Body physics Stage 2: physical arm segments (chase-driven upper arm + forearm, hierarchical budgets, layer 5)

- PhysicsArms: per side 2.0 kg upper arm + 1.2 kg forearm RigidBody3D PD-chasing their
  analytic 2-bone IK pose, solved per physics tick from the physical chest shoulders to the
  physical hands; no joints (Box3D has no usable shoulder/wrist joint; AGENTS forbids joints
  between tracked bodies). Budgets from a 60 kg mass stat: hand 90 < forearm 180 < upper
  arm 360 < chest 2400 N. Owns calibration + reach budget (moved from BodyRig); 1 m snap,
  non-finite reset, BodyMath.RotationError servo.
- Layer 5 = player arms (mask 1); props/table/floor masks include it; no self-collision.
- BodyRig: visuals only, arms drawn shoulder -> physical elbow -> wrist.
- ArmSpike: track gap 0, swing lag <= 6 cm, arm pushes prop with hand clear, forearm blocked
  by bar without tunneling and quiet when pinned, teleport snap, NaN reset — PASS x3.
- CollapseSpike now runs the full body (49/49 x3); all six spikes PASS x3.
- HANDOFF/AGENTS: Stage 2 entry (item 10), layers, tuning table, Stage 2b next steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/ArmSpike.cs

