python
@@ -479,6 +479,12 @@ Other language changes
   :func:`textwrap.dedent`.
   (Contributed by Jon Crall and Steven Sun in :gh:`103998`.)
 
+* Improve error message when an object supporting the synchronous (resp.
+  asynchronous) context manager protocol is entered using :keyword:`async
+  with` (resp. :keyword:`with`) instead of :keyword:`with` (resp.
+  :keyword:`async with`).
+  (Contributed by Bénédikt Tran in :gh:`128398`.)
+
 
 .. _whatsnew314-pep765:
 

diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
@@ -279,6 +279,7 @@ PyAPI_DATA(const conversion_func) _PyEval_ConversionFuncs[];
 typedef struct _special_method {
     PyObject *name;
     const char *error;
+    const char *error_suggestion;  // improved optional suggestion
 } _Py_SpecialMethod;
 
 PyAPI_DATA(const _Py_SpecialMethod) _Py_SpecialMethods[];
@@ -309,6 +310,16 @@ PyAPI_FUNC(PyObject *) _PyEval_LoadName(PyThreadState *tstate, _PyInterpreterFra
 PyAPI_FUNC(int)
 _Py_Check_ArgsIterable(PyThreadState *tstate, PyObject *func, PyObject *args);
 
+/*
+ * Indicate whether a special method of given 'oparg' can use the (improved)
+ * alternative error message instead. Only methods loaded by LOAD_SPECIAL
+ * support alternative error messages.
+ *
+ * Symbol is exported for the JIT (see discussion on GH-132218).
+ */
+PyAPI_FUNC(int)
+_PyEval_SpecialMethodCanSuggest(PyObject *self, int oparg);
+
 /* Bits that can be set in PyThreadState.eval_breaker */
 #define _PY_GIL_DROP_REQUEST_BIT (1U << 0)
 #define _PY_SIGNALS_PENDING_BIT (1U << 1)

diff --git a/Lib/test/test_with.py b/Lib/test/test_with.py
@@ -1,16 +1,27 @@
-"""Unit tests for the with statement specified in PEP 343."""
+"""Unit tests for the 'with/async with' statements specified in PEP 343/492."""
 
 
 __author__ = "Mike Bland"
 __email__ = "mbland at acm dot org"
 
+import re
 import sys
 import traceback
 import unittest
 from collections import deque
 from contextlib import _GeneratorContextManager, contextmanager, nullcontext
 
 
+def do_with(obj):
+    with obj:
+        pass
+
+
+async def do_async_with(obj):
+    async with obj:
+        pass
+
+
 class MockContextManager(_GeneratorContextManager):
     def __init__(self, *args):
         super().__init__(*args)
@@ -110,34 +121,77 @@ def fooNotDeclared():
             with foo: pass
         self.assertRaises(NameError, fooNotDeclared)
 
-    def testEnterAttributeError1(self):
-        class LacksEnter(object):
-            def __exit__(self, type, value, traceback):
-                pass
-
-        def fooLacksEnter():
-            foo = LacksEnter()
-            with foo: pass
-        self.assertRaisesRegex(TypeError, 'the context manager', fooLacksEnter)
-
-    def testEnterAttributeError2(self):
-        class LacksEnterAndExit(object):
-            pass
+    def testEnterAttributeError(self):
+        class LacksEnter:
+            def __exit__(self, type, value, traceback): ...
 
-        def fooLacksEnterAndExit():
-            foo = LacksEnterAndExit()
-            with foo: pass
-        self.assertRaisesRegex(TypeError, 'the context manager', fooLacksEnterAndExit)
+        with self.assertRaisesRegex(TypeError, re.escape((
+            "object does not support the context manager protocol "
+            "(missed __enter__ method)"
+        ))):
+            do_with(LacksEnter())
 
