diff --git a/Include/internal/pycore_codecs.h b/Include/internal/pycore_codecs.h
@@ -45,7 +45,7 @@ extern int _PyCodec_UnregisterError(const char *name);
    in Python 3.5+?
 
  */
-extern PyObject* _PyCodec_LookupTextEncoding(
+PyAPI_FUNC(PyObject*) _PyCodec_LookupTextEncoding(
    const char *encoding,
    const char *alternate_command);
 

diff --git a/Lib/codecs.py b/Lib/codecs.py
@@ -93,7 +93,8 @@ class CodecInfo(tuple):
 
     def __new__(cls, encode, decode, streamreader=None, streamwriter=None,
         incrementalencoder=None, incrementaldecoder=None, name=None,
-        *, _is_text_encoding=None):
+        *, _is_text_encoding=None,
+        _is_multibyte=None):
         self = tuple.__new__(cls, (encode, decode, streamreader, streamwriter))
         self.name = name
         self.encode = encode
@@ -104,6 +105,8 @@ def __new__(cls, encode, decode, streamreader=None, streamwriter=None,
         self.streamreader = streamreader
         if _is_text_encoding is not None:
             self._is_text_encoding = _is_text_encoding
+        if _is_multibyte is not None:
+            self._is_multibyte = _is_multibyte
         return self
 
     def __repr__(self):

diff --git a/Lib/encodings/big5.py b/Lib/encodings/big5.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/big5hkscs.py b/Lib/encodings/big5hkscs.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/cp932.py b/Lib/encodings/cp932.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/cp949.py b/Lib/encodings/cp949.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/cp950.py b/Lib/encodings/cp950.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/euc_jis_2004.py b/Lib/encodings/euc_jis_2004.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/euc_jisx0213.py b/Lib/encodings/euc_jisx0213.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/euc_jp.py b/Lib/encodings/euc_jp.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/euc_kr.py b/Lib/encodings/euc_kr.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/gb18030.py b/Lib/encodings/gb18030.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/gb2312.py b/Lib/encodings/gb2312.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/gbk.py b/Lib/encodings/gbk.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/hz.py b/Lib/encodings/hz.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/idna.py b/Lib/encodings/idna.py
@@ -385,4 +385,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamwriter=StreamWriter,
         streamreader=StreamReader,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp.py b/Lib/encodings/iso2022_jp.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp_1.py b/Lib/encodings/iso2022_jp_1.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp_2.py b/Lib/encodings/iso2022_jp_2.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp_2004.py b/Lib/encodings/iso2022_jp_2004.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp_3.py b/Lib/encodings/iso2022_jp_3.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_jp_ext.py b/Lib/encodings/iso2022_jp_ext.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/iso2022_kr.py b/Lib/encodings/iso2022_kr.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/johab.py b/Lib/encodings/johab.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/punycode.py b/Lib/encodings/punycode.py
@@ -250,4 +250,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamwriter=StreamWriter,
         streamreader=StreamReader,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/raw_unicode_escape.py b/Lib/encodings/raw_unicode_escape.py
@@ -43,4 +43,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamwriter=StreamWriter,
         streamreader=StreamReader,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/shift_jis.py b/Lib/encodings/shift_jis.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/shift_jis_2004.py b/Lib/encodings/shift_jis_2004.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/shift_jisx0213.py b/Lib/encodings/shift_jisx0213.py
@@ -36,4 +36,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/unicode_escape.py b/Lib/encodings/unicode_escape.py
@@ -43,4 +43,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamwriter=StreamWriter,
         streamreader=StreamReader,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_16.py b/Lib/encodings/utf_16.py
@@ -152,4 +152,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_16_be.py b/Lib/encodings/utf_16_be.py
@@ -39,4 +39,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_16_le.py b/Lib/encodings/utf_16_le.py
@@ -39,4 +39,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_32.py b/Lib/encodings/utf_32.py
@@ -147,4 +147,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_32_be.py b/Lib/encodings/utf_32_be.py
@@ -34,4 +34,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_32_le.py b/Lib/encodings/utf_32_le.py
@@ -34,4 +34,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_7.py b/Lib/encodings/utf_7.py
@@ -35,4 +35,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_8.py b/Lib/encodings/utf_8.py
@@ -39,4 +39,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/encodings/utf_8_sig.py b/Lib/encodings/utf_8_sig.py
@@ -127,4 +127,5 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
@@ -1892,6 +1892,7 @@ def test_copy(self):
         self.assertIsNot(dup, orig)
         self.assertEqual(dup, orig)
         self.assertTrue(orig._is_text_encoding)
+        self.assertTrue(orig._is_multibyte)
         self.assertEqual(dup.encode, orig.encode)
         self.assertEqual(dup.name, orig.name)
         self.assertEqual(dup.incrementalencoder, orig.incrementalencoder)
@@ -1912,6 +1913,7 @@ def test_deepcopy(self):
         self.assertIsNot(dup, orig)
         self.assertEqual(dup, orig)
         self.assertTrue(orig._is_text_encoding)
+        self.assertTrue(orig._is_multibyte)
         self.assertEqual(dup.encode, orig.encode)
         self.assertEqual(dup.name, orig.name)
         self.assertEqual(dup.incrementalencoder, orig.incrementalencoder)
@@ -1940,6 +1942,7 @@ def test_pickle(self):
                      unpickled_codec_info.incrementalencoder
                 )
                 self.assertTrue(unpickled_codec_info._is_text_encoding)
+                self.assertTrue(unpickled_codec_info._is_multibyte)
 