@@ -0,0 +1,277 @@
+using System.Collections.Generic;
+using Godot;
+
+// Headless regression for body-physics Stage 2 (PhysicsArms): chase-driven arm segments.
+// Run: godot-mono --headless --fixed-fps 90 --path . scenes/arm_spike.tscn
+public partial class ArmSpike : Node3D
+{
+    private enum Phase { Settle, Track, Swing, Push, Pin, Teleport, NanGuard, Done }
+
+    private const float GapLimit = 0.04f;
+    private const float SwingGapLimit = 0.12f;
+    private const float ChestDisturbLimit = 0.01f;
+    private const float PushMinDisplacement = 0.05f;
+    private const float PushHandClearance = 0.08f;
+    private const float PinSpeedLimit = 0.3f;
+    private const float PinBarY = 1.22f;
+    private const float PinObstructionMin = 0.01f;
+    private const float TeleportGapLimit = 0.04f;
+    private static readonly Vector3 HandSize = new(0.035f, 0.09f, 0.12f);
+
+    private Node3D _head;
+    private PhysicsTorso _torso;
+    private PhysicsArms _arms;
+    private readonly RigidBody3D[] _hands = new RigidBody3D[2];
+    private readonly Transform3D[] _handTarget = new Transform3D[2];
+    private readonly Vector3[] _handRest = new Vector3[2];
+    private RigidBody3D _box;
+    private Vector3 _boxStart;
+    private StaticBody3D _slab;
+
+    private Phase _phase = Phase.Settle;
+    private float _phaseT;
+    private float _maxGap;
+    private float _maxChestErr;
+    private float _minHandBoxDist;
+    private float _maxPinSpeed;
+    private float _maxPinObstruction;
+    private readonly float[] _pinPrevY = { float.MinValue, float.MinValue };
+    private bool _pinTunneled;
+    private bool _pinFinite = true;
+    private readonly List<string> _names = new();
+    private readonly List<bool> _ok = new();
+
+    public override void _Ready()
+    {
+        StaticBody3D floor = new() { CollisionMask = 0b11101 };
+        floor.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = new Vector3(20f, 0.1f, 20f) }, Position = new Vector3(0f, -0.05f, 0f) });
+        AddChild(floor);
+
+        _head = new Node3D { Position = new Vector3(0f, 1.65f, 0f) };
+        AddChild(_head);
+        _torso = new PhysicsTorso { HeadSource = _head };
+        AddChild(_torso);
+        _torso.Body.GlobalPosition = new Vector3(0f, 1.23f, 0.12f);
+
+        for (int side = 0; side < 2; side++)
+        {
+            float x = side == 0 ? -0.2f : 0.2f;
+            _handRest[side] = new Vector3(x, 1.1f, -0.35f);
+            _handTarget[side] = new Transform3D(Basis.Identity, _handRest[side]);
+            _hands[side] = new RigidBody3D
+            {
+                Mass = 0.4f,
+                GravityScale = 0f,
+                ContinuousCd = true,
+                Position = _handRest[side],
+                CollisionLayer = 0b1000,
+                CollisionMask = 0b1001,
+            };
+            _hands[side].AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = HandSize } });
+            AddChild(_hands[side]);
+        }
+
+        _arms = new PhysicsArms { HeadSource = _head, Torso = _torso, HandLeft = _hands[0], HandRight = _hands[1] };
+        AddChild(_arms);
+        GD.Print("ARM: arm spike scene ready");
+    }
+
+    public override void _PhysicsProcess(double delta)
+    {
+        float dt = (float)delta;
+        _phaseT += dt;
+        for (int side = 0; side < 2; side++)
+            PhysicsHands.Drive(_hands[side], _handTarget[side], dt);
+        if (!_arms.Calibrated || _arms.UpperArm(0) == null)
+            return;
+
+        switch (_phase)
+        {
+            case Phase.Settle:
+                if (_phaseT > 1f)
+                    Start(Phase.Track, "track — hands circle, segments must stay connected, chest undisturbed");
+                break;
+
+            case Phase.Track:
+                for (int side = 0; side < 2; side++)
+                {
+                    float w = side == 0 ? 2.1f : 2.7f;
+                    _handTarget[side].Origin = _handRest[side] + new Vector3(0.15f * Mathf.Sin(w * _phaseT), 0.15f * Mathf.Cos(w * _phaseT), 0.1f * Mathf.Sin(0.5f * w * _phaseT));
+                }
+                if (_phaseT > 0.5f)
+                {
+                    _maxGap = Mathf.Max(_maxGap, MaxGap());
+                    _maxChestErr = Mathf.Max(_maxChestErr, _torso.Body.GlobalPosition.DistanceTo(_torso.TargetPosition));
+                }
+                if (_phaseT > 3f)
+                {
+                    Record($"track joint gap max {_maxGap:F3} m (limit {GapLimit})", _maxGap < GapLimit);
+                    Record($"chest undisturbed by own arms (err max {_maxChestErr:F4} m, limit {ChestDisturbLimit})", _maxChestErr < ChestDisturbLimit);
+                    Start(Phase.Swing, "swing — fast 0.4 m arm swings at 2 Hz, bounded lag");
+                }
+                break;
+
+            case Phase.Swing:
+                _handTarget[1].Origin = _handRest[1] + new Vector3(0.2f * Mathf.Sin(12.6f * _phaseT), 0.1f * Mathf.Sin(6.3f * _phaseT), 0f);
+                _maxGap = Mathf.Max(_maxGap, MaxGap());
+                if (_phaseT > 2f)
+                {
+                    _handTarget[1].Origin = _handRest[1];
+                    Record($"swing joint gap max {_maxGap:F3} m (limit {SwingGapLimit})", _maxGap < SwingGapLimit);
+                    Start(Phase.Push, "push — right forearm swings up through a 1 kg box (hand kept clear)");
+                    // Box just above the forearm's middle; the hand rises 0.3 m ~0.2 m away from it.
+                    _boxStart = (_arms.ElbowTarget(1) + _arms.WristTarget(1)) * 0.5f + Vector3.Up * 0.11f;
+                    _box = new RigidBody3D { Mass = 1f, GravityScale = 0f, Position = _boxStart, CollisionMask = 0b11101 };
+                    _box.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = Vector3.One * 0.08f } });
+                    AddChild(_box);
+                    _minHandBoxDist = float.MaxValue;
+                }
+                break;
+
+            case Phase.Push:
+            {
+                float k = Mathf.Min(_phaseT / 0.6f, 1f);
+                _handTarget[1].Origin = _handRest[1] + Vector3.Up * (0.3f * k);
+                _minHandBoxDist = Mathf.Min(_minHandBoxDist, _hands[1].GlobalPosition.DistanceTo(_box.GlobalPosition));
+                if (_phaseT > 1.5f)
+                {
+                    float moved = _box.GlobalPosition.DistanceTo(_boxStart);
+                    Record($"arm segment pushes prop (box moved {moved:F3} m, limit > {PushMinDisplacement}; hand-box min dist {_minHandBoxDist:F3} m, limit > {PushHandClearance})",
+                        moved > PushMinDisplacement && _minHandBoxDist > PushHandClearance);
+                    _box.QueueFree();
+                    _handTarget[1].Origin = _handRest[1];
+                    // Two short static bars just above the forearms' middles (x clear of the chest).
+                    // Hands at (±0.2, 1.17, -0.09) put the forearm centers at ~(±0.22, 1.16, 0.09)
+                    // with the elbows swung back; then the hands rise 0.25 m in front of the bars,
+                    // so the forearm targets sweep up through them.
+                    // The bars appear once the forearms have settled in the pin pose (t = 0.8 s).
+                    _maxPinObstruction = 0f;
+                    Start(Phase.Pin, "pin — forearms blocked under static bars while the hands rise past them");
+                }
+                break;
+            }
+
+            case Phase.Pin:
+            {
+                if (_slab == null && _phaseT >= 0.8f)
+                {
+                    _slab = new StaticBody3D { CollisionMask = 0b11101 };
+                    foreach (float x in new[] { -0.22f, 0.22f })
+                        _slab.AddChild(new CollisionShape3D { Position = new Vector3(x, 0f, 0f), Shape = new BoxShape3D { Size = new Vector3(0.14f, 0.04f, 0.04f) } });
+                    AddChild(_slab);
+                    _slab.GlobalPosition = new Vector3(0f, PinBarY, 0.09f);
+                    for (int side = 0; side < 2; side++)
+                        GD.Print($"ARM: pin start side {side}: forearm center {_arms.Forearm(side).GlobalPosition} (bar y {PinBarY})");
+                }
+                float rise = Mathf.Clamp((_phaseT - 1f) / 0.5f, 0f, 1f) * 0.25f;
+                for (int side = 0; side < 2; side++)
+                    _handTarget[side].Origin = new Vector3(side == 0 ? -0.2f : 0.2f, 1.17f + rise, -0.09f);
+                if (_slab != null)
+                {
+                    for (int side = 0; side < 2; side++)
+                    {
+                        RigidBody3D fore = _arms.Forearm(side);
+                        _pinFinite &= BodyMath.IsFinite(fore) && BodyMath.IsFinite(_arms.UpperArm(side));
+                        if (_phaseT > 2.5f)
+                        {
+                            _maxPinSpeed = Mathf.Max(_maxPinSpeed, fore.LinearVelocity.Length());
+                            _maxPinSpeed = Mathf.Max(_maxPinSpeed, _arms.UpperArm(side).LinearVelocity.Length());
+                        }
+                        // Tunneling = the forearm center rises past the bar while still over it in x
+                        // (bar half-width 0.07 + forearm half-width 0.03). Sliding off sideways and
+                        // then rising is honest physics.
+                        Vector3 c = fore.GlobalPosition;
+                        bool overBar = Mathf.Abs(Mathf.Abs(c.X) - 0.22f) < 0.1f;
+                        if (_pinPrevY[side] > float.MinValue && _pinPrevY[side] < PinBarY && c.Y >= PinBarY && overBar)
+                            _pinTunneled = true;
+                        if (_pinPrevY[side] < PinBarY && c.Y >= PinBarY)
+                            GD.Print($"ARM: side {side} forearm cleared the bar at x {c.X:F3} (over bar: {overBar}) t {_phaseT:F2}");
+                        _pinPrevY[side] = c.Y;
+                        Vector3 foreTarget = (_arms.ElbowTarget(side) + _arms.WristTarget(side)) * 0.5f;
+                        _maxPinObstruction = Mathf.Max(_maxPinObstruction, _arms.Forearm(side).GlobalPosition.DistanceTo(foreTarget));
+                    }
+                }
+                if (_phaseT > 3.5f)
+                {
+                    Record($"forearm blocked by bar, no tunneling (deviation max {_maxPinObstruction:F3} m, limit > {PinObstructionMin}; tunneled {_pinTunneled})",
+                        _maxPinObstruction > PinObstructionMin && !_pinTunneled);
+                    Record($"pinned under bar: finite and quiet in the last 1 s (segment |v| max {_maxPinSpeed:F3} m/s, limit {PinSpeedLimit})", _pinFinite && _maxPinSpeed < PinSpeedLimit);
+                    _slab.QueueFree();
+                    for (int side = 0; side < 2; side++)
+                        _handRest[side] += new Vector3(3f, 0f, 0f);
+                    _head.GlobalPosition += new Vector3(3f, 0f, 0f);
+                    for (int side = 0; side < 2; side++)
+                    {
+                        _handTarget[side].Origin = _handRest[side];
+                        _hands[side].GlobalPosition = _handRest[side];
+                        _hands[side].LinearVelocity = Vector3.Zero;
+                    }
+                    Start(Phase.Teleport, "teleport — head + hands jump 3 m, arms must snap");
+                }
+                break;
+            }
+
+            case Phase.Teleport:
+                if (_phaseT > 0.4f)
+                {
+                    float gap = MaxGap();
+                    Record($"teleport re-attach gap {gap:F3} m (limit {TeleportGapLimit})", gap < TeleportGapLimit);
+                    _arms.Forearm(0).AngularVelocity = new Vector3(float.NaN, 0f, 0f);
+                    Start(Phase.NanGuard, "nan guard — inject NaN into the left forearm, it must reset");
+                }
+                break;
+
+            case Phase.NanGuard:
+                if (_phaseT > 0.3f)
+                {
+                    bool finite = BodyMath.IsFinite(_arms.Forearm(0));
+                    float gap = MaxGap();
+                    Record($"non-finite segment reset (finite {finite}, gap {gap:F3} m)", finite && gap < GapLimit);
+                    Finish();
+                }
+                break;
+        }
+    }
+
+    private float MaxGap()
+    {
+        float gap = 0f;
+        for (int side = 0; side < 2; side++)
+        {
+            RigidBody3D upper = _arms.UpperArm(side);
+            RigidBody3D fore = _arms.Forearm(side);
+            Vector3 upperProx = upper.GlobalPosition - upper.GlobalTransform.Basis.Y * (_arms.UpperLen * 0.5f);
+            Vector3 upperDist = upper.GlobalPosition + upper.GlobalTransform.Basis.Y * (_arms.UpperLen * 0.5f);
+            Vector3 foreProx = fore.GlobalPosition - fore.GlobalTransform.Basis.Y * (_arms.ForeLen * 0.5f);
+            gap = Mathf.Max(gap, upperProx.DistanceTo(_arms.Shoulder(side)));
+            gap = Mathf.Max(gap, upperDist.DistanceTo(foreProx));
+            gap = Mathf.Max(gap, _arms.Wrist(side).DistanceTo(_arms.WristTarget(side)));
+        }
+        return gap;
+    }
+
+    private void Start(Phase phase, string what)
+    {
+        _phase = phase;
+        _phaseT = 0f;
+        _maxGap = 0f;
+        GD.Print($"ARM: {what}");
+    }
+
+    private void Record(string name, bool ok)
+    {
+        _names.Add(name);
+        _ok.Add(ok);
+        GD.Print($"ARM: {name} → {(ok ? "PASS" : "FAIL")}");
+    }
+
+    private void Finish()
+    {
+        _phase = Phase.Done;
+        bool overall = true;
+        foreach (bool ok in _ok)
+            overall &= ok;
+        GD.Print($"ARM: overall {(overall ? "PASS — physical arms functional on Box3D" : "CHECK — inspect failures above")}");
+        GetTree().Quit();
+    }
+}

