diff --git a/Doc/c-api/float.rst b/Doc/c-api/float.rst
index 6e83e01344a9b2..929b56bd8e8203 100644
--- a/Doc/c-api/float.rst
+++ b/Doc/c-api/float.rst
@@ -190,24 +190,23 @@ The pack and unpack functions provide an efficient platform-independent way to
 store floating-point values as byte strings. The Pack routines produce a bytes
 string from a C :c:expr:`double`, and the Unpack routines produce a C
 :c:expr:`double` from such a bytes string. The suffix (2, 4 or 8) specifies the
-number of bytes in the bytes string.
+number of bytes in the bytes string:
 
-On platforms that appear to use IEEE 754 formats these functions work by
-copying bits. On other platforms, the 2-byte format is identical to the IEEE
-754 binary16 half-precision format, the 4-byte format (32-bit) is identical to
-the IEEE 754 binary32 single precision format, and the 8-byte format to the
-IEEE 754 binary64 double precision format, although the packing of INFs and
-NaNs (if such things exist on the platform) isn't handled correctly, and
-attempting to unpack a bytes string containing an IEEE INF or NaN will raise an
-exception.
+* The 2-byte format is the IEEE 754 binary16 half-precision format.
+* The 4-byte format is the IEEE 754 binary32 single-precision format.
+* The 8-byte format is the IEEE 754 binary64 double-precision format.
 
-Note that NaN type may not be preserved on IEEE platforms (signaling NaNs become
-quiet NaNs), for example on x86 systems in 32-bit mode.
+The NaN type may not be preserved on some platforms while unpacking (signaling
+NaNs become quiet NaNs), for example on x86 systems in 32-bit mode.
 
+It's assumed that the :c:expr:`double` type has the IEEE 754 binary64 double
+precision format.  What happens if it's not true is partly accidental (alas).
 On non-IEEE platforms with more precision, or larger dynamic range, than IEEE
 754 supports, not all values can be packed; on non-IEEE platforms with less
-precision, or smaller dynamic range, not all values can be unpacked. What
-happens in such cases is partly accidental (alas).
+precision, or smaller dynamic range, not all values can be unpacked.  The
+packing of special numbers like INFs and NaNs (if such things exist on the
+platform) may not be handled correctly, and attempting to unpack a bytes string
+containing an IEEE INF or NaN may raise an exception.
 
 .. versionadded:: 3.11
 
@@ -217,9 +216,9 @@ Pack functions
 The pack routines write 2, 4 or 8 bytes, starting at *p*. *le* is an
 :c:expr:`int` argument, non-zero if you want the bytes string in little-endian
 format (exponent last, at ``p+1``, ``p+3``, or ``p+6`` and ``p+7``), zero if you
-want big-endian format (exponent first, at *p*). The :c:macro:`PY_BIG_ENDIAN`
-constant can be used to use the native endian: it is equal to ``1`` on big
-endian processor, or ``0`` on little endian processor.
+want big-endian format (exponent first, at *p*). Use the :c:macro:`!PY_LITTLE_ENDIAN`
+constant to select the native endian: it is equal to ``0`` on big
+endian processor, or ``1`` on little endian processor.
 
 Return value: ``0`` if all is OK, ``-1`` if error (and an exception is set,
 most likely :exc:`OverflowError`).
@@ -236,6 +235,9 @@ most likely :exc:`OverflowError`).
 
    Pack a C double as the IEEE 754 binary64 double precision format.
 
+   .. impl-detail::
+      This function always succeeds in CPython.
+
 
 Unpack functions
 ^^^^^^^^^^^^^^^^
@@ -243,14 +245,17 @@ Unpack functions
 The unpack routines read 2, 4 or 8 bytes, starting at *p*.  *le* is an
 :c:expr:`int` argument, non-zero if the bytes string is in little-endian format
 (exponent last, at ``p+1``, ``p+3`` or ``p+6`` and ``p+7``), zero if big-endian
-(exponent first, at *p*). The :c:macro:`PY_BIG_ENDIAN` constant can be used to
-use the native endian: it is equal to ``1`` on big endian processor, or ``0``
+(exponent first, at *p*). Use the :c:macro:`!PY_LITTLE_ENDIAN` constant to
+select the native endian: it is equal to ``0`` on big endian processor, or ``1``
 on little endian processor.
 
