@@ -1797,6 +1797,9 @@ object.
    The instance must be destroyed by :c:func:`PyUnicodeWriter_Finish` on
    success, or :c:func:`PyUnicodeWriter_Discard` on error.
 
+   The API is **not thread safe**. To share a writer with multiple threads, a
+   critical section or a lock is needed.
+
 .. c:function:: PyUnicodeWriter* PyUnicodeWriter_Create(Py_ssize_t length)
 
    Create a Unicode writer instance.

diff --git a/Include/internal/pycore_unicodeobject.h b/Include/internal/pycore_unicodeobject.h
@@ -16,7 +16,9 @@ extern "C" {
 #define _Py_MAX_UNICODE 0x10ffff
 
 
-extern int _PyUnicode_IsModifiable(PyObject *unicode);
+// Export for '_multibytecodec' shared extension. _PyUnicodeWriter_CanWrite()
+// calls this function when assertions are enabled.
+PyAPI_FUNC(int) _PyUnicode_IsModifiable(PyObject *unicode);
 extern void _PyUnicodeWriter_InitWithBuffer(
     _PyUnicodeWriter *writer,
     PyObject *buffer);
@@ -105,12 +107,31 @@ _PyUnicode_EnsureUnicode(PyObject *obj)
     return 0;
 }
 
+#ifndef NDEBUG
+static inline int
+_PyUnicodeWriter_CanWrite(_PyUnicodeWriter *writer)
+{
+    // Code adapted from _PyUnicode_IsModifiable()
+    assert(!writer->readonly);
+    PyObject *buffer = writer->buffer;
+    assert(buffer != NULL);
+    // Do not use _PyObject_IsUniquelyReferenced(): the caller can have its own
+    // lock to prevent a writer being used by two theads at the same time.
+    assert(Py_REFCNT(buffer) == 1);
+    assert(PyUnstable_Unicode_GET_CACHED_HASH(buffer) == -1);
+    assert(!PyUnicode_CHECK_INTERNED(buffer));
+    assert(!_Py_IsImmortal(buffer));
+    return 1;
+}
+#endif
+
 static inline int
 _PyUnicodeWriter_WriteCharInline(_PyUnicodeWriter *writer, Py_UCS4 ch)
 {
     assert(ch <= _Py_MAX_UNICODE);
     if (_PyUnicodeWriter_Prepare(writer, 1, ch) < 0)
         return -1;
+    assert(_PyUnicodeWriter_CanWrite(writer));
     PyUnicode_WRITE(writer->kind, writer->data, writer->pos, ch);
     writer->pos++;
     return 0;

diff --git a/Lib/test/test_capi/test_unicode.py b/Lib/test/test_capi/test_unicode.py
@@ -1981,6 +1981,17 @@ def test_substring_empty(self):
         writer.write_substring("abc", 1, 1)
         self.assertEqual(writer.finish(), '')
 
+    def test_singletons(self):
+        writer = self.create_writer(5)
+        self.assertIs(writer.finish(), '')
+
+        for ch in range(256):
+            with self.subTest(ch=ch):
+                ch = chr(ch)
+                writer = self.create_writer(0)
+                writer.write_substring(ch + 'xxx', 0, 1)
+                self.assertIs(writer.finish(), ch)
+
 
 @unittest.skipIf(ctypes is None, 'need ctypes')
 class PyUnicodeWriterFormatTest(unittest.TestCase):

diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -2220,6 +2220,7 @@ long_to_decimal_string_internal(PyObject *aa,
             Py_DECREF(scratch);
             return -1;
         }
+        assert(_PyUnicodeWriter_CanWrite(writer));
     }
     else if (bytes_writer) {
         *bytes_str = PyBytesWriter_GrowAndUpdatePointer(bytes_writer, strlen,
@@ -2390,8 +2391,10 @@ long_format_binary(PyObject *aa, int base, int alternate,
     }
 
     if (writer) {
-        if (_PyUnicodeWriter_Prepare(writer, sz, 'x') == -1)
+        if (_PyUnicodeWriter_Prepare(writer, sz, 'x') == -1) {
             return -1;
+        }
+        assert(_PyUnicodeWriter_CanWrite(writer));
     }
     else if (bytes_writer) {
         *bytes_str = PyBytesWriter_GrowAndUpdatePointer(bytes_writer, sz,