scripts/ArmSpike.cs.uid

@@ -0,0 +1 @@
+uid://ksf2lcey06ag

scripts/BodyRig.cs

@@ -1,52 +1,39 @@
 using Godot;
 
+// Placeholder rig VISUALS only: head cube on the HMD, arm boxes drawn from the physical body
+// (PhysicsTorso shoulders → PhysicsArms elbow → hand wrist). Arm IK, calibration and the reach
+// budget live in PhysicsArms; the torso mesh lives in PhysicsTorso.
 public partial class BodyRig : Node3D
 {
     private const float HeadBackOffset = 0.06f;
     private const float HeadSize = 0.15f;
-    private const float ShoulderRise = 0.25f;
-    private const float ReachBudgetMax = 0.25f;
-    private const float ReachVerticalMin = -0.05f;
-    private const float ReachVerticalMax = 0.12f;
-    private const float ReachPushSpeed = 0.8f;
-    private const float ReachDecaySpeed = 0.4f;
-    private const float ReachDecayFactor = 0.92f;
     private const float WristOffset = 0.06f;
-    private const float SoftIKFactor = 0.98f;
-    private const float PoleHandWeight = 0.25f;
-    private const float PoleSmoothRate = 10f;
 
     private XRCamera3D _camera;
     private XRController3D _left;
     private XRController3D _right;
-    private PhysicsTorso _torso;
+    private PhysicsArms _arms;
 
     private MeshInstance3D _headMesh;
     private readonly MeshInstance3D[] _upperArmMeshes = new MeshInstance3D[2];
     private readonly MeshInstance3D[] _forearmMeshes = new MeshInstance3D[2];
 