     def testExitAttributeError(self):
-        class LacksExit(object):
-            def __enter__(self):
-                pass
-
-        def fooLacksExit():
-            foo = LacksExit()
-            with foo: pass
-        self.assertRaisesRegex(TypeError, 'the context manager.*__exit__', fooLacksExit)
+        class LacksExit:
+            def __enter__(self): ...
+
+        msg = re.escape((
+            "object does not support the context manager protocol "
+            "(missed __exit__ method)"
+        ))
+        # a missing __exit__ is reported missing before a missing __enter__
+        with self.assertRaisesRegex(TypeError, msg):
+            do_with(object())
+        with self.assertRaisesRegex(TypeError, msg):
+            do_with(LacksExit())
+
+    def testWithForAsyncManager(self):
+        class AsyncManager:
+            async def __aenter__(self): ...
+            async def __aexit__(self, type, value, traceback): ...
+
+        with self.assertRaisesRegex(TypeError, re.escape((
+            "object does not support the context manager protocol "
+            "(missed __exit__ method) but it supports the asynchronous "
+            "context manager protocol. Did you mean to use 'async with'?"
+        ))):
+            do_with(AsyncManager())
+
+    def testAsyncEnterAttributeError(self):
+        class LacksAsyncEnter:
+            async def __aexit__(self, type, value, traceback): ...
+
+        with self.assertRaisesRegex(TypeError, re.escape((
+            "object does not support the asynchronous context manager protocol "
+            "(missed __aenter__ method)"
+        ))):
+            do_async_with(LacksAsyncEnter()).send(None)
+
+    def testAsyncExitAttributeError(self):
+        class LacksAsyncExit:
+            async def __aenter__(self): ...
+
+        msg = re.escape((
+            "object does not support the asynchronous context manager protocol "
+            "(missed __aexit__ method)"
+        ))
+        # a missing __aexit__ is reported missing before a missing __aenter__
+        with self.assertRaisesRegex(TypeError, msg):
+            do_async_with(object()).send(None)
+        with self.assertRaisesRegex(TypeError, msg):
+            do_async_with(LacksAsyncExit()).send(None)
+
+    def testAsyncWithForSyncManager(self):
+        class SyncManager:
+            def __enter__(self): ...
+            def __exit__(self, type, value, traceback): ...
+
+        with self.assertRaisesRegex(TypeError, re.escape((
+            "object does not support the asynchronous context manager protocol "
+            "(missed __aexit__ method) but it supports the context manager "
+            "protocol. Did you mean to use 'with'?"
+        ))):
+            do_async_with(SyncManager()).send(None)
 
     def assertRaisesSyntaxError(self, codestr):
         def shouldRaiseSyntaxError(s):
@@ -190,6 +244,7 @@ def shouldThrow():
                 pass
         self.assertRaises(RuntimeError, shouldThrow)
 
+
 class ContextmanagerAssertionMixin(object):
 
     def setUp(self):

diff --git a/Lib/unittest/async_case.py b/Lib/unittest/async_case.py
@@ -75,9 +75,17 @@ async def enterAsyncContext(self, cm):
             enter = cls.__aenter__
             exit = cls.__aexit__
         except AttributeError:
-            raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does "
-                            f"not support the asynchronous context manager protocol"
-                           ) from None
+            msg = (f"'{cls.__module__}.{cls.__qualname__}' object does "
+                   "not support the asynchronous context manager protocol")
+            try:
+                cls.__enter__
+                cls.__exit__
+            except AttributeError:
+                pass
+            else:
+                msg += (" but it supports the context manager protocol. "
+                        "Did you mean to use enterContext()?")
+            raise TypeError(msg) from None
         result = await enter(cm)
         self.addAsyncCleanup(exit, cm, None, None, None)
         return result

diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
@@ -111,8 +111,17 @@ def _enter_context(cm, addcleanup):
         enter = cls.__enter__
         exit = cls.__exit__
     except AttributeError:
-        raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does "
-                        f"not support the context manager protocol") from None
+        msg = (f"'{cls.__module__}.{cls.__qualname__}' object does "
+               "not support the context manager protocol")
+        try:
+            cls.__aenter__
+            cls.__aexit__
+        except AttributeError:
+            pass
+        else:
+            msg += (" but it supports the asynchronous context manager "
+                    "protocol. Did you mean to use enterAsyncContext()?")
+        raise TypeError(msg) from None
     result = enter(cm)
     addcleanup(exit, cm, None, None, None)
     return result

