diff --git a/sentry_sdk/ai/utils.py b/sentry_sdk/ai/utils.py
index 3cfae61546..bcd76f884f 100644
--- a/sentry_sdk/ai/utils.py
+++ b/sentry_sdk/ai/utils.py
@@ -682,18 +682,19 @@ def truncate_messages_by_size(
     In the single message case, the serialized message size may exceed `max_bytes`, because
     truncation is based only on character count in that case.
     """
-    serialized_json = json.dumps(messages, separators=(",", ":"))
+    normalized_messages = _normalize_data(messages, unpack=False)
+    serialized_json = json.dumps(normalized_messages, separators=(",", ":"))
     current_size = len(serialized_json.encode("utf-8"))
 
     if current_size <= max_bytes:
-        return messages, 0
+        return normalized_messages, 0
 
-    truncation_index = _find_truncation_index(messages, max_bytes)
-    if truncation_index < len(messages):
-        truncated_messages = messages[truncation_index:]
+    truncation_index = _find_truncation_index(normalized_messages, max_bytes)
+    if truncation_index < len(normalized_messages):
+        truncated_messages = normalized_messages[truncation_index:]
     else:
-        truncation_index = len(messages) - 1
-        truncated_messages = messages[-1:]
+        truncation_index = len(normalized_messages) - 1
+        truncated_messages = normalized_messages[-1:]
 
     if len(truncated_messages) == 1:
         truncated_messages[0] = _truncate_single_message_content_if_present(