diff --git a/.jules/bolt.md b/.jules/bolt.md
index d54cf10fc..b184c8933 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -61,3 +61,7 @@
 ## 2026-07-13 - Array.from mapping optimization
 **Learning:** Using `Array.from({ length: N }).map(...)` creates an intermediate array of `undefined` values which requires memory allocation and garbage collection, adding O(N) unnecessary overhead in frequently re-rendered UI components.
 **Action:** Use `Array.from({ length: N }, (_, index) => ...)` to map elements directly during array creation, avoiding intermediate allocations.
+
+## 2026-07-20 - Eliminate intermediate list allocations in Python sum(zip(A, A[1:]))
+**Learning:** Using `sum(1 for a, b in zip(list, list[1:]))` creates an intermediate list slice (`list[1:]`) and zip object, allocating O(N) memory and causing garbage collection overhead.
+**Action:** Calculate the metric (e.g. state changes) inside the primary loop by caching the previous state in a local variable (e.g., `last_chord`) to achieve O(1) memory and avoid intermediate list slice allocations.
diff --git a/services/analysis-engine/src/bandscope_analysis/chords/section_harmony.py b/services/analysis-engine/src/bandscope_analysis/chords/section_harmony.py
index a61a7d001..b3447408f 100644
--- a/services/analysis-engine/src/bandscope_analysis/chords/section_harmony.py
+++ b/services/analysis-engine/src/bandscope_analysis/chords/section_harmony.py
@@ -84,14 +84,17 @@ def _summarize_one_section(
         the portion of their duration that overlaps the window.
     """
     durations: dict[str, float] = {}
-    overlapping_chords: list[str] = []
+    chord_changes = 0
+    last_chord: str | None = None
 
     for seg_start, seg_end, chord in segments:
         overlap = min(seg_end, section_end) - max(seg_start, section_start)
         if overlap <= 0.0:
             continue
         durations[chord] = durations.get(chord, 0.0) + overlap
-        overlapping_chords.append(chord)
+        if last_chord is not None and last_chord != chord:
+            chord_changes += 1
+        last_chord = chord
 
     chords: list[ChordDuration] = [
         {"chord": chord, "duration": duration}
@@ -104,12 +107,6 @@ def _summarize_one_section(
             main_chord = entry["chord"]
             break
 
-    chord_changes = sum(
-        1
-        for previous, current in zip(overlapping_chords, overlapping_chords[1:], strict=False)
-        if previous != current
-    )
-
     return {
         "start_time": section_start,
         "end_time": section_end,