diff --git a/src/k1link/compute/lidar_replay.py b/src/k1link/compute/lidar_replay.py
index 394c0a7..f78299c 100644
--- a/src/k1link/compute/lidar_replay.py
+++ b/src/k1link/compute/lidar_replay.py
@@ -398,12 +398,16 @@ def build_lidar_replay_pack_v2(
             pose_coverage_threshold_ms=pose_coverage_threshold_ms,
         )
         _write_json(staging / LIDAR_QUALITY_REPORT_NAME, quality)
-        equivalence = _equivalence_report(
-            pack_id,
-            logical_sha256,
-            source,
-            arrays,
-        )
+        persisted_arrays = _MaterializedNpz(arrays_path)
+        try:
+            equivalence = _equivalence_report_from_arrays(
+                pack_id,
+                logical_sha256,
+                arrays,
+                persisted_arrays,
+            )
+        finally:
+            persisted_arrays.close()
         _write_json(staging / LIDAR_EQUIVALENCE_REPORT_NAME, equivalence)
         artifacts = [
             _artifact_descriptor("lidar-arrays", arrays_path, "application/x-npz"),
@@ -816,7 +820,22 @@ def _equivalence_report(
     capture_path: Path,
     replay_arrays: Any,
 ) -> dict[str, object]:
-    source_arrays = _capture_arrays(capture_path)
+    return _equivalence_report_from_arrays(
+        pack_id,
+        logical_sha256,
+        _capture_arrays(capture_path),
+        replay_arrays,
+    )
+
+
+def _equivalence_report_from_arrays(
+    pack_id: str,
+    logical_sha256: str,
+    source_arrays: Any,
+    replay_arrays: Any,
+) -> dict[str, object]:
+    """Compare one source-derived array set with its persisted replay image."""
+
     comparisons: dict[str, dict[str, object]] = {}
     mismatch_count = 0
     for name in sorted(_ARRAY_DTYPES):
diff --git a/tests/test_lidar_replay.py b/tests/test_lidar_replay.py
index 845f64b..dfe7ac0 100644
--- a/tests/test_lidar_replay.py
+++ b/tests/test_lidar_replay.py
@@ -10,6 +10,7 @@ import pytest
 from fastapi import APIRouter
 from fastapi.routing import APIRoute
 
+import k1link.compute.lidar_replay as lidar_replay_module
 from k1link.compute import (
     K1_LIDAR_PACK_V2_PROFILE,
     K1LocalSurfaceV1,
@@ -253,6 +254,35 @@ def test_lidar_replay_v2_retains_fields_and_passes_equivalence(tmp_path: Path) -
     assert build_lidar_replay_pack_v2(capture, tmp_path / "packs") == output
 
 
+def test_lidar_replay_build_decodes_source_once(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    capture = _capture(tmp_path)
+    original_capture_arrays = lidar_replay_module._capture_arrays
+    calls: list[Path] = []
+
+    def counting_capture_arrays(path: Path) -> dict[str, Any]:
+        calls.append(path)
+        return original_capture_arrays(path)
+
+    monkeypatch.setattr(
+        lidar_replay_module,
+        "_capture_arrays",
+        counting_capture_arrays,
+    )
+
+    output = build_lidar_replay_pack_v2(capture, tmp_path / "packs")
+    pack = LidarReplayPackV2(output)
+    try:
+        assert pack.equivalence["status"] == "passed"
+        assert pack.equivalence["array_mismatches"] == 0
+    finally:
+        pack.close()
+
+    assert calls == [capture.resolve(strict=True)]
+
+
 def test_lidar_replay_v2_unblocks_detector_input_but_not_mapping() -> None:
     assessments = {
         item.stage: item for item in assess_lidar_profile(K1_LIDAR_PACK_V2_PROFILE)