 Return value: The unpacked double.  On error, this is ``-1.0`` and
 :c:func:`PyErr_Occurred` is true (and an exception is set, most likely
 :exc:`OverflowError`).
 
+.. impl-detail::
+    These functions always succeed in CPython.
+
 .. c:function:: double PyFloat_Unpack2(const char *p, int le)
 
    Unpack the IEEE 754 binary16 half-precision format as a C double.
diff --git a/Doc/tools/.nitignore b/Doc/tools/.nitignore
index 54b92f1172e80c..ffe98d332d6e93 100644
--- a/Doc/tools/.nitignore
+++ b/Doc/tools/.nitignore
@@ -3,7 +3,6 @@
 # Keep lines sorted lexicographically to help avoid merge conflicts.
 
 Doc/c-api/descriptor.rst
-Doc/c-api/float.rst
 Doc/c-api/init_config.rst
 Doc/c-api/intro.rst
 Doc/c-api/stable.rst
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 5689ecbffc4b30..69283f67487b18 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -1358,7 +1358,7 @@ The JIT avoids :term:`reference count`\ s where possible. This generally
 reduces the cost of most operations in Python.
 
 (Contributed by Ken Jin, Donghee Na, Zheao Li, Hai Zhu, Savannah Ostrowski,
-Reiden Ong, Noam Cohen, Tomas Roun, PuQing, and Cajetan Rodrigues in :gh:`134584`.)
+Reiden Ong, Noam Cohen, Tomas Roun, PuQing, Cajetan Rodrigues, and Sacul in :gh:`134584`.)
 
 .. rubric:: Better machine code generation
 
diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
index 05951771c3b92e..df127b55be4068 100644
--- a/Lib/test/test_capi/test_opt.py
+++ b/Lib/test/test_capi/test_opt.py
@@ -2832,6 +2832,18 @@ def testfunc(n):
         self.assertIn("_GUARD_TYPE_VERSION", uops)
         self.assertNotIn("_CHECK_ATTR_CLASS", uops)
 
+    def test_load_common_constant(self):
+        def testfunc(n):
+            for _ in range(n):
+                x = list(i for i in ())
+            return x
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, list(()))
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+        self.assertIn("_BUILD_LIST", uops)
+        self.assertNotIn("_LOAD_COMMON_CONSTANT", uops)
+
     def test_load_small_int(self):
         def testfunc(n):
             x = 0
diff --git a/Lib/test/test_capi/test_unicode.py b/Lib/test/test_capi/test_unicode.py
index 55120448a8a3ec..5dee25756fe289 100644
--- a/Lib/test/test_capi/test_unicode.py
+++ b/Lib/test/test_capi/test_unicode.py
@@ -1765,13 +1765,15 @@ def test_basic(self):
         writer.write_utf8(b'var', -1)
 
         # test PyUnicodeWriter_WriteChar()
-        writer.write_char('=')
+        writer.write_char(ord('='))
 
         # test PyUnicodeWriter_WriteSubstring()
         writer.write_substring("[long]", 1, 5)
+        # CRASHES writer.write_substring(NULL, 0, 0)
 
         # test PyUnicodeWriter_WriteStr()
         writer.write_str(" value ")
+        # CRASHES writer.write_str(NULL)
 
         # test PyUnicodeWriter_WriteRepr()
         writer.write_repr("repr")
@@ -1786,14 +1788,28 @@ def test_repr_null(self):
         self.assertEqual(writer.finish(),
                          "var=<NULL>")
 
+    def test_write_char(self):
+        writer = self.create_writer(0)
+        writer.write_char(0)
+        writer.write_char(ord('$'))
+        writer.write_char(0x20ac)
+        writer.write_char(0x10_ffff)
+        self.assertRaises(ValueError, writer.write_char, 0x11_0000)
+        self.assertRaises(ValueError, writer.write_char, 0xFFFF_FFFF)
+        self.assertEqual(writer.finish(),
+                         "\0$\u20AC\U0010FFFF")
+
     def test_utf8(self):
         writer = self.create_writer(0)
         writer.write_utf8(b"ascii", -1)
-        writer.write_char('-')
+        writer.write_char(ord('-'))
         writer.write_utf8(b"latin1=\xC3\xA9", -1)
-        writer.write_char('-')
+        writer.write_char(ord('-'))
         writer.write_utf8(b"euro=\xE2\x82\xAC", -1)
-        writer.write_char('.')
+        writer.write_char(ord('.'))
+        writer.write_utf8(NULL, 0)
+        # CRASHES writer.write_utf8(NULL, 1)
+        # CRASHES writer.write_utf8(NULL, -1)
         self.assertEqual(writer.finish(),
                          "ascii-latin1=\xE9-euro=\u20AC.")
 
@@ -1801,6 +1817,9 @@ def test_ascii(self):
         writer = self.create_writer(0)
         writer.write_ascii(b"Hello ", -1)
         writer.write_ascii(b"", 0)
+        writer.write_ascii(NULL, 0)
+        # CRASHES writer.write_ascii(NULL, 1)
+        # CRASHES writer.write_ascii(NULL, -1)
         writer.write_ascii(b"Python! <truncated>", 6)
         self.assertEqual(writer.finish(), "Hello Python")
 
@@ -1817,6 +1836,9 @@ def test_recover_utf8_error(self):
         # write fails with an invalid string
         with self.assertRaises(UnicodeDecodeError):
             writer.write_utf8(b"invalid\xFF", -1)
+        with self.assertRaises(UnicodeDecodeError):
+            s = "truncated\u20AC".encode()
+            writer.write_utf8(s, len(s) - 1)
 
         # retry write with a valid string
         writer.write_utf8(b"valid", -1)
@@ -1828,13 +1850,19 @@ def test_decode_utf8(self):
         # test PyUnicodeWriter_DecodeUTF8Stateful()
         writer = self.create_writer(0)
         writer.decodeutf8stateful(b"ign\xFFore", -1, b"ignore")
-        writer.write_char('-')
+        writer.write_char(ord('-'))
         writer.decodeutf8stateful(b"replace\xFF", -1, b"replace")
-        writer.write_char('-')
+        writer.write_char(ord('-'))
 
         # incomplete trailing UTF-8 sequence
         writer.decodeutf8stateful(b"incomplete\xC3", -1, b"replace")
 
+        writer.decodeutf8stateful(NULL, 0, b"replace")
+        # CRASHES writer.decodeutf8stateful(NULL, 1, b"replace")
+        # CRASHES writer.decodeutf8stateful(NULL, -1, b"replace")
+        with self.assertRaises(UnicodeDecodeError):
+            writer.decodeutf8stateful(b"default\xFF", -1, NULL)
+
         self.assertEqual(writer.finish(),
                          "ignore-replace\uFFFD-incomplete\uFFFD")
 
@@ -1845,12 +1873,12 @@ def test_decode_utf8_consumed(self):
         # valid string
         consumed = writer.decodeutf8stateful(b"text", -1, b"strict", True)
         self.assertEqual(consumed, 4)
-        writer.write_char('-')
+        writer.write_char(ord('-'))
 
         # non-ASCII
         consumed = writer.decodeutf8stateful(b"\xC3\xA9-\xE2\x82\xAC", 6, b"strict", True)
         self.assertEqual(consumed, 6)
-        writer.write_char('-')
+        writer.write_char(ord('-'))
 
         # invalid UTF-8 (consumed is 0 on error)
         with self.assertRaises(UnicodeDecodeError):
@@ -1859,54 +1887,92 @@ def test_decode_utf8_consumed(self):
         # ignore error handler
         consumed = writer.decodeutf8stateful(b"more\xFF", -1, b"ignore", True)
         self.assertEqual(consumed, 5)
-        writer.write_char('-')
+        writer.write_char(ord('-'))
 
