diff --git a/data_only_viz/lidar_receiver.py b/data_only_viz/lidar_receiver.py
new file mode 100644
index 0000000..60b3fbd
--- /dev/null
+++ b/data_only_viz/lidar_receiver.py
@@ -0,0 +1,44 @@
+"""TCP receiver for iPhone ARBodyTracker LiDAR ARMeshAnchor stream.
+
+Wire format (per frame, after the 4-byte big-endian length prefix consumed
+by the socket reader):
+
+    [uint64 BE timestamp_ns]
+    [uint32 BE vertex_count]
+    [float32 LE x y z] * vertex_count
+
+The decoder is pure and side-effect-free so it can be unit-tested without a
+socket. The socket reader lives in a separate class (LidarTCPReader) so its
+threading model is independently testable.
+"""
+from __future__ import annotations
+
+import struct
+from dataclasses import dataclass
+
+import numpy as np
+
+_HEADER = struct.Struct(">QI")  # timestamp_ns, vertex_count
+
+
+@dataclass(frozen=True)
+class LidarFrame:
+    """One decoded LiDAR frame from the iPhone."""
+
+    timestamp_ns: int
+    points: np.ndarray  # shape (N, 3), float32, ARKit world frame (meters)
+
+
+def decode_frame(body: bytes) -> LidarFrame:
+    """Decode a frame body (length prefix already stripped)."""
+    if len(body) < _HEADER.size:
+        raise ValueError(f"truncated frame: header needs {_HEADER.size} bytes, got {len(body)}")
+    timestamp_ns, vertex_count = _HEADER.unpack_from(body, 0)
+    if vertex_count == 0:
+        raise ValueError("vertex_count must be > 0")
+    expected = _HEADER.size + vertex_count * 12
+    if len(body) < expected:
+        raise ValueError(f"truncated frame: need {expected} bytes for {vertex_count} verts, got {len(body)}")
+    raw = body[_HEADER.size : expected]
+    pts = np.frombuffer(raw, dtype="<f4").reshape(vertex_count, 3).astype(np.float32, copy=True)
+    return LidarFrame(timestamp_ns=int(timestamp_ns), points=pts)
diff --git a/data_only_viz/tests/test_lidar_receiver.py b/data_only_viz/tests/test_lidar_receiver.py
new file mode 100644
index 0000000..72b32fe
--- /dev/null
+++ b/data_only_viz/tests/test_lidar_receiver.py
@@ -0,0 +1,50 @@
+"""Unit tests for the iPhone LiDAR TCP frame decoder."""
+from __future__ import annotations
+
+import struct
+
+import numpy as np
+import pytest
+
+
+def _encode_frame(points: np.ndarray, timestamp_ns: int) -> bytes:
+    """Mimic the iPhone-side encoder for round-trip testing."""
+    n = points.shape[0]
+    body = struct.pack(">Q", timestamp_ns) + struct.pack(">I", n) + points.astype("<f4").tobytes()
+    header = struct.pack(">I", len(body))
+    return header + body
+
+
+def test_decode_lidar_frame_roundtrip() -> None:
+    from data_only_viz.lidar_receiver import LidarFrame, decode_frame
+
+    pts = np.array([[0.1, 0.2, 0.3], [-1.0, 2.0, 5.5]], dtype=np.float32)
+    payload = _encode_frame(pts, timestamp_ns=1_700_000_000_000_000_000)
+
+    body = payload[4:]
+    frame = decode_frame(body)
+
+    assert isinstance(frame, LidarFrame)
+    assert frame.timestamp_ns == 1_700_000_000_000_000_000
+    np.testing.assert_allclose(frame.points, pts, atol=1e-6)
+
+
+def test_decode_lidar_frame_rejects_truncated() -> None:
+    from data_only_viz.lidar_receiver import decode_frame
+
+    pts = np.array([[1.0, 2.0, 3.0]], dtype=np.float32)
+    body = (
+        struct.pack(">Q", 0) +
+        struct.pack(">I", 1) +
+        pts.astype("<f4").tobytes()[:8]  # truncated
+    )
+    with pytest.raises(ValueError, match="truncated"):
+        decode_frame(body)
+
+
+def test_decode_lidar_frame_rejects_zero_vertex_count() -> None:
+    from data_only_viz.lidar_receiver import decode_frame
+
+    body = struct.pack(">Q", 0) + struct.pack(">I", 0)
+    with pytest.raises(ValueError, match="vertex_count"):
+        decode_frame(body)