-    private bool _calibrated;
-    private float _upperLen;
-    private float _foreLen;
-    private float _shoulderHalfWidth;
-    private Vector3 _torsoOffset = Vector3.Zero;
+    private bool _sized;
     private Basis _yawBasis = Basis.Identity;
-    private Basis _torsoBasis = Basis.Identity;
-    private readonly Vector3[] _lastPole = new Vector3[2];
 
     public override void _Ready()
     {
         _camera = GetNode<XRCamera3D>("../XROrigin3D/XRCamera3D");
         _left = GetNode<XRController3D>("../XROrigin3D/LeftHand");
         _right = GetNode<XRController3D>("../XROrigin3D/RightHand");
-        _torso = GetNode<PhysicsTorso>("../PhysicsTorso");
+        _arms = GetNode<PhysicsArms>("../PhysicsArms");
 
         _headMesh = MakeMesh(new BoxMesh { Size = new Vector3(HeadSize, HeadSize, HeadSize) }, new Color(0.8f, 0.8f, 0.8f));
-        _upperArmMeshes[0] = MakeMesh(new BoxMesh { Size = new Vector3(0.07f, 0.3f, 0.07f) }, new Color(0.3f, 0.3f, 0.33f));
-        _upperArmMeshes[1] = MakeMesh(new BoxMesh { Size = new Vector3(0.07f, 0.3f, 0.07f) }, new Color(0.3f, 0.3f, 0.33f));
-        _forearmMeshes[0] = MakeMesh(new BoxMesh { Size = new Vector3(0.06f, 0.3f, 0.06f) }, new Color(0.3f, 0.3f, 0.33f));
-        _forearmMeshes[1] = MakeMesh(new BoxMesh { Size = new Vector3(0.06f, 0.3f, 0.06f) }, new Color(0.3f, 0.3f, 0.33f));
-
+        for (int side = 0; side < 2; side++)
+        {
+            _upperArmMeshes[side] = MakeMesh(new BoxMesh { Size = new Vector3(0.07f, 0.3f, 0.07f) }, new Color(0.3f, 0.3f, 0.33f));
+            _forearmMeshes[side] = MakeMesh(new BoxMesh { Size = new Vector3(0.06f, 0.3f, 0.06f) }, new Color(0.3f, 0.3f, 0.33f));
+        }
         SetRigVisible(false);
     }
 
@@ -70,44 +57,37 @@ public partial class BodyRig : Node3D
     public override void _Process(double delta)
     {
         UpdateYawBasis();
-        float dt = (float)delta;
-
-        if (!_calibrated)
-        {
-            float headHeight = _camera.GlobalPosition.Y;
-            if (headHeight <= 0.5f)
-                return;
-            Calibrate(headHeight);
-        }
-        if (_torso.Body == null)
+        if (!_arms.Calibrated || _arms.UpperArm(0) == null)
             return;
+        if (!_sized)
+        {
+            _sized = true;
+            for (int side = 0; side < 2; side++)
+            {
+                ((BoxMesh)_upperArmMeshes[side].Mesh).Size = new Vector3(0.07f, _arms.UpperLen, 0.07f);
+                ((BoxMesh)_forearmMeshes[side].Mesh).Size = new Vector3(0.06f, _arms.ForeLen, 0.06f);
+            }
+            SetRigVisible(true);
+        }
 
         Vector3 headPos = _camera.GlobalPosition - _yawBasis * Vector3.Forward * HeadBackOffset;
         _headMesh.GlobalTransform = new Transform3D(_yawBasis, headPos);
 
-        _torsoBasis = _torso.Body.GlobalTransform.Basis;
-        Vector3 torsoCenter = _torso.Body.GlobalPosition;
+        PlaceArm(0, PhysicsHands.GetBody(_left));
+        PlaceArm(1, PhysicsHands.GetBody(_right));
+    }
 
-        Vector3 topCenter = torsoCenter + Vector3.Up * ShoulderRise;
-        Vector3 leftShoulder = topCenter - _torsoBasis.X * _shoulderHalfWidth;
-        Vector3 rightShoulder = topCenter + _torsoBasis.X * _shoulderHalfWidth;
-
-        Vector3 push = SolveArm(0, leftShoulder, -_torsoBasis.X, PhysicsHands.GetBody(_left), _upperArmMeshes[0], _forearmMeshes[0], dt, out bool decayLeft);
-        push += SolveArm(1, rightShoulder, _torsoBasis.X, PhysicsHands.GetBody(_right), _upperArmMeshes[1], _forearmMeshes[1], dt, out bool decayRight);
-
-        if (push != Vector3.Zero)
-        {
-            _torsoOffset += push * (dt * ReachPushSpeed);
-        }
-        else if (decayLeft || decayRight)
-        {
-            _torsoOffset = _torsoOffset.MoveToward(Vector3.Zero, ReachDecaySpeed * dt);
-        }
-
-        Vector3 horizontal = new(_torsoOffset.X, 0f, _torsoOffset.Z);
-        horizontal = horizontal.LimitLength(ReachBudgetMax);
-        _torsoOffset = new Vector3(horizontal.X, Mathf.Clamp(_torsoOffset.Y, ReachVerticalMin, ReachVerticalMax), horizontal.Z);
-        _torso.ReachOffset = _torsoOffset;
+    private void PlaceArm(int side, RigidBody3D hand)
+    {
+        Vector3 shoulder = _arms.Shoulder(side);
+        Vector3 elbow = _arms.Elbow(side);
+        Vector3 wrist = hand != null
+            ? hand.GlobalPosition + hand.GlobalTransform.Basis * new Vector3(0f, 0f, WristOffset)
+            : _arms.Wrist(side);
+        if (!shoulder.IsFinite() || !elbow.IsFinite() || !wrist.IsFinite())
+            return;
+        PlaceSegment(_upperArmMeshes[side], shoulder, elbow);
+        PlaceSegment(_forearmMeshes[side], elbow, wrist);
     }
 
     private void UpdateYawBasis()
@@ -119,97 +99,21 @@ public partial class BodyRig : Node3D
         _yawBasis = Basis.LookingAt(fwd.Normalized(), Vector3.Up);
     }
 