         # incomplete trailing UTF-8 sequence
         consumed = writer.decodeutf8stateful(b"incomplete\xC3", -1, b"ignore", True)
         self.assertEqual(consumed, 10)
+        writer.write_char(ord('-'))
 
-        self.assertEqual(writer.finish(), "text-\xE9-\u20AC-more-incomplete")
+        consumed = writer.decodeutf8stateful(NULL, 0, b"replace", True)
+        self.assertEqual(consumed, 0)
+        # CRASHES writer.decodeutf8stateful(NULL, 1, b"replace", True)
+        # CRASHES writer.decodeutf8stateful(NULL, -1, b"replace", True)
+        consumed = writer.decodeutf8stateful(b"default\xC3", -1, NULL, True)
+        self.assertEqual(consumed, 7)
+
+        self.assertEqual(writer.finish(), "text-\xE9-\u20AC-more-incomplete-default")
 
     def test_widechar(self):
+        from _testcapi import SIZEOF_WCHAR_T
+
+        if SIZEOF_WCHAR_T == 2:
+            encoding = 'utf-16le' if sys.byteorder == 'little' else 'utf-16be'
+        elif SIZEOF_WCHAR_T == 4:
+            encoding = 'utf-32le' if sys.byteorder == 'little' else 'utf-32be'
+
         writer = self.create_writer(0)
-        writer.write_widechar("latin1=\xE9")
-        writer.write_widechar("-")
-        writer.write_widechar("euro=\u20AC")
-        writer.write_char("-")
-        writer.write_widechar("max=\U0010ffff")
-        writer.write_char('.')
+        writer.write_widechar("latin1=\xE9".encode(encoding))
+        writer.write_char(ord("-"))
+        writer.write_widechar("euro=\u20AC".encode(encoding))
+        writer.write_char(ord("-"))
+        writer.write_widechar("max=\U0010ffff".encode(encoding))
+        writer.write_char(ord("-"))
+        writer.write_widechar("zeroes=".encode(encoding).ljust(SIZEOF_WCHAR_T * 10, b'\0'),
+                              10)
+        writer.write_char(ord('.'))
+
+        if SIZEOF_WCHAR_T == 4:
+            invalid = (b'\x00\x00\x11\x00' if sys.byteorder == 'little' else
+                       b'\x00\x11\x00\x00')
+            with self.assertRaises(ValueError):
+                writer.write_widechar("invalid=".encode(encoding) + invalid)
+        writer.write_widechar(b'', -5)
+        writer.write_widechar(NULL, 0)
+        # CRASHES writer.write_widechar(NULL, 1)
+        # CRASHES writer.write_widechar(NULL, -1)
+
         self.assertEqual(writer.finish(),
-                         "latin1=\xE9-euro=\u20AC-max=\U0010ffff.")
+                         "latin1=\xE9-euro=\u20AC-max=\U0010ffff-zeroes=\0\0\0.")
 
     def test_ucs4(self):
+        encoding = 'utf-32le' if sys.byteorder == 'little' else 'utf-32be'
+
         writer = self.create_writer(0)
-        writer.write_ucs4("ascii IGNORED", 5)
-        writer.write_char("-")
-        writer.write_ucs4("latin1=\xe9", 8)
-        writer.write_char("-")
-        writer.write_ucs4("euro=\u20ac", 6)
-        writer.write_char("-")
-        writer.write_ucs4("max=\U0010ffff", 5)
-        writer.write_char(".")
+        writer.write_ucs4("ascii IGNORED".encode(encoding), 5)
+        writer.write_char(ord("-"))
+        writer.write_ucs4("latin1=\xe9".encode(encoding))
+        writer.write_char(ord("-"))
+        writer.write_ucs4("euro=\u20ac".encode(encoding))
+        writer.write_char(ord("-"))
+        writer.write_ucs4("max=\U0010ffff".encode(encoding))
+        writer.write_char(ord("."))
         self.assertEqual(writer.finish(),
                          "ascii-latin1=\xE9-euro=\u20AC-max=\U0010ffff.")
 