diff --git a/Objects/unicode_writer.c b/Objects/unicode_writer.c
@@ -350,6 +350,8 @@ _PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer, PyObject *str)
         if (_PyUnicodeWriter_PrepareInternal(writer, len, maxchar) == -1)
             return -1;
     }
+
+    assert(_PyUnicodeWriter_CanWrite(writer));
     _PyUnicode_FastCopyCharacters(writer->buffer, writer->pos,
                                   str, 0, len);
     writer->pos += len;
@@ -428,6 +430,7 @@ _PyUnicodeWriter_WriteSubstring(_PyUnicodeWriter *writer, PyObject *str,
     if (_PyUnicodeWriter_Prepare(writer, len, maxchar) < 0) {
         return -1;
     }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     _PyUnicode_FastCopyCharacters(writer->buffer, writer->pos,
                                   str, start, len);
@@ -485,8 +488,10 @@ _PyUnicodeWriter_WriteASCIIString(_PyUnicodeWriter *writer,
         return 0;
     }
 
-    if (_PyUnicodeWriter_Prepare(writer, len, 127) == -1)
+    if (_PyUnicodeWriter_Prepare(writer, len, 127) == -1) {
         return -1;
+    }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     switch (writer->kind)
     {
@@ -591,6 +596,7 @@ _PyUnicodeWriter_WriteLatin1String(_PyUnicodeWriter *writer,
     maxchar = ucs1lib_find_max_char((const Py_UCS1*)str, (const Py_UCS1*)str + len);
     if (_PyUnicodeWriter_Prepare(writer, len, maxchar) == -1)
         return -1;
+    assert(_PyUnicodeWriter_CanWrite(writer));
     unicode_write_cstr(writer->buffer, writer->pos, str, len);
     writer->pos += len;
     return 0;

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
@@ -1743,18 +1743,21 @@ unicode_is_singleton(PyObject *unicode)
 }
 #endif
 
+// If this function is updated, update also _PyUnicodeWriter_CanWrite().
 int
 _PyUnicode_IsModifiable(PyObject *unicode)
 {
     assert(_PyUnicode_CHECK(unicode));
+    if (!PyUnicode_CheckExact(unicode))
+        return 0;
+    // On Free Threading, this test fails if called from a thread other
+    // than the one which created the str object.
     if (!_PyObject_IsUniquelyReferenced(unicode))
         return 0;
     if (PyUnicode_HASH(unicode) != -1)
         return 0;
     if (PyUnicode_CHECK_INTERNED(unicode))
         return 0;
-    if (!PyUnicode_CheckExact(unicode))
-        return 0;
 #ifdef Py_DEBUG
     /* singleton refcount is greater than 1 */
     assert(!unicode_is_singleton(unicode));
@@ -2008,6 +2011,7 @@ PyUnicodeWriter_WriteWideChar(PyUnicodeWriter *pub_writer,
     if (_PyUnicodeWriter_Prepare(writer, size - num_surrogates, maxchar) < 0) {
         return -1;
     }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     int kind = writer->kind;
     void *data = (Py_UCS1*)writer->data + writer->pos * kind;
@@ -2266,6 +2270,7 @@ PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *pub_writer,
     if (_PyUnicodeWriter_Prepare(writer, size, max_char) < 0) {
         return -1;
     }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     int kind = writer->kind;
     void *data = (Py_UCS1*)writer->data + writer->pos * kind;
@@ -2552,8 +2557,10 @@ unicode_fromformat_write_str(_PyUnicodeWriter *writer, PyObject *str,
     else
         maxchar = writer->maxchar;
 
-    if (_PyUnicodeWriter_Prepare(writer, arglen, maxchar) == -1)
+    if (_PyUnicodeWriter_Prepare(writer, arglen, maxchar) == -1) {
         return -1;
+    }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     fill = Py_MAX(width - length, 0);
     if (fill && !(flags & F_LJUST)) {
@@ -2843,8 +2850,10 @@ unicode_fromformat_arg(_PyUnicodeWriter *writer,
         Py_ssize_t spacepad = Py_MAX(width - precision - sign, 0);
         Py_ssize_t zeropad = Py_MAX(precision - len, 0);
 
-        if (_PyUnicodeWriter_Prepare(writer, width, 127) == -1)
+        if (_PyUnicodeWriter_Prepare(writer, width, 127) == -1) {
             return NULