000003u, name: "GhostNpc",
+            itemType: (uint)ItemType.Creature,
+            physicsState: 0x14u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x14u, parsed!.Value.PhysicsState);
+    }
+
+    [Fact]
+    public void TryParse_ObjectDescriptionFlags_PlayerKillerBitsSurface()
+    {
+        // BF_PLAYER (0x8) | BF_PLAYER_KILLER (0x20) → a PK player.
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000004u, name: "+PkPlayer",
+            itemType: (uint)ItemType.Creature,
+            objectDescriptionFlags: 0x8u | 0x20u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x28u, parsed!.Value.ObjectDescriptionFlags);
+    }
+
+    [Fact]
+    public void TryParse_ObjectDescriptionFlags_PkLiteBit()
+    {
+        // BF_PLAYER (0x8) | BF_PKLITE_PKSTATUS (0x2000000) → a PK-lite player.
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000005u, name: "+PklPlayer",
+            itemType: (uint)ItemType.Creature,
+            objectDescriptionFlags: 0x8u | 0x2000000u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x2000008u, parsed!.Value.ObjectDescriptionFlags);
+    }
+
     private static byte[] BuildMinimalCreateObjectWithWeenieHeader(
         uint guid,
         string name,
-        uint itemType)
+        uint itemType,
+        uint physicsState = 0,
+        uint objectDescriptionFlags = 0)
     {
         var bytes = new List<byte>();
         WriteU32(bytes, CreateObject.Opcode);
@@ -38,9 +91,9 @@ public sealed class CreateObjectTests
         bytes.Add(0);
         bytes.Add(0);
 
-        // PhysicsData: no flags, empty physics state, then 9 sequence stamps.
-        WriteU32(bytes, 0);
+        // PhysicsData: physics flags = 0, then PhysicsState u32, then 9 seq stamps.
         WriteU32(bytes, 0);
+        WriteU32(bytes, physicsState);
         for (int i = 0; i < 9; i++)
             WriteU16(bytes, 0);
         Align4(bytes);
@@ -51,7 +104,7 @@ public sealed class CreateObjectTests
         WritePackedDword(bytes, 0x1234); // WeenieClassId
         WritePackedDword(bytes, 0);      // IconId via known-type writer
         WriteU32(bytes, itemType);
-        WriteU32(bytes, 0);              // ObjectDescriptionFlags
+        WriteU32(bytes, objectDescriptionFlags);
         Align4(bytes);
 
         return bytes.ToArray();

