Fix chord spelling for non-C candidate roots

README.md

@@ -48,9 +48,11 @@ Built on [MERT-v2-FullSong](https://huggingface.co/m-a-p/MERT-v2-FullSong), with
 
 **Update log:**
 
+**2026-09-24:** Fixed a bug in key-aware chord spelling. Candidate chord tones were spelled incorrectly, which favored sharp-side names for chromatic chords (e.g., `B#` instead of `C` in E major, `A#` instead of `Bb` in G major). Only chord label names change; pitches, timing, key labels, and model weights are unchanged.
+
 **2026-09-21:** Fixed missing enharmonic spelling correction for key and chord labels. Chord spelling now follows the local key. This updates key/chord labels and ABC notation without changing musical pitches or timing.
 
-If you downloaded the repository or model before this update (2026-09-21), please update to the latest repository code. **Model weights are unchanged.**
+If you downloaded the repository or model before this update (2026-09-24), please update to the latest repository code. **Model weights are unchanged.**
 
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chord_spelling_sheetsage2.py

@@ -102,6 +102,25 @@ def get_chord_spelling_table():
     CHORD_SPELLING_TABLE = chord_spelling_table
     return chord_spelling_table
 
+# Circle-of-fifths offsets of major-scale degrees 1-7 from the tonic.
+MAJOR_SCALE_FIFTHS = np.array([0, 2, 4, -1, 1, 3, 5])
+
+def spell_chord_tones(root_spelling, quality):
+    '''
+    Spell every chord tone from a root spelling.
+    Template intervals are relative to the root's own major scale, so adding
+    them on the circle of fifths keeps each tone's pitch class; e.g., E:maj
+    gives E, G#, B, and Db:hdim7 gives Db, Fb, Abb, Cb.
+    :param root_spelling: Numpy array of shape (2,) with note letter id and accidental offset.
+    :param quality: Chord quality in QUALITIES.
+    :return: Numpy array of shape (N, 2); the first row is the root.
+    '''
+    intervals = get_chord_spelling_table()[quality]
+    letters = (root_spelling[0] + intervals[:, 0]) % 7
+    root_pos = CIRCLE_OF_FIFTH_INV[root_spelling[0]] + root_spelling[1] * 7
+    tone_pos = root_pos + MAJOR_SCALE_FIFTHS[intervals[:, 0]] + intervals[:, 1] * 7
+    return np.stack([letters, (tone_pos - CIRCLE_OF_FIFTH_INV[letters]) // 7], axis=1)
+
 def score_spelling_under_key(spelling, key):
     '''
     Score the chord spelling under a given key.
@@ -144,7 +163,7 @@ def correct_chord_spelling(label, key_name):
             possible_root_id,
             (root_semitone - mir_eval.chord.pitch_class_to_semitone(possible_root) + 6) % 12 - 6
         ])
-        chord_spelling = get_chord_spelling_table()[quality] + root_spelling[None, :]
+        chord_spelling = spell_chord_tones(root_spelling, quality)
         scores.append(score_spelling_under_key(chord_spelling, key_spelling))
     best_root_id = np.argmin(scores)
     best_root = NOTE_NAMES[best_root_id]

tests/test_spelling.py

@@ -7,7 +7,8 @@ import unittest
 import mir_eval.chord
 import numpy as np
 
-from ..chord_spelling_sheetsage2 import correct_chord_rows, correct_chord_spelling, normalize_key_name
+from ..chord_spelling_sheetsage2 import (QUALITIES, correct_chord_rows, correct_chord_spelling,
+                                        normalize_key_name, spell_chord_tones)
 from ..tokenization_sheetsage2 import SheetSage2Tokenizer as Tokenizer, FULL_CHORD_VOCABULARY
 from ..exports_sheetsage2 import export_result
 from ..notation_sheetsage2 import get_key_accidentals, note_to_abc
@@ -36,6 +37,24 @@ class SpellingTests(unittest.TestCase):
                               ('C:min', 'C:min'), ('N', 'N'), ('X', 'X')]:
             self.assertEqual(correct_chord_spelling(raw, 'C:minor'), expected)
 
+    def test_chord_tones_keep_pitch_classes(self):
+        naturals = [0, 2, 4, 5, 7, 9, 11]
+        for quality, chroma in QUALITIES.items():
+            offsets = [i for i, value in enumerate(chroma) if value > 0]
+            for root_pc in range(12):
+                for letter in range(7):
+                    acc = (root_pc - naturals[letter] + 6) % 12 - 6
+                    tones = spell_chord_tones(np.array([letter, acc]), quality)
+                    self.assertEqual([(naturals[l] + a) % 12 for l, a in tones],
+                                     [(root_pc + o) % 12 for o in offsets])
+
+    def test_chromatic_chords_use_key_side_spelling(self):
+        for raw, key, expected in [('C:maj', 'E:major', 'C:maj'), ('A#:maj', 'G:major', 'Bb:maj'),
+                                   ('D#:maj', 'G:major', 'Eb:maj'), ('G#:7', 'C:major', 'Ab:7'),
+                                   ('A#:min', 'C:major', 'Bb:min'), ('C#:hdim7', 'G:major', 'C#:hdim7'),
+                                   ('F:hdim7', 'G#:minor', 'E#:hdim7')]:
+            self.assertEqual(correct_chord_spelling(raw, key), expected)
+
     def test_local_midpoint_including_boundary_and_missing_keys(self):
         rows = [[0, 2, 'G#:maj'], [2, 6, 'G#:maj'], [6, 8, 'G#:maj']]
         keys = [[0, 4, 'C:minor'], [4, 8, 'E:major']]