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-04-07-13-46-57.gh-issue-128398.gJ2zIF.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-04-07-13-46-57.gh-issue-128398.gJ2zIF.rst
@@ -0,0 +1,4 @@
+Improve error message when an object supporting the synchronous (resp.
+asynchronous) context manager protocol is entered using :keyword:`async
+with` (resp. :keyword:`with`) instead of :keyword:`with` (resp.
+:keyword:`async with`). Patch by Bénédikt Tran.
@@ -3425,9 +3425,12 @@ dummy_func(
             PyObject *attr_o = _PyObject_LookupSpecialMethod(owner_o, name, &self_or_null_o);
             if (attr_o == NULL) {
                 if (!_PyErr_Occurred(tstate)) {
-                    _PyErr_Format(tstate, PyExc_TypeError,
-                                  _Py_SpecialMethods[oparg].error,
-                                  Py_TYPE(owner_o)->tp_name);
+                    const char *errfmt = _PyEval_SpecialMethodCanSuggest(owner_o, oparg)
+                        ? _Py_SpecialMethods[oparg].error_suggestion
+                        : _Py_SpecialMethods[oparg].error;
+                    assert(!_PyErr_Occurred(tstate));
+                    assert(errfmt != NULL);
+                    _PyErr_Format(tstate, PyExc_TypeError, errfmt, owner_o);
                 }
                 ERROR_IF(true, error);
             }

@@ -545,23 +545,51 @@ const conversion_func _PyEval_ConversionFuncs[4] = {
 const _Py_SpecialMethod _Py_SpecialMethods[] = {
     [SPECIAL___ENTER__] = {
         .name = &_Py_ID(__enter__),
-        .error = "'%.200s' object does not support the "
-                 "context manager protocol (missed __enter__ method)",
+        .error = (
+            "'%T' object does not support the context manager protocol "
+            "(missed __enter__ method)"
+        ),
+        .error_suggestion = (
+            "'%T' object does not support the context manager protocol "
+            "(missed __enter__ method) but it supports the asynchronous "
+            "context manager protocol. Did you mean to use 'async with'?"
+        )
     },
     [SPECIAL___EXIT__] = {
         .name = &_Py_ID(__exit__),
-        .error = "'%.200s' object does not support the "
-                 "context manager protocol (missed __exit__ method)",
+        .error = (
+            "'%T' object does not support the context manager protocol "
+            "(missed __exit__ method)"
+        ),
+        .error_suggestion = (
+            "'%T' object does not support the context manager protocol "
+            "(missed __exit__ method) but it supports the asynchronous "
+            "context manager protocol. Did you mean to use 'async with'?"
+        )
     },
     [SPECIAL___AENTER__] = {
         .name = &_Py_ID(__aenter__),
-        .error = "'%.200s' object does not support the asynchronous "
-                 "context manager protocol (missed __aenter__ method)",
+        .error = (
+            "'%T' object does not support the asynchronous "
+            "context manager protocol (missed __aenter__ method)"
+        ),
+        .error_suggestion = (
+            "'%T' object does not support the asynchronous context manager "
+            "protocol (missed __aenter__ method) but it supports the context "
+            "manager protocol. Did you mean to use 'with'?"
+        )
     },
     [SPECIAL___AEXIT__] = {
         .name = &_Py_ID(__aexit__),
-        .error = "'%.200s' object does not support the asynchronous "
-                 "context manager protocol (missed __aexit__ method)",
+        .error = (
+            "'%T' object does not support the asynchronous "
+            "context manager protocol (missed __aexit__ method)"
+        ),
+        .error_suggestion = (
+            "'%T' object does not support the asynchronous context manager "
+            "protocol (missed __aexit__ method) but it supports the context "
+            "manager protocol. Did you mean to use 'with'?"
+        )
     }
 };
 
@@ -3380,3 +3408,33 @@ _PyEval_LoadName(PyThreadState *tstate, _PyInterpreterFrame *frame, PyObject *na
     }
     return value;
 }
+
+/* Check if a 'cls' provides the given special method. */
+static inline int
+type_has_special_method(PyTypeObject *cls, PyObject *name)
+{
+    // _PyType_Lookup() does not set an exception and returns a borrowed ref
+    assert(!PyErr_Occurred());
+    PyObject *r = _PyType_Lookup(cls, name);
+    return r != NULL && Py_TYPE(r)->tp_descr_get != NULL;
+}
+
+int
+_PyEval_SpecialMethodCanSuggest(PyObject *self, int oparg)
+{
+    PyTypeObject *type = Py_TYPE(self);
+    switch (oparg) {
+        case SPECIAL___ENTER__:
+        case SPECIAL___EXIT__: {
+            return type_has_special_method(type, &_Py_ID(__aenter__))
+                   && type_has_special_method(type, &_Py_ID(__aexit__));
+        }
+        case SPECIAL___AENTER__:
+        case SPECIAL___AEXIT__: {
+            return type_has_special_method(type, &_Py_ID(__enter__))
+                   && type_has_special_method(type, &_Py_ID(__exit__));
+        }
+        default:
+            Py_FatalError("unsupported special method");
+    }
+}
diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h