tests/AcDream.Core.Tests/Physics/CollisionExemptionTests.cs

@@ -0,0 +1,169 @@
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Unit tests for <see cref="CollisionExemption"/> — Commit C of the
+/// 2026-04-29 live-entity collision port. Covers retail's
+/// <c>CPhysicsObj::FindObjCollisions</c> exemption block, ported
+/// line-for-line from
+/// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:276782-276839,276971</c>.
+///
+/// <para>
+/// Behaviour matrix (target / mover columns):
+/// </para>
+/// <list type="table">
+///   <listheader><term>Mover</term><term>Target</term><description>Skip?</description></listheader>
+///   <item><term>any</term><term>ETHEREAL+IGNORE_COLLISIONS</term><description>YES (early-out)</description></item>
+///   <item><term>IsViewer</term><term>IsCreature</term><description>YES (camera ray-test passes through)</description></item>
+///   <item><term>IGNORE_CREATURES</term><term>IsCreature</term><description>YES (mover walks through creatures)</description></item>
+///   <item><term>IsPlayer</term><term>IsPlayer (no PK)</term><description>YES (non-PK pair walks through)</description></item>
+///   <item><term>IsPlayer + IsPK</term><term>IsPlayer + IsPK</term><description>NO (PK pair collides)</description></item>
+///   <item><term>IsPlayer + IsPKLite</term><term>IsPlayer + IsPKLite</term><description>NO (PKLite pair collides)</description></item>
+///   <item><term>IsPlayer</term><term>IsPlayer + IsImpenetrable</term><description>NO (Impenetrable target always collides)</description></item>
+///   <item><term>IsPlayer + IsPK</term><term>IsPlayer (no PK)</term><description>YES (mismatched PK skip)</description></item>
+///   <item><term>IsPlayer</term><term>IsCreature (NPC)</term><description>NO (player vs NPC always collides)</description></item>
+/// </list>
+/// </summary>
+public class CollisionExemptionTests
+{
+    private const uint ETHEREAL_PS = 0x4u;
+    private const uint IGNORE_COLLISIONS_PS = 0x10u;
+
+    [Fact]
+    public void EtherealAndIgnoreCollisions_AlwaysSkipped()
+    {
+        // Target with both bits set is exempted from any mover.
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: ETHEREAL_PS | IGNORE_COLLISIONS_PS,
+            targetFlags: EntityCollisionFlags.None,
+            moverState: ObjectInfoState.IsPlayer));
+    }
+
+    [Fact]
+    public void EtherealOnly_NotSkipped()
+    {
+        // Target with ETHEREAL but NOT IGNORE_COLLISIONS does not bail
+        // out at the first gate — collision proceeds. (Step-down marks
+        // obstruction_ethereal, but does not exempt.)
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: ETHEREAL_PS,
+            targetFlags: EntityCollisionFlags.None,
+            moverState: ObjectInfoState.IsPlayer));
+    }
+
+    [Fact]
+    public void Viewer_VsCreature_Skipped()
+    {
+        // Camera-ray viewer transitions through creatures.
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsCreature,
+            moverState: ObjectInfoState.IsViewer));
+    }
+
+    [Fact]
+    public void Viewer_VsNonCreature_NotSkipped()
+    {
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.None,
+            moverState: ObjectInfoState.IsViewer));
+    }
+
+    [Fact]
+    public void IgnoreCreatures_VsCreature_Skipped()
+    {
+        // Per acclient_2013_pseudo_c.txt:276971 — an arrow with
+        // IGNORE_CREATURES doesn't get blocked by the very monster it's
+        // tracking towards (until missile_ignore filters its target).
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsCreature,
+            moverState: ObjectInfoState.IgnoreCreatures));
+    }
+
+    [Fact]
+    public void NonPkPlayer_VsNonPkPlayer_Skipped()
+    {
+        // The user-visible payoff: two ordinary players walk through each
+        // other instead of blocking.
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer,
+            moverState: ObjectInfoState.IsPlayer));
+    }
+
+    [Fact]
+    public void Pk_VsPk_NotSkipped()
+    {
+        // Two PK players collide.
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPK,
+            moverState: ObjectInfoState.IsPlayer | ObjectInfoState.IsPK));
+    }
+
+    [Fact]
+    public void PkLite_VsPkLite_NotSkipped()
+    {
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPKLite,
+            moverState: ObjectInfoState.IsPlayer | ObjectInfoState.IsPKLite));
+    }
+
+    [Fact]
+    public void Pk_VsNonPk_Skipped()
+    {
+        // Mismatched PK status: still exempt — only matching pair collides.
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer,
+            moverState: ObjectInfoState.IsPlayer | ObjectInfoState.IsPK));
+    }
+
+    [Fact]
+    public void Pk_VsPkLite_Skipped()
+    {
+        // PK and PKLite are different pools — pair doesn't match.
+        Assert.True(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPKLite,
+            moverState: ObjectInfoState.IsPlayer | ObjectInfoState.IsPK));
+    }
+
+    [Fact]
+    public void ImpenetrableTarget_VsAnyPlayer_NotSkipped()
+    {
+        // Impenetrable target ("Free" PK status) always collides with
+        // any player mover — regardless of mover's PK state.
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsImpenetrable,
+            moverState: ObjectInfoState.IsPlayer));
+    }
+
+    [Fact]
+    public void Player_VsCreature_NotSkipped()
+    {
+        // PvP exemption only applies player-on-player. Player vs creature
+        // (NPC, monster) is the normal blocking case.
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsCreature,
+            moverState: ObjectInfoState.IsPlayer));
+    }
+
+    [Fact]
+    public void NonPlayerMover_VsPlayer_NotSkipped()
+    {
+        // PvP rule requires BOTH to be players. Mover is not a player
+        // (e.g., dead-reckoned remote NPC) → no exemption applies.
+        Assert.False(CollisionExemption.ShouldSkip(
+            targetState: 0u,
+            targetFlags: EntityCollisionFlags.IsPlayer,
+            moverState: ObjectInfoState.None));
+    }
+}

