diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index b749c60..bdd3672 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -53,7 +53,7 @@ jobs:
           - os: windows-latest
             python: 3.6
           - os: windows-latest
-            python: 3.12
+            python: 3.13
 
           # macOS: test only new Python
           - os: macos-latest
diff --git a/pythoncapi_compat.h b/pythoncapi_compat.h
index acaadf3..34a84c9 100644
--- a/pythoncapi_compat.h
+++ b/pythoncapi_compat.h
@@ -1537,7 +1537,7 @@ static inline int PyUnicode_Equal(PyObject *str1, PyObject *str2)
     }
 
 #if PY_VERSION_HEX >= 0x030d0000 && !defined(PYPY_VERSION)
-    PyAPI_FUNC(int) _PyUnicode_Equal(PyObject *str1, PyObject *str2);
+    extern int _PyUnicode_Equal(PyObject *str1, PyObject *str2);
 
     return _PyUnicode_Equal(str1, str2);
 #elif PY_VERSION_HEX >= 0x03060000 && !defined(PYPY_VERSION)
@@ -1564,7 +1564,7 @@ static inline PyObject* PyBytes_Join(PyObject *sep, PyObject *iterable)
 static inline Py_hash_t Py_HashBuffer(const void *ptr, Py_ssize_t len)
 {
 #if PY_VERSION_HEX >= 0x03000000 && !defined(PYPY_VERSION)
-    PyAPI_FUNC(Py_hash_t) _Py_HashBytes(const void *src, Py_ssize_t len);
+    extern Py_hash_t _Py_HashBytes(const void *src, Py_ssize_t len);
 
     return _Py_HashBytes(ptr, len);
 #else
diff --git a/tests/setup.py b/tests/setup.py
index 2e10c15..98f7798 100755
--- a/tests/setup.py
+++ b/tests/setup.py
@@ -13,38 +13,53 @@
 
 
 # C++ is only supported on Python 3.6 and newer
-TEST_CPP = (sys.version_info >= (3, 6))
+TEST_CXX = (sys.version_info >= (3, 6))
 
 SRC_DIR = os.path.normpath(os.path.join(os.path.dirname(__file__), '..'))
 
 # Windows uses MSVC compiler
 MSVC = (os.name == "nt")
 
-# C compiler flags for GCC and clang
-COMMON_FLAGS = [
-    # Treat warnings as error
-    '-Werror',
-    # Enable all warnings
-    '-Wall', '-Wextra',
-    # Extra warnings
-    '-Wconversion',
-    # /usr/lib64/pypy3.7/include/pyport.h:68:20: error: redefinition of typedef
-    # 'Py_hash_t' is a C11 feature
-    "-Wno-typedef-redefinition",
-]
-CFLAGS = COMMON_FLAGS + [
-    # Use C99 for pythoncapi_compat.c which initializes PyModuleDef with a
-    # mixture of designated and non-designated initializers
-    '-std=c99',
-]
-CPPFLAGS = list(COMMON_FLAGS)
-# FIXME: _Py_CAST() emits C++ compilers on Python 3.12.
-# See: https://github.com/python/cpython/issues/94731
-if 0:
-    CPPFLAGS.extend((
-        '-Wold-style-cast',
-        '-Wzero-as-null-pointer-constant',
-    ))
+if not MSVC:
+    # C compiler flags for GCC and clang
+    COMMON_FLAGS = [
+        # Treat warnings as error
+        '-Werror',
+        # Enable all warnings
+        '-Wall', '-Wextra',
+        # Extra warnings
+        '-Wconversion',
+        # /usr/lib64/pypy3.7/include/pyport.h:68:20: error: redefinition of typedef
+        # 'Py_hash_t' is a C11 feature
+        "-Wno-typedef-redefinition",
+    ]
+    CFLAGS = COMMON_FLAGS + [
+        # Use C99 for pythoncapi_compat.c which initializes PyModuleDef with a
+        # mixture of designated and non-designated initializers
+        '-std=c99',
+    ]
+    CXXFLAGS = list(COMMON_FLAGS)
+    # FIXME: _Py_CAST() emits C++ compilers on Python 3.12.
+    # See: https://github.com/python/cpython/issues/94731
+    if 0:
+        CXXFLAGS.extend((
+            '-Wold-style-cast',
+            '-Wzero-as-null-pointer-constant',
+        ))
+else:
+    COMMON_FLAGS = [
+        # Treat all compiler warnings as compiler errors
+        '/WX',
+    ]
+    if sys.version_info >= (3, 13):
+        COMMON_FLAGS.append(
+            # Display warnings level 1 to 4
+            '/W4'
+        )
+    CFLAGS = COMMON_FLAGS + [
+        '/std:c99'
+    ]
+    CXXFLAGS = list(COMMON_FLAGS)
 
 
 def main():
@@ -68,9 +83,8 @@ def main():
 
     cflags = ['-I' + SRC_DIR]
     cppflags = list(cflags)
-    if not MSVC:
-        cflags.extend(CFLAGS)
-        cppflags.extend(CPPFLAGS)
+    cflags.extend(CFLAGS)
+    cppflags.extend(CXXFLAGS)
 
     # C extension
     c_ext = Extension(
@@ -79,7 +93,7 @@ def main():
         extra_compile_args=cflags)
     extensions = [c_ext]
 
-    if TEST_CPP:
+    if TEST_CXX:
         # C++ extension
 
         # MSVC has /std flag but doesn't support /std:c++11
@@ -89,7 +103,10 @@ def main():
                 ('test_pythoncapi_compat_cpp11ext', '-std=c++11'),
             ]
         else:
-            versions = [('test_pythoncapi_compat_cppext', None)]
+            versions = [
+                ('test_pythoncapi_compat_cpp03ext', '/std:c++03'),
+                ('test_pythoncapi_compat_cpp11ext', '/std:c++11'),
+            ]
         for name, flag in versions:
             flags = list(cppflags)
             if flag is not None: