Reject aux notes that can't be written in wrann

docs/changes.rst

@@ -41,6 +41,9 @@ Unreleased
 **Fix resample_sig for signals starting with NaN**
   ``resample_sig`` (and the functions that use it, such as ``resample_singlechan`` and ``resample_multichan``) returned only NaN values if the signal started with NaN. Leading NaN values now take the first valid value before resampling, as trailing ones already took the last.
 
+**Reject aux notes that can't be written in wrann**
+  ``wrann`` now raises a ``ValueError`` if an ``aux_note`` is longer than 255 characters or contains characters outside Latin-1, instead of writing a corrupt annotation file. A longer note used to be read back as many extra annotations, and other characters were silently replaced.
+
 Version 4.3.1 (Feb 2026)
 -----------------------------
 

src/wfdb/io/annotation.py

@@ -544,6 +544,22 @@ def check_field(self, field):
                             + " field must not contain tabs or newlines"
                         )
 
+            if field == "aux_note":
+                # Each note is stored as Latin-1 bytes after a one-byte length.
+                for e in uniq_elements:
+                    try:
+                        n_bytes = len(e.encode("latin-1"))
+                    except UnicodeEncodeError:
+                        raise ValueError(
+                            f"aux_note {e!r} contains characters that can't be "
+                            "written (only Latin-1 characters are supported)"
+                        ) from None
+                    if n_bytes > 255:
+                        raise ValueError(
+                            f"aux_note values must be at most 255 characters, "
+                            f"but one has {n_bytes}"
+                        )
+
         elif field == "sample":
             if len(self.sample) == 1:
                 sampdiffs = np.array([self.sample[0]])
@@ -1670,9 +1686,10 @@ def field2bytes(field, value, typecodes):
         # - First byte stores length of aux_note field
         # - Second byte stores 63*4 indicator
         # - Then store the aux_note string characters
-        data_bytes = [len(value), 252] + [ord(i) for i in value]
+        encoded = value.encode("latin-1")
+        data_bytes = [len(encoded), 252] + list(encoded)
         # Zero pad odd length aux_note strings
-        if len(value) % 2:
+        if len(encoded) % 2:
             data_bytes.append(0)
 
     return data_bytes
@@ -1727,7 +1744,8 @@ def wrann(
         annotation.
     aux_note : list, optional
         A list containing the auxiliary information string (or None for
-        annotations without notes) for each annotation.
+        annotations without notes) for each annotation. Each string can have
+        at most 255 characters, all in the Latin-1 character set.
     label_store : ndarray, optional
         A numpy array containing the integer values used to store the
         annotation labels. If this field is present, `symbol` must not be

tests/test_annotation.py

@@ -3,6 +3,7 @@
 
 import numpy as np
 import pandas as pd
+import pytest
 
 import wfdb
 
@@ -287,3 +288,37 @@ def test_5(self, temp_path):
         )
         ann = wfdb.rdann(os.path.join(temp_path, "CustomLabel"), "atr")
         assert ann.symbol == ["z", "l", "v", "r"]
+
+
+def test_aux_note_round_trip(temp_path):
+    """Latin-1 aux notes up to 255 characters are read back unchanged."""
+    aux_note = ["x" * 255, "37°C, Müller", "(AFIB"]
+    wfdb.wrann(
+        "aux",
+        "atr",
+        np.array([1, 2, 3]),
+        symbol=["+", "N", "+"],
+        aux_note=aux_note,
+        write_dir=temp_path,
+    )
+    ann = wfdb.rdann(os.path.join(temp_path, "aux"), "atr")
+    assert ann.sample.tolist() == [1, 2, 3]
+    assert ann.aux_note == aux_note
+
+
+@pytest.mark.parametrize(
+    "aux_note, match",
+    [("x" * 256, "at most 255 characters"), ("(€)", "Latin-1")],
+)
+def test_aux_note_rejected(temp_path, aux_note, match):
+    """Aux notes that can't be stored raise an error instead of being corrupted."""
+    with pytest.raises(ValueError, match=match):
+        wfdb.wrann(
+            "bad_aux",
+            "atr",
+            np.array([1, 2]),
+            symbol=["+", "N"],
+            aux_note=[aux_note, ""],
+            write_dir=temp_path,
+        )
+    assert not os.path.exists(os.path.join(temp_path, "bad_aux.atr"))