         # Test some special characters
         writer = self.create_writer(0)
         # Lone surrogate character
-        writer.write_ucs4("lone\uDC80", 5)
-        writer.write_char("-")
+        writer.write_ucs4("lone\uDC80".encode(encoding, 'surrogatepass'))
+        writer.write_char(ord("-"))
         # Surrogate pair
-        writer.write_ucs4("pair\uDBFF\uDFFF", 5)
-        writer.write_char("-")
-        writer.write_ucs4("null[\0]", 7)
+        writer.write_ucs4("pair\uD83D\uDC0D".encode(encoding, 'surrogatepass'))
+        writer.write_char(ord("-"))
+        writer.write_ucs4("null[\0]".encode(encoding), 7)
+        invalid = (b'\x00\x00\x11\x00' if sys.byteorder == 'little' else
+                   b'\x00\x11\x00\x00')
+        # CRASHES writer.write_ucs4("invalid".encode(encoding) + invalid)
+        writer.write_ucs4(NULL, 0)
+        # CRASHES writer.write_ucs4(NULL, 1)
         self.assertEqual(writer.finish(),
-                         "lone\udc80-pair\udbff-null[\0]")
+                         "lone\udc80-pair\ud83d\udc0d-null[\x00]")
 
         # invalid size
         writer = self.create_writer(0)
         with self.assertRaises(ValueError):
-            writer.write_ucs4("text", -1)
+            writer.write_ucs4("text".encode(encoding), -1)
+        self.assertRaises(ValueError, writer.write_ucs4, b'', -1)
+        self.assertRaises(ValueError, writer.write_ucs4, NULL, -1)
 
     def test_substring_empty(self):
         writer = self.create_writer(0)
@@ -1932,7 +1998,7 @@ def test_format(self):
         from ctypes import c_int
         writer = self.create_writer(0)
         self.writer_format(writer, b'%s %i', b'abc', c_int(123))
-        writer.write_char('.')
+        writer.write_char(ord('.'))
         self.assertEqual(writer.finish(), 'abc 123.')
 
     def test_recover_error(self):
diff --git a/Modules/_testcapi/unicode.c b/Modules/_testcapi/unicode.c
index 668adc5085b4fe..915c9230f66b52 100644
--- a/Modules/_testcapi/unicode.c
+++ b/Modules/_testcapi/unicode.c
@@ -301,16 +301,12 @@ writer_write_char(PyObject *self_raw, PyObject *args)
         return NULL;
     }
 
-    PyObject *str;
-    if (!PyArg_ParseTuple(args, "U", &str)) {
+    unsigned int ch;
+    if (!PyArg_ParseTuple(args, "I", &ch)) {
         return NULL;
     }
-    if (PyUnicode_GET_LENGTH(str) != 1) {
-        PyErr_SetString(PyExc_ValueError, "expect a single character");
-    }
-    Py_UCS4 ch = PyUnicode_READ_CHAR(str, 0);
 
-    if (PyUnicodeWriter_WriteChar(self->writer, ch) < 0) {
+    if (PyUnicodeWriter_WriteChar(self->writer, (Py_UCS4)ch) < 0) {
         return NULL;
     }
     Py_RETURN_NONE;
@@ -325,9 +321,9 @@ writer_write_utf8(PyObject *self_raw, PyObject *args)
         return NULL;
     }
 
-    char *str;
-    Py_ssize_t size;
-    if (!PyArg_ParseTuple(args, "yn", &str, &size)) {
+    const char *str;
+    Py_ssize_t bsize, size;
+    if (!PyArg_ParseTuple(args, "z#n", &str, &bsize, &size)) {
         return NULL;
     }
 
@@ -346,9 +342,9 @@ writer_write_ascii(PyObject *self_raw, PyObject *args)
         return NULL;
     }
 
-    char *str;
-    Py_ssize_t size;
-    if (!PyArg_ParseTuple(args, "yn", &str, &size)) {
+    const char *str;
+    Py_ssize_t bsize, size;
+    if (!PyArg_ParseTuple(args, "z#n", &str, &bsize, &size)) {
         return NULL;
     }
 
