Add HTTP status context to XML parsing errors using PEP 678 notes

When XML parsing fails due to empty or malformed responses, users now
receive the actual HTTP error response from the service alongside the
parsing error, providing useful information about what actually happened.

Before:
  ResponseParserError: Unable to parse response (no element found:
  line 1, column 0), invalid XML received. Further retries may succeed: b''

After:
  ResponseParserError: Unable to parse response (no element found:
  line 1, column 0), invalid XML received. Further retries may succeed: b''
  HTTP 413: Content Too Large

This exposes the real service error (HTTP 413: Content Too Large) that
was previously hidden behind cryptic XML parsing failures, giving users
actionable information about why their request failed.

Author: adev-code

botocore/parsers.py

@@ -592,7 +592,15 @@ def _handle_blob(self, shape, text):
 class QueryParser(BaseXMLResponseParser):
     def _do_error_parse(self, response, shape):
         xml_contents = response['body']
-        root = self._parse_xml_string_to_dom(xml_contents)
+        try:
+            root = self._parse_xml_string_to_dom(xml_contents)
+        except ResponseParserError as e:
+            status_message = http.client.responses.get(
+                response['status_code'], ''
+            )
+            if status_message:
+                e.add_note(f"HTTP {response['status_code']}: {status_message}")
+            raise
         parsed = self._build_name_to_xml_node(root)
         self._replace_nodes(parsed)
         # Once we've converted xml->dict, we need to make one or two
@@ -1447,7 +1455,15 @@ def _parse_error_from_http_status(self, response):
 
     def _parse_error_from_body(self, response):
         xml_contents = response['body']
-        root = self._parse_xml_string_to_dom(xml_contents)
+        try:
+            root = self._parse_xml_string_to_dom(xml_contents)
+        except ResponseParserError as e:
+            status_message = http.client.responses.get(
+                response['status_code'], ''
+            )
+            if status_message:
+                e.add_note(f"HTTP {response['status_code']}: {status_message}")
+            raise
         parsed = self._build_name_to_xml_node(root)
         self._replace_nodes(parsed)
         if root.tag == 'Error':

tests/unit/test_parsers.py

@@ -1621,6 +1621,48 @@ def test_can_parse_route53_with_missing_message(self):
         # still populate an empty string.
         self.assertEqual(error['Message'], '')
 
+    def test_query_parser_empty_body_4xx_error_with_notes(self):
+        parser = parsers.QueryParser()
+        response = {
+            'body': b'',
+            'headers': {
+                'Content-Length': '0',
+                'Date': 'Fri, 21 Nov 2025 18:18:28 GMT',
+                'Connection': 'close',
+            },
+            'status_code': 413,
+        }
+
+        with self.assertRaises(parsers.ResponseParserError) as cm:
+            parser._do_error_parse(response, None)
+
+        exception = cm.exception
+
+        self.assertTrue(hasattr(exception, '__notes__'))
+        self.assertEqual(len(exception.__notes__), 1)
+        self.assertEqual(exception.__notes__[0], "HTTP 413: Content Too Large")
+
+    def test_parse_error_from_body_empty_body_4xx_error_with_notes(self):
+        parser = parsers.RestXMLParser()
+        response = {
+            'body': b'',
+            'headers': {
+                'Content-Length': '0',
+                'Date': 'Fri, 21 Nov 2025 18:18:28 GMT',
+                'Connection': 'close',
+            },
+            'status_code': 413,
+        }
+
+        with self.assertRaises(parsers.ResponseParserError) as cm:
+            parser._parse_error_from_body(response)
+
+        exception = cm.exception
+
+        self.assertTrue(hasattr(exception, '__notes__'))
+        self.assertEqual(len(exception.__notes__), 1)
+        self.assertEqual(exception.__notes__[0], "HTTP 413: Content Too Large")
+
 
 def _generic_test_bodies():
     generic_html_body = (