-    private void Calibrate(float headHeight)
-    {
-        _calibrated = true;
-        float armTotal = Mathf.Clamp(headHeight * 0.46f, 0.5f, 0.72f);
-        _upperLen = 0.5f * armTotal;
-        _foreLen = 0.5f * armTotal;
-        _shoulderHalfWidth = 0.16f + headHeight * 0.02f;
-
-        ((BoxMesh)_upperArmMeshes[0].Mesh).Size = new Vector3(0.07f, _upperLen, 0.07f);
-        ((BoxMesh)_upperArmMeshes[1].Mesh).Size = new Vector3(0.07f, _upperLen, 0.07f);
-        ((BoxMesh)_forearmMeshes[0].Mesh).Size = new Vector3(0.06f, _foreLen, 0.06f);
-        ((BoxMesh)_forearmMeshes[1].Mesh).Size = new Vector3(0.06f, _foreLen, 0.06f);
-
-        SetRigVisible(true);
-        GD.Print($"BodyRig: proportions calibrated from head height {headHeight:F2} m " +
-                 $"(arm total {armTotal:F2} m, upper {_upperLen:F2} m, fore {_foreLen:F2} m, shoulder half-width {_shoulderHalfWidth:F2} m)");
-    }
-
-    private Vector3 SolveArm(int index, Vector3 shoulder, Vector3 side, RigidBody3D body, MeshInstance3D upperMesh, MeshInstance3D foreMesh, float delta, out bool decay)
-    {
-        Vector3 rawTarget = body != null
-            ? body.GlobalPosition + body.GlobalTransform.Basis * new Vector3(0f, 0f, WristOffset)
-            : shoulder + _torsoBasis * new Vector3(0f, -0.25f, -0.3f);
-
-        float reach = _upperLen + _foreLen;
-        Vector3 shoulderToTarget = rawTarget - shoulder;
-        if (shoulderToTarget.LengthSquared() < 0.0001f)
-            shoulderToTarget = _torsoBasis * new Vector3(0f, -0.25f, -0.3f);
-
-        float rawDist = shoulderToTarget.Length();
-        float d = Mathf.Clamp(rawDist, 0.01f, reach * SoftIKFactor);
-        Vector3 dir = shoulderToTarget / rawDist;
-        Vector3 clampedTarget = shoulder + dir * d;
-
-        Vector3 push = Vector3.Zero;
-        decay = false;
-        if (rawDist > reach)
-        {
-            push = (rawTarget - clampedTarget).Normalized();
-        }
-        else if (rawDist < reach * ReachDecayFactor)
-        {
-            decay = true;
-        }
-
-        float cosA = (_upperLen * _upperLen + d * d - _foreLen * _foreLen) / (2f * _upperLen * d);
-        float angle = Mathf.Acos(Mathf.Clamp(cosA, -1f, 1f));
-
-        Vector3 handForward = body != null ? body.GlobalTransform.Basis * Vector3.Forward : Vector3.Zero;
-        Vector3 pole = Vector3.Down * 2f + _torsoBasis * Vector3.Back * 0.8f + side * 0.3f;
-        pole -= dir * pole.Dot(dir);
-        pole -= handForward * PoleHandWeight;
-        Vector3 poleDir;
-        if (pole.LengthSquared() < 0.0001f)
-            poleDir = _lastPole[index] == Vector3.Zero ? Vector3.Down : _lastPole[index];
-        else
-        {
-            poleDir = _lastPole[index].Lerp(pole, 1f - Mathf.Exp(-delta * PoleSmoothRate));
-            if (poleDir.LengthSquared() < 0.0001f)
-                poleDir = _lastPole[index];
-            poleDir = poleDir.Normalized();
-            _lastPole[index] = poleDir;
-        }
-
-        Vector3 axis = dir.Cross(poleDir);
-        if (axis.LengthSquared() < 0.0001f)
-            axis = dir.Cross(Vector3.Up);
-        if (axis.LengthSquared() < 0.0001f)
-            axis = Vector3.Right;
-        axis = axis.Normalized();
-
-        Vector3 elbowPos = shoulder + dir.Rotated(axis, angle) * _upperLen;
-        Vector3 foreEnd = elbowPos + (clampedTarget - elbowPos).Normalized() * _foreLen;
-
-        PlaceSegment(upperMesh, shoulder, elbowPos);
-        PlaceSegment(foreMesh, elbowPos, foreEnd);
-
-        return push;
-    }
-
+    // Stretches the unit-length box between two joints so the drawn arm stays connected even
+    // when a physical segment is held back by contact.
     private static void PlaceSegment(MeshInstance3D mesh, Vector3 from, Vector3 to)
     {
         Vector3 segment = to - from;
-        if (segment.LengthSquared() < 0.000001f)
+        float length = segment.Length();
+        if (length < 0.001f)
             return;
-        Vector3 y = segment.Normalized();
+        Vector3 y = segment / length;
         Vector3 x = Vector3.Up.Cross(y);
         if (x.LengthSquared() < 0.0001f)
             x = Vector3.Right.Cross(y);
         x = x.Normalized();
         Vector3 z = x.Cross(y);
-        mesh.GlobalTransform = new Transform3D(new Basis(x, y, z), (from + to) * 0.5f);
+        float scale = length / ((BoxMesh)mesh.Mesh).Size.Y;
+        mesh.GlobalTransform = new Transform3D(new Basis(x, y * scale, z), (from + to) * 0.5f);
     }
 }

scripts/CollapseSpike.cs

@@ -1,7 +1,8 @@
 using System.Collections.Generic;
 using Godot;
 