@@ -367,19 +363,23 @@ writer_write_widechar(PyObject *self_raw, PyObject *args)
         return NULL;
     }
 
-    PyObject *str;
-    if (!PyArg_ParseTuple(args, "U", &str)) {
-        return NULL;
-    }
+    const char *s;
+    Py_ssize_t bsize;
+    Py_ssize_t size = -100;
 
-    Py_ssize_t size;
-    wchar_t *wstr = PyUnicode_AsWideCharString(str, &size);
-    if (wstr == NULL) {
+    if (!PyArg_ParseTuple(args, "z#|n", &s, &bsize, &size)) {
         return NULL;
     }
+    if (size == -100) {
+        if (bsize % SIZEOF_WCHAR_T) {
+            PyErr_SetString(PyExc_AssertionError,
+                            "invalid size in writer.write_widechar()");
+            return NULL;
+        }
+        size = bsize / SIZEOF_WCHAR_T;
+    }
 
-    int res = PyUnicodeWriter_WriteWideChar(self->writer, wstr, size);
-    PyMem_Free(wstr);
+    int res = PyUnicodeWriter_WriteWideChar(self->writer, (const wchar_t *)s, size);
     if (res < 0) {
         return NULL;
     }
@@ -395,21 +395,23 @@ writer_write_ucs4(PyObject *self_raw, PyObject *args)
         return NULL;
     }
 
-    PyObject *str;
-    Py_ssize_t size;
-    if (!PyArg_ParseTuple(args, "Un", &str, &size)) {
-        return NULL;
-    }
-    Py_ssize_t len = PyUnicode_GET_LENGTH(str);
-    size = Py_MIN(size, len);
+    const char *s;
+    Py_ssize_t bsize;
+    Py_ssize_t size = -100;
 
-    Py_UCS4 *ucs4 = PyUnicode_AsUCS4Copy(str);
-    if (ucs4 == NULL) {
+    if (!PyArg_ParseTuple(args, "z#|n", &s, &bsize, &size)) {
         return NULL;
     }
+    if (size == -100) {
+        if (bsize % sizeof(Py_UCS4)) {
+            PyErr_SetString(PyExc_AssertionError,
+                            "invalid size in writer.write_ucs4()");
+            return NULL;
+        }
+        size = bsize / sizeof(Py_UCS4);
+    }
 
-    int res = PyUnicodeWriter_WriteUCS4(self->writer, ucs4, size);
-    PyMem_Free(ucs4);
+    int res = PyUnicodeWriter_WriteUCS4(self->writer, (const Py_UCS4 *)s, size);
     if (res < 0) {
         return NULL;
     }
@@ -418,18 +420,14 @@ writer_write_ucs4(PyObject *self_raw, PyObject *args)
 
 
 static PyObject*
-writer_write_str(PyObject *self_raw, PyObject *args)
+writer_write_str(PyObject *self_raw, PyObject *obj)
 {
     WriterObject *self = (WriterObject *)self_raw;
     if (writer_check(self) < 0) {
         return NULL;
     }
 
-    PyObject *obj;
-    if (!PyArg_ParseTuple(args, "O", &obj)) {
-        return NULL;
-    }
-
+    NULLABLE(obj);
     if (PyUnicodeWriter_WriteStr(self->writer, obj) < 0) {
         return NULL;
     }
@@ -438,19 +436,14 @@ writer_write_str(PyObject *self_raw, PyObject *args)
 
 
 static PyObject*
-writer_write_repr(PyObject *self_raw, PyObject *args)
+writer_write_repr(PyObject *self_raw, PyObject *obj)
 {
     WriterObject *self = (WriterObject *)self_raw;
     if (writer_check(self) < 0) {
         return NULL;
     }
 
-    PyObject *obj;
-    if (!PyArg_ParseTuple(args, "O", &obj)) {
-        return NULL;
-    }
     NULLABLE(obj);
-
     if (PyUnicodeWriter_WriteRepr(self->writer, obj) < 0) {
         return NULL;
     }
@@ -468,9 +461,10 @@ writer_write_substring(PyObject *self_raw, PyObject *args)
 
