diff --git a/.jules/bolt.md b/.jules/bolt.md
index d54cf10fc..a13b4b1a1 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.
+
+## 2026-08-23 - Vectorize Python loop over audio frames in probability matrix construction
+**Learning:** Computing observation probabilities per frame using a `for i in range(n_frames)` loop in pure Python generates significant overhead when handling audio segments (e.g. 50k+ frames), creating a slow bottleneck before Viterbi decoding.
+**Action:** Replace sequential per-frame condition checks with fully vectorized NumPy operations (`np.pad`, `|` boolean mask, and array slicing assignments) to compute logic globally over the time axis, changing slow Python loops into fast C-level operations.
diff --git a/services/analysis-engine/src/bandscope_analysis/chords/chord_recognizer.py b/services/analysis-engine/src/bandscope_analysis/chords/chord_recognizer.py
index 8f6466924..124760416 100644
--- a/services/analysis-engine/src/bandscope_analysis/chords/chord_recognizer.py
+++ b/services/analysis-engine/src/bandscope_analysis/chords/chord_recognizer.py
@@ -294,6 +294,13 @@ def _build_observation_probs(
             Observation probabilities of shape (25, n_frames).
         """
         n_frames = chromagram.shape[1]
+
+        if similarity.shape != (24, n_frames):
+            raise ValueError(
+                f"similarity shape {similarity.shape} does not match "
+                f"expected shape (24, {n_frames})"
+            )
+
         obs_probs = np.zeros((_NUM_CHORD_STATES, n_frames))
 
         # Chord observation likelihoods from template similarity
@@ -306,17 +313,20 @@ def _build_observation_probs(
 
         # N (no-chord) observation probability based on noise indicators
         chroma_vars = np.var(chromagram, axis=0)
-        for i in range(n_frames):
-            rms_val = rms[i] if i < len(rms) else 0.0
-            chroma_var = chroma_vars[i]
-            max_sim = similarity[:, i].max() if similarity.shape[1] > i else 0.0
-
-            # High N probability when signal is low/flat
-            if max_sim < 0.3 or rms_val < 0.01 or chroma_var < 0.02:
-                obs_probs[:24, i] *= 0.1
-                obs_probs[_NO_CHORD_STATE, i] = 0.9
-            else:
-                obs_probs[_NO_CHORD_STATE, i] = 0.05
+
+        rms_vals = np.pad(rms, (0, max(0, n_frames - len(rms))))[:n_frames]
+        max_sims = (
+            similarity[:, :n_frames].max(axis=0)
+            if similarity.shape[1] >= n_frames
+            else np.pad(similarity[:, :].max(axis=0), (0, max(0, n_frames - similarity.shape[1])))
+        )
+
+        condition = (max_sims < 0.3) | (rms_vals < 0.01) | (chroma_vars[:n_frames] < 0.02)
+
+        # High N probability when signal is low/flat
+        obs_probs[:24, condition] *= 0.1
+        obs_probs[_NO_CHORD_STATE, condition] = 0.9
+        obs_probs[_NO_CHORD_STATE, ~condition] = 0.05
 
         # Normalize columns
         col_sums = obs_probs.sum(axis=0, keepdims=True) + 1e-12
diff --git a/services/analysis-engine/tests/test_chord_observation_contract.py b/services/analysis-engine/tests/test_chord_observation_contract.py
new file mode 100644
index 000000000..748d63a5c
--- /dev/null
+++ b/services/analysis-engine/tests/test_chord_observation_contract.py
@@ -0,0 +1,28 @@
+"""Test observation probability contracts for the chord recognizer."""
+
+import numpy as np
+import pytest
+
+from bandscope_analysis.chords.chord_recognizer import ChordRecognizer
+
+
+def test_observation_probs_reject_similarity_frame_mismatch() -> None:
+    """Reject similarity arrays that would otherwise broadcast across frames."""
+    recognizer = ChordRecognizer()
+    chromagram = np.ones((12, 5), dtype=np.float64)
+    similarity = np.ones((24, 1), dtype=np.float64)
+    rms = np.ones(5, dtype=np.float64)
+
+    with pytest.raises(ValueError, match="similarity shape"):
+        recognizer._build_observation_probs(chromagram, similarity, rms)
+
+
+def test_observation_probs_reject_similarity_chord_state_mismatch() -> None:
+    """Reject similarity arrays that do not contain all 24 chord templates."""
+    recognizer = ChordRecognizer()
+    chromagram = np.ones((12, 5), dtype=np.float64)
+    similarity = np.ones((23, 5), dtype=np.float64)
+    rms = np.ones(5, dtype=np.float64)
+
+    with pytest.raises(ValueError, match="similarity shape"):
+        recognizer._build_observation_probs(chromagram, similarity, rms)