tests/AcDream.Core.Tests/Physics/EntityCollisionFlagsTests.cs

@@ -0,0 +1,89 @@
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Unit tests for <see cref="EntityCollisionFlags"/> — Commit A of the
+/// 2026-04-29 live-entity collision port. Verifies retail-faithful
+/// conversion from PWD bitfield bits to the boolean collision-decision
+/// flags that <c>FindObjCollisions</c> consumes for the PvP exemption.
+///
+/// <para>Bit positions confirmed against:</para>
+/// <list type="bullet">
+///   <item><c>docs/research/named-retail/acclient_2013_pseudo_c.txt:406898-406918</c>
+///         (<c>ACCWeenieObject::IsPKLite/IsPK/IsImpenetrable</c>) — the
+///         retail client itself reads bits 25 / 5 / 21 of <c>pwd._bitfield</c>.</item>
+///   <item><c>docs/research/named-retail/acclient.h:6431-6463</c>
+///         (<c>PublicWeenieDesc::BitfieldIndex</c>) — names the bits:
+///         <c>BF_PLAYER=0x8</c>, <c>BF_PLAYER_KILLER=0x20</c>,
+///         <c>BF_FREE_PKSTATUS=0x200000</c>, <c>BF_PKLITE_PKSTATUS=0x2000000</c>.</item>
+///   <item><c>docs/research/named-retail/acclient_2013_pseudo_c.txt:441868-441890</c>
+///         (<c>PublicWeenieDesc::SetPlayerKillerStatus</c>) — confirms
+///         the writer maps PKStatusEnum values onto these exact bits.</item>
+/// </list>
+/// </summary>
+public class EntityCollisionFlagsTests
+{
+    [Fact]
+    public void FromPwdBitfield_AllZeros_NoFlags()
+    {
+        Assert.Equal(EntityCollisionFlags.None, EntityCollisionFlagsExt.FromPwdBitfield(0u));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_PlayerBit_SetsIsPlayer()
+    {
+        // BF_PLAYER = 0x8 (bit 3)
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x8u);
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsPlayer));
+        Assert.False(flags.HasFlag(EntityCollisionFlags.IsPK));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_PlayerKillerBit_SetsIsPK()
+    {
+        // BF_PLAYER_KILLER = 0x20 (bit 5)
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x20u);
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsPK));
+        Assert.False(flags.HasFlag(EntityCollisionFlags.IsPKLite));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_PkLiteBit_SetsIsPKLite()
+    {
+        // BF_PKLITE_PKSTATUS = 0x2000000 (bit 25)
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x2000000u);
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsPKLite));
+        Assert.False(flags.HasFlag(EntityCollisionFlags.IsPK));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_FreePkStatusBit_SetsIsImpenetrable()
+    {
+        // BF_FREE_PKSTATUS = 0x200000 (bit 21)
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x200000u);
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsImpenetrable));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_PlayerAndPK_SetsBoth()
+    {
+        // A PK player: BF_PLAYER (0x8) | BF_PLAYER_KILLER (0x20) = 0x28
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x28u);
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsPlayer));
+        Assert.True(flags.HasFlag(EntityCollisionFlags.IsPK));
+        Assert.False(flags.HasFlag(EntityCollisionFlags.IsPKLite));
+        Assert.False(flags.HasFlag(EntityCollisionFlags.IsImpenetrable));
+    }
+
+    [Fact]
+    public void FromPwdBitfield_UnrelatedBits_Ignored()
+    {
+        // Set BF_OPENABLE (0x1), BF_INSCRIBABLE (0x2), BF_STUCK (0x4) — none
+        // map to collision flags. A creature spawn might have BF_ATTACKABLE
+        // (0x10) set; that's ItemType-derived IsCreature, not a PvP flag.
+        var flags = EntityCollisionFlagsExt.FromPwdBitfield(0x1u | 0x2u | 0x4u | 0x10u);
+        Assert.Equal(EntityCollisionFlags.None, flags);
+    }
+}

tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs

@@ -177,4 +177,79 @@ public class ShadowObjectRegistryTests
         var objs = reg.GetObjectsInCell(cellId);
         Assert.Contains(objs, e => e.EntityId == 99u);
     }
+
+    // -----------------------------------------------------------------------
+    // UpdatePosition — Commit A of 2026-04-29 live-entity collision port
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void UpdatePosition_MovedEntity_NewCellOccupied()
+    {
+        // Entity starts at local (12, 12) — cell (0,0).
+        var reg = new ShadowObjectRegistry();
+        reg.Register(42u, 0x01000010u, new Vector3(12f, 12f, 50f), Quaternion.Identity, 0.5f, OffX, OffY, LbId);
+
+        // Move to local (60, 12) — cell (2, 0). Same landblock.
+        reg.UpdatePosition(42u, new Vector3(60f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+
+        // Old cell empty, new cell holds the entity.
+        Assert.Empty(reg.GetObjectsInCell(LbId | 1u));   // cell (0,0)
+        var newCell = reg.GetObjectsInCell(LbId | 17u);  // cell (2,0) → 2*8+0+1
+        Assert.Single(newCell);
+        Assert.Equal(42u, newCell[0].EntityId);
+        Assert.Equal(1, reg.TotalRegistered);            // not duplicated
+    }
+
+    [Fact]
+    public void UpdatePosition_PreservesFlags()
+    {
+        // Register with PK flags + PhysicsState; UpdatePosition must keep them.
+        var reg = new ShadowObjectRegistry();
+        reg.Register(50u, 0x01000011u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId,
+                     state: 0x4u, // ETHEREAL_PS
+                     flags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPK);
+
+        reg.UpdatePosition(50u, new Vector3(60f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+
+        var newCell = reg.GetObjectsInCell(LbId | 17u);
+        Assert.Single(newCell);
+        Assert.Equal(0x4u, newCell[0].State);
+        Assert.Equal(EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPK, newCell[0].Flags);
+    }
+
+    [Fact]
+    public void UpdatePosition_UnregisteredEntity_NoOp()
+    {
+        var reg = new ShadowObjectRegistry();
+        // Should not throw, should not register a new entity.
+        reg.UpdatePosition(99u, new Vector3(12f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+        Assert.Equal(0, reg.TotalRegistered);
+    }
+
+    [Fact]
+    public void Register_WithStateAndFlags_StoredOnEntry()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(60u, 0x01000012u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId,
+                     state: 0x10u, // IGNORE_COLLISIONS_PS
+                     flags: EntityCollisionFlags.IsImpenetrable);
+
+        var entry = reg.GetObjectsInCell(LbId | 1u)[0];
+        Assert.Equal(0x10u, entry.State);
+        Assert.Equal(EntityCollisionFlags.IsImpenetrable, entry.Flags);
+    }
+
+    [Fact]
+    public void Register_DefaultStateAndFlags_AreZeroAndNone()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(70u, 0x01000013u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId);
+
+        var entry = reg.GetObjectsInCell(LbId | 1u)[0];
+        Assert.Equal(0u, entry.State);
+        Assert.Equal(EntityCollisionFlags.None, entry.Flags);
+    }
 }