         # Test a CodecInfo with _is_text_encoding equal to false.
         codec_info = codecs.lookup('base64')

diff --git a/Lib/test/test_pyexpat.py b/Lib/test/test_pyexpat.py
@@ -325,11 +325,15 @@ def test_supported_encodings2(self, encoding):
         ])
 
     @support.subTests('encoding', [
-        'UTF-7',
+        'UTF8', 'UTF-7',
+        "unicode-escape", "raw-unicode-escape",
         "Big5-HKSCS", "Big5",
         "cp932", "cp949", "cp950",
         "EUC_JIS-2004", "EUC_JISX0213", "EUC-JP", "EUC-KR",
         "GB18030", "GB2312", "GBK",
+        "HZ-GB-2312",
+        "ISO-2022-JP", "ISO-2022-JP-1", "ISO-2022-JP-2004",
+        "ISO-2022-JP-2", "ISO-2022-JP-3", "ISO-2022-JP-EXT",
         "ISO-2022-KR",
         "johab",
         "Shift_JIS", "Shift_JIS-2004", "Shift_JISX0213",

diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
@@ -1039,9 +1039,11 @@ def bxml(encoding, body=''):
             'cp932', 'cp949', 'cp950',
             'euc-jp', 'euc-jis-2004', 'euc-jisx0213', 'euc-kr',
             'gb2312', 'gbk', 'gb18030',
-            'iso2022-kr', 'johab',
+            'iso2022-jp', 'iso2022-jp-1', 'iso2022-jp-2', 'iso2022-jp-2004',
+            'iso2022-jp-3', 'iso2022-jp-ext',
+            'iso2022-kr', 'johab', 'hz',
             'shift-jis', 'shift-jis-2004', 'shift-jisx0213',
-            'utf-7',
+            'utf-7', 'utf-8-sig', 'utf8',
         ]
         for encoding in unsupported_ascii_compatible_encodings:
             with self.subTest(encoding=encoding):

diff --git a/Misc/NEWS.d/next/Library/2026-04-23-14-46-30.gh-issue-148821.cR4kMa.rst b/Misc/NEWS.d/next/Library/2026-04-23-14-46-30.gh-issue-148821.cR4kMa.rst
@@ -0,0 +1,4 @@
+The :mod:`XML parser <xml.parsers.expat>` now raises :exc:`ValueError` for known
+unsupported multi-byte encodings such us "UTF8", "ISO-2022-JP" or
+"raw-unicode-escape" instead of failing later, when encounter non-ASCII
+data.
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
@@ -4,6 +4,7 @@
 
 #include "Python.h"
 #include "pycore_ceval.h"         // _Py_EnterRecursiveCall()
+#include "pycore_codecs.h"        // _PyCodec_LookupTextEncoding()
 #include "pycore_import.h"        // _PyImport_SetModule()
 #include "pycore_pyhash.h"        // _Py_HashSecret
 #include "pycore_traceback.h"     // _PyTraceback_Add()
@@ -1465,6 +1466,32 @@ PyUnknownEncodingHandler(void *encodingHandlerData,
     if (PyErr_Occurred())
         return XML_STATUS_ERROR;
 
+    PyObject *codec = _PyCodec_LookupTextEncoding(name, NULL);
+    if (codec == NULL) {
+        return XML_STATUS_ERROR;
+    }
+    if (!PyTuple_CheckExact(codec)) {
+        PyObject *attr;
+        if (PyObject_GetOptionalAttrString(codec, "_is_multibyte", &attr) < 0) {
+            Py_DECREF(codec);
+            return XML_STATUS_ERROR;
+        }
+        if (attr != NULL) {
+            int is_multibyte = PyObject_IsTrue(attr);
+            Py_DECREF(attr);
+            if (is_multibyte != 0) {  // true or error
+                Py_DECREF(codec);
+                if (is_multibyte > 0) {  // true
+                    PyErr_Format(PyExc_ValueError,
+                                 "multi-byte encodings are not supported: '%s'",
+                                 name);
+                }
+                return XML_STATUS_ERROR;
+            }
+        }
+    }
+    Py_DECREF(codec);
+
     u = PyUnicode_Decode((const char*) template_buffer, 256, name, "replace");
     if (u == NULL) {
         Py_XDECREF(u);
@@ -1473,8 +1500,9 @@ PyUnknownEncodingHandler(void *encodingHandlerData,
 
     if (PyUnicode_GET_LENGTH(u) != 256) {
         Py_DECREF(u);
-        PyErr_SetString(PyExc_ValueError,
-                        "multi-byte encodings are not supported");
+        PyErr_Format(PyExc_ValueError,
+                     "multi-byte encodings are not supported: '%s'",
+                     name);
         return XML_STATUS_ERROR;
     }
 

diff --git a/Python/codecs.c b/Python/codecs.c
@@ -10,6 +10,7 @@ Copyright (c) Corporation for National Research Initiatives.
 
 #include "Python.h"
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
+#include "pycore_codecs.h"        // export _PyCodec_LookupTextEncoding()
 #include "pycore_interp.h"        // PyInterpreterState.codec_search_path
 #include "pycore_pyerrors.h"      // _PyErr_FormatNote()
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()

@@ -51,6 +51,7 @@ def getregentry():
         incrementaldecoder=IncrementalDecoder,
         streamreader=StreamReader,
         streamwriter=StreamWriter,
+        _is_multibyte=True,
     )
 """)