-// Headless regression for the Stage-1 playtest collapse (2026-10-01). Two families:
+// Headless regression for the Stage-1 playtest collapse (2026-10-01), now with the full Stage-2
+// body (chest + PhysicsArms + hands). Two families:
 // - Stacking: seated chest overlapping the static table, 50 kg anvil rigid-welded to two
 //   chase hands, the logged grab sequence (L, R two-handed, L release, L re-grab), the real
 //   PropShop with the chest plowing through the props. Did NOT reproduce the NaN — kept as
@@ -39,6 +40,7 @@ public partial class CollapseSpike : Node3D
     private Node3D _world;
     private Node3D _head;
     private PhysicsTorso _torso;
+    private PhysicsArms _arms;
     private readonly RigidBody3D[] _hands = new RigidBody3D[2];
     private readonly Node3D[] _targets = new Node3D[2];
     private readonly ConeTwistJoint3D[] _welds = new ConeTwistJoint3D[2];
@@ -189,6 +191,9 @@ public partial class CollapseSpike : Node3D
             _world.AddChild(_targets[h]);
             _targetBase[h] = handPos;
         }
+
+        _arms = new PhysicsArms { HeadSource = _head, Torso = _torso, HandLeft = _hands[0], HandRight = _hands[1] };
+        _world.AddChild(_arms);
     }
 
     public override void _PhysicsProcess(double delta)
@@ -258,6 +263,15 @@ public partial class CollapseSpike : Node3D
         Check("handR", _hands[1]);
         if (_anvil != null)
             Check("anvil", _anvil);
+        for (int side = 0; side < 2; side++)
+        {
+            if (_arms.UpperArm(side) == null)
+                continue;
+            Check($"upper arm {side}", _arms.UpperArm(side));
+            Check($"forearm {side}", _arms.Forearm(side));
+            if (_arms.Forearm(side).LinearVelocity.Length() > RunawaySpeed || _arms.UpperArm(side).LinearVelocity.Length() > RunawaySpeed)
+                Fail($"arm {side} runaway");
+        }
         foreach (Node n in GetTree().GetNodesInGroup("prop"))
         {
             if (n is not RigidBody3D prop || prop.IsQueuedForDeletion())