     PyObject *str;
     Py_ssize_t start, end;
-    if (!PyArg_ParseTuple(args, "Unn", &str, &start, &end)) {
+    if (!PyArg_ParseTuple(args, "Onn", &str, &start, &end)) {
         return NULL;
     }
+    NULLABLE(str);
 
     if (PyUnicodeWriter_WriteSubstring(self->writer, str, start, end) < 0) {
         return NULL;
@@ -488,10 +482,10 @@ writer_decodeutf8stateful(PyObject *self_raw, PyObject *args)
     }
 
     const char *str;
-    Py_ssize_t len;
+    Py_ssize_t bsize, len;
     const char *errors;
     int use_consumed = 0;
-    if (!PyArg_ParseTuple(args, "yny|i", &str, &len, &errors, &use_consumed)) {
+    if (!PyArg_ParseTuple(args, "z#nz#|p", &str, &bsize, &len, &errors, &bsize, &use_consumed)) {
         return NULL;
     }
 
@@ -544,8 +538,8 @@ static PyMethodDef writer_methods[] = {
     {"write_ascii", _PyCFunction_CAST(writer_write_ascii), METH_VARARGS},
     {"write_widechar", _PyCFunction_CAST(writer_write_widechar), METH_VARARGS},
     {"write_ucs4", _PyCFunction_CAST(writer_write_ucs4), METH_VARARGS},
-    {"write_str", _PyCFunction_CAST(writer_write_str), METH_VARARGS},
-    {"write_repr", _PyCFunction_CAST(writer_write_repr), METH_VARARGS},
+    {"write_str", _PyCFunction_CAST(writer_write_str), METH_O},
+    {"write_repr", _PyCFunction_CAST(writer_write_repr), METH_O},
     {"write_substring", _PyCFunction_CAST(writer_write_substring), METH_VARARGS},
     {"decodeutf8stateful", _PyCFunction_CAST(writer_decodeutf8stateful), METH_VARARGS},
     {"get_pointer", _PyCFunction_CAST(writer_get_pointer), METH_VARARGS},
diff --git a/Platforms/emscripten/__main__.py b/Platforms/emscripten/__main__.py
index a7c52c2d59f10d..6a7963413da31a 100644
--- a/Platforms/emscripten/__main__.py
+++ b/Platforms/emscripten/__main__.py
@@ -413,6 +413,11 @@ def make_mpdec(context, working_dir):
     write_library_config(prefix, "mpdec", mpdec_config, context.quiet)
 
 
+def make_dependencies(context):
+    make_emscripten_libffi(context)
+    make_mpdec(context)
+
+
 def calculate_node_path():
     node_version = os.environ.get("PYTHON_NODE_VERSION", None)
     if node_version is None:
@@ -573,7 +578,10 @@ def run_emscripten_python(context):
     if args and args[0] == "--":
         args = args[1:]
 
-    os.execv(str(exec_script), [str(exec_script)] + args)
+    if context.test:
+        args = load_config_toml()["test-args"] + args
+
+    os.execv(str(exec_script), [str(exec_script), *args])
 
 
 def build_target(context):
@@ -665,6 +673,11 @@ def main():
         help="Clone libffi repo, configure and build it for emscripten",
     )
 
+    make_dependencies_cmd = subcommands.add_parser(
+        "make-dependencies",
+        help="Build all static library dependencies",
+    )
+
     make_build = subcommands.add_parser(
         "make-build-python", help="Run `make` for the build Python"
     )
@@ -685,6 +698,15 @@ def main():
         "run",
         help="Run the built emscripten Python",
     )
+    run.add_argument(
+        "--test",
+        action="store_true",
+        default=False,
+        help=(
+            "If passed, will add the default test arguments to the beginning of the command. "
+            "Default arguments loaded from Platforms/emscripten/config.toml"
+        )
+    )
     run.add_argument(
         "args",
         nargs=argparse.REMAINDER,
@@ -694,6 +716,7 @@ def main():
         )
     )
     add_cross_build_dir_option(run)
+
     clean = subcommands.add_parser(
         "clean", help="Delete files and directories created by this script"
     )
@@ -714,6 +737,7 @@ def main():
         configure_build,
         make_libffi_cmd,
         make_mpdec_cmd,
+        make_dependencies_cmd,
         make_build,
         configure_host,
         make_host,
@@ -781,6 +805,7 @@ def main():
         "install-emscripten": install_emscripten,
         "make-libffi": make_emscripten_libffi,
         "make-mpdec": make_mpdec,
+        "make-dependencies": make_dependencies,
         "configure-build-python": configure_build_python,
         "make-build-python": make_build_python,
         "configure-host": configure_emscripten_python,
diff --git a/Platforms/emscripten/config.toml b/Platforms/emscripten/config.toml
index 4e76b5bf9f7d2b..c474078fb48ba3 100644
--- a/Platforms/emscripten/config.toml
+++ b/Platforms/emscripten/config.toml
@@ -3,6 +3,14 @@
 # Python versions.
 emscripten-version = "4.0.12"
 node-version = "24"
+test-args = [
+    "-m", "test",
+    "-v",
+    "-uall",
+    "--rerun",
+    "--single-process",
+    "-W",
+]
 
 [libffi]
 url = "https://github.com/libffi/libffi/releases/download/v{version}/libffi-{version}.tar.gz"
diff --git a/Python/optimizer_bytecodes.c b/Python/optimizer_bytecodes.c
index 9cb701d6a86f59..fff05f06f647c2 100644
--- a/Python/optimizer_bytecodes.c
+++ b/Python/optimizer_bytecodes.c
@@ -631,6 +631,13 @@ dummy_func(void) {
         value = PyJitRef_Borrow(sym_new_const(ctx, val));
     }
 
+    op(_LOAD_COMMON_CONSTANT, (-- value)) {
+        assert(oparg < NUM_COMMON_CONSTANTS);
+        PyObject *val = _PyInterpreterState_GET()->common_consts[oparg];
+        ADD_OP(_LOAD_CONST_INLINE_BORROW, 0, (uintptr_t)val);
+        value = PyJitRef_Borrow(sym_new_const(ctx, val));
+    }
+
     op(_LOAD_SMALL_INT, (-- value)) {
         PyObject *val = PyLong_FromLong(oparg);
         assert(val);
diff --git a/Python/optimizer_cases.c.h b/Python/optimizer_cases.c.h
index 481e099a3a37b0..1d1c62f9949e96 100644
--- a/Python/optimizer_cases.c.h
+++ b/Python/optimizer_cases.c.h
@@ -1435,7 +1435,10 @@
 
         case _LOAD_COMMON_CONSTANT: {
             JitOptRef value;
-            value = sym_new_not_null(ctx);
+            assert(oparg < NUM_COMMON_CONSTANTS);
+            PyObject *val = _PyInterpreterState_GET()->common_consts[oparg];
+            ADD_OP(_LOAD_CONST_INLINE_BORROW, 0, (uintptr_t)val);
+            value = PyJitRef_Borrow(sym_new_const(ctx, val));
             CHECK_STACK_BOUNDS(1);
             stack_pointer[0] = value;
             stack_pointer += 1;
diff --git a/Tools/check-c-api-docs/ignored_c_api.txt b/Tools/check-c-api-docs/ignored_c_api.txt
index 02a3031e52fb8b..e464162c52a371 100644
--- a/Tools/check-c-api-docs/ignored_c_api.txt
+++ b/Tools/check-c-api-docs/ignored_c_api.txt
@@ -31,7 +31,6 @@ PY_DWORD_MAX
 PY_FORMAT_SIZE_T
 PY_INT32_T
 PY_INT64_T
-PY_LITTLE_ENDIAN
 PY_LLONG_MAX
 PY_LLONG_MIN
 PY_LONG_LONG
@@ -39,6 +38,7 @@ PY_SIZE_MAX
 PY_UINT32_T
 PY_UINT64_T
 PY_ULLONG_MAX
+PY_BIG_ENDIAN
 # unicodeobject.h
 Py_UNICODE_SIZE
 # cpython/methodobject.h