scripts/PhysicsArms.cs

@@ -0,0 +1,258 @@
+using Godot;
+
+// Body-physics Stage 2: physical arm segments. Upper arm + forearm per side are chase-driven
+// RigidBody3Ds (no joints — Box3D has no usable 3-DOF shoulder/wrist joint, and AGENTS.md
+// forbids joints between tracked bodies). Each segment PD-chases its analytic 2-bone IK pose,
+// solved every physics tick from the PHYSICAL chest shoulders to the PHYSICAL hand bodies.
+// Hierarchical budgets (Nimso's muscle model): hand 90 N < forearm < upper arm < chest 2400 N.
+// The arm IK + reach budget (torso lean on overreach) live here; BodyRig only draws.
+public partial class PhysicsArms : Node
+{
+    private const float BodyMassStat = 60f;
+    private const float ForearmMass = 1.2f;
+    private const float UpperArmMass = 2.0f;
+    private const float ForearmForcePerKg = 3f;   // 180 N at 60 kg
+    private const float UpperArmForcePerKg = 6f;  // 360 N at 60 kg
+    private const float SegmentMaxSpin = 40f;
+    private const float ForearmMaxAlpha = 1500f;
+    private const float UpperArmMaxAlpha = 1000f;
+    private const float SegmentInset = 0.04f;
+    public const float SnapDistance = 1f;
+
+    private const float ShoulderRise = 0.25f;
+    private const float ReachBudgetMax = 0.25f;
+    private const float ReachVerticalMin = -0.05f;
+    private const float ReachVerticalMax = 0.12f;
+    private const float ReachPushSpeed = 0.8f;
+    private const float ReachDecaySpeed = 0.4f;
+    private const float ReachDecayFactor = 0.92f;
+    private const float WristOffset = 0.06f;
+    private const float SoftIKFactor = 0.98f;
+    private const float PoleHandWeight = 0.25f;
+    private const float PoleSmoothRate = 10f;
+
+    public static float ForearmMaxForce => BodyMassStat * ForearmForcePerKg;
+    public static float UpperArmMaxForce => BodyMassStat * UpperArmForcePerKg;
+
+    // Injected by headless spikes; resolved from the main scene otherwise.
+    public Node3D HeadSource;
+    public PhysicsTorso Torso;
+    public RigidBody3D HandLeft;
+    public RigidBody3D HandRight;
+
+    public bool Calibrated { get; private set; }
+    public float UpperLen { get; private set; }
+    public float ForeLen { get; private set; }
+    public float ShoulderHalfWidth { get; private set; }
+
+    private readonly RigidBody3D[] _upper = new RigidBody3D[2];
+    private readonly RigidBody3D[] _fore = new RigidBody3D[2];
+    private readonly Vector3[] _shoulder = new Vector3[2];
+    private readonly Vector3[] _elbowTarget = new Vector3[2];
+    private readonly Vector3[] _wristTarget = new Vector3[2];
+    private readonly Vector3[] _lastPole = new Vector3[2];
+    private Vector3 _torsoOffset;
+    private Basis _torsoBasis = Basis.Identity;
+
+    public RigidBody3D UpperArm(int side) => _upper[side];
+    public RigidBody3D Forearm(int side) => _fore[side];
+    public Vector3 Shoulder(int side) => _shoulder[side];
+    public Vector3 ElbowTarget(int side) => _elbowTarget[side];
+    public Vector3 WristTarget(int side) => _wristTarget[side];
+
+    // Physical elbow: where the two segment bodies meet (mean of their joint-side ends).
+    public Vector3 Elbow(int side) =>
+        (SegmentEnd(_upper[side], UpperLen, 1f) + SegmentEnd(_fore[side], ForeLen, -1f)) * 0.5f;
+
+    public Vector3 Wrist(int side) => SegmentEnd(_fore[side], ForeLen, 1f);
+
+    public override void _Ready()
+    {
+        if (HeadSource == null)
+        {
+            HeadSource = GetNode<XRCamera3D>("../XROrigin3D/XRCamera3D");
+            Torso = GetNode<PhysicsTorso>("../PhysicsTorso");
+            HandLeft = PhysicsHands.GetBody(GetNode<XRController3D>("../XROrigin3D/LeftHand"));
+            HandRight = PhysicsHands.GetBody(GetNode<XRController3D>("../XROrigin3D/RightHand"));
+        }
+    }
+
+    public override void _PhysicsProcess(double delta)
+    {
+        float dt = (float)delta;
+        if (!Calibrated)
+        {
+            float headHeight = HeadSource.GlobalPosition.Y;
+            if (headHeight <= 0.5f || Torso?.Body == null)
+                return;
+            Calibrate(headHeight);
+        }
+
+        _torsoBasis = Torso.Body.GlobalTransform.Basis;
+        Vector3 topCenter = Torso.Body.GlobalPosition + Vector3.Up * ShoulderRise;
+        _shoulder[0] = topCenter - _torsoBasis.X * ShoulderHalfWidth;
+        _shoulder[1] = topCenter + _torsoBasis.X * ShoulderHalfWidth;
+
+        Vector3 push = SolveArm(0, -_torsoBasis.X, HandLeft, dt, out bool decayLeft);
+        push += SolveArm(1, _torsoBasis.X, HandRight, dt, out bool decayRight);
+        UpdateReachBudget(push, decayLeft || decayRight, dt);
+
+        for (int side = 0; side < 2; side++)
+        {
+            Drive(_upper[side], SegmentTarget(_shoulder[side], _elbowTarget[side], _upper[side]), UpperArmMaxForce, UpperArmMaxAlpha, dt);
+            Drive(_fore[side], SegmentTarget(_elbowTarget[side], _wristTarget[side], _fore[side]), ForearmMaxForce, ForearmMaxAlpha, dt);
+        }
+    }
+
+    private void Calibrate(float headHeight)
+    {
+        Calibrated = true;
+        float armTotal = Mathf.Clamp(headHeight * 0.46f, 0.5f, 0.72f);
+        UpperLen = 0.5f * armTotal;
+        ForeLen = 0.5f * armTotal;
+        ShoulderHalfWidth = 0.16f + headHeight * 0.02f;
+
+        for (int side = 0; side < 2; side++)
+        {
+            string name = side == 0 ? "Left" : "Right";
+            _upper[side] = MakeSegment($"{name}UpperArmBody", UpperArmMass, new Vector3(0.07f, UpperLen - SegmentInset, 0.07f));
+            _fore[side] = MakeSegment($"{name}ForearmBody", ForearmMass, new Vector3(0.06f, ForeLen - SegmentInset, 0.06f));
+        }
+        GD.Print($"PhysicsArms: proportions calibrated from head height {headHeight:F2} m " +
+                 $"(arm total {armTotal:F2} m, upper {UpperLen:F2} m, fore {ForeLen:F2} m, shoulder half-width {ShoulderHalfWidth:F2} m); " +
+                 $"budgets fore {ForearmMaxForce:F0} N, upper {UpperArmMaxForce:F0} N");
+    }
+
+    private RigidBody3D MakeSegment(string name, float mass, Vector3 size)
+    {
+        RigidBody3D body = new()
+        {
+            Name = name,
+            Mass = mass,
+            GravityScale = 0f,
+            ContinuousCd = true,
+            CollisionLayer = 0b10000,  // layer 5 = player arms
+            CollisionMask = 0b1,       // world + props only; own chest/hands/arms excluded
+        };
+        body.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = size } });
+        AddChild(body);
+        // Spawn somewhere finite; the first Drive snaps it onto its target (> SnapDistance).
+        body.GlobalPosition = Torso.Body.GlobalPosition + Vector3.Down * 100f;
+        return body;
+    }
+
+    private static void Drive(RigidBody3D body, Transform3D target, float maxForce, float maxAlpha, float dt)
+    {
+        if (BodyMath.ResetIfNonFinite(body, target, "PhysicsArms"))
+            return;
+        if (body.GlobalPosition.DistanceTo(target.Origin) > SnapDistance)
+        {
+            body.GlobalTransform = target;
+            body.LinearVelocity = Vector3.Zero;
+            body.AngularVelocity = Vector3.Zero;
+            return;
+        }
+
+        Vector3 desiredVelocity = (target.Origin - body.GlobalPosition) / dt;
+        Vector3 force = ((desiredVelocity - body.LinearVelocity) / dt * body.Mass).LimitLength(maxForce);
+        body.LinearVelocity += force / body.Mass * dt;
+
+        Vector3 rotErr = BodyMath.RotationError(target.Basis.GetRotationQuaternion(),
+            body.GlobalTransform.Basis.GetRotationQuaternion());
+        Vector3 spinDesired = (rotErr / dt).LimitLength(SegmentMaxSpin);
+        body.AngularVelocity += (spinDesired - body.AngularVelocity).LimitLength(maxAlpha * dt);
+    }
+
+    // Segment pose from joint A to joint B: local +Y runs A→B; twist follows the body's current
+    // X axis so the long-axis roll never flips (a box arm has no meaningful roll).
+    private static Transform3D SegmentTarget(Vector3 a, Vector3 b, RigidBody3D body)
+    {
+        Vector3 y = b - a;
+        if (y.LengthSquared() < 1e-8f)
+            y = Vector3.Down;
+        y = y.Normalized();
+        Vector3 x = body.GlobalTransform.Basis.X;
+        x -= y * x.Dot(y);
+        if (x.LengthSquared() < 1e-6f)
+        {
+            x = y.Cross(Vector3.Forward);
+            if (x.LengthSquared() < 1e-6f)
+                x = y.Cross(Vector3.Right);
+        }
+        x = x.Normalized();
+        return new Transform3D(new Basis(x, y, x.Cross(y)), (a + b) * 0.5f);
+    }
+
+    private static Vector3 SegmentEnd(RigidBody3D body, float length, float sign) =>
+        body.GlobalPosition + body.GlobalTransform.Basis.Y * (sign * length * 0.5f);
+
+    private void UpdateReachBudget(Vector3 push, bool decay, float dt)
+    {
+        if (push != Vector3.Zero)
+            _torsoOffset += push * (dt * ReachPushSpeed);
+        else if (decay)
+            _torsoOffset = _torsoOffset.MoveToward(Vector3.Zero, ReachDecaySpeed * dt);
+
+        Vector3 horizontal = new Vector3(_torsoOffset.X, 0f, _torsoOffset.Z).LimitLength(ReachBudgetMax);
+        _torsoOffset = new Vector3(horizontal.X, Mathf.Clamp(_torsoOffset.Y, ReachVerticalMin, ReachVerticalMax), horizontal.Z);
+        Torso.ReachOffset = _torsoOffset;
+    }
+
+    // Analytic 2-bone IK (moved from BodyRig): soft-IK clamp, torso-frame elbow pole with the
+    // shoulder→hand axis projected out, hand-forward push, exp-smoothed pole.
+    private Vector3 SolveArm(int index, Vector3 side, RigidBody3D hand, float delta, out bool decay)
+    {
+        Vector3 shoulder = _shoulder[index];
+        Vector3 rawTarget = hand != null
+            ? hand.GlobalPosition + hand.GlobalTransform.Basis * new Vector3(0f, 0f, WristOffset)
+            : shoulder + _torsoBasis * new Vector3(0f, -0.25f, -0.3f);
+
+        float reach = UpperLen + ForeLen;
+        Vector3 shoulderToTarget = rawTarget - shoulder;
+        if (shoulderToTarget.LengthSquared() < 0.0001f)
+            shoulderToTarget = _torsoBasis * new Vector3(0f, -0.25f, -0.3f);
+
+        float rawDist = shoulderToTarget.Length();
+        float d = Mathf.Clamp(rawDist, 0.01f, reach * SoftIKFactor);
+        Vector3 dir = shoulderToTarget / rawDist;
+        Vector3 clampedTarget = shoulder + dir * d;
+
+        Vector3 push = Vector3.Zero;
+        decay = false;
+        if (rawDist > reach)
+            push = (rawTarget - clampedTarget).Normalized();
+        else if (rawDist < reach * ReachDecayFactor)
+            decay = true;
+
+        float cosA = (UpperLen * UpperLen + d * d - ForeLen * ForeLen) / (2f * UpperLen * d);
+        float angle = Mathf.Acos(Mathf.Clamp(cosA, -1f, 1f));
+
+        Vector3 handForward = hand != null ? hand.GlobalTransform.Basis * Vector3.Forward : Vector3.Zero;
+        Vector3 pole = Vector3.Down * 2f + _torsoBasis * Vector3.Back * 0.8f + side * 0.3f;
+        pole -= dir * pole.Dot(dir);
+        pole -= handForward * PoleHandWeight;
+        Vector3 poleDir;
+        if (pole.LengthSquared() < 0.0001f)
+            poleDir = _lastPole[index] == Vector3.Zero ? Vector3.Down : _lastPole[index];
+        else
+        {
+            poleDir = _lastPole[index].Lerp(pole, 1f - Mathf.Exp(-delta * PoleSmoothRate));
+            if (poleDir.LengthSquared() < 0.0001f)
+                poleDir = _lastPole[index];
+            poleDir = poleDir.Normalized();
+            _lastPole[index] = poleDir;
+        }
+
+        Vector3 axis = dir.Cross(poleDir);
+        if (axis.LengthSquared() < 0.0001f)
+            axis = dir.Cross(Vector3.Up);
+        if (axis.LengthSquared() < 0.0001f)
+            axis = Vector3.Right;
+        axis = axis.Normalized();
+
+        Vector3 elbow = shoulder + dir.Rotated(axis, angle) * UpperLen;
+        _elbowTarget[index] = elbow;
+        _wristTarget[index] = elbow + (clampedTarget - elbow).Normalized() * ForeLen;
+        return push;
+    }
+}

scripts/PhysicsArms.cs.uid

@@ -0,0 +1 @@
+uid://boeceet3aku24

scripts/PropShop.cs

@@ -45,7 +45,7 @@ public partial class PropShop : Node3D
     {
         Color wood = new(0.45f, 0.3f, 0.18f);
         StaticBody3D table = new();
-        table.CollisionMask = 0b1101;  // world + player body + hands
+        table.CollisionMask = 0b11101;  // world + player body + hands + arms
         table.AddChild(new CollisionShape3D
         {
             Position = new Vector3(0, 0.725f, -0.6f),
@@ -96,7 +96,7 @@ public partial class PropShop : Node3D
             ContinuousCd = spec.Mass < 0.1f,
             ContactMonitor = true,
             MaxContactsReported = 4,
-            CollisionMask = 0b1101,  // world + player body + hands
+            CollisionMask = 0b11101,  // world + player body + hands + arms
         };
         body.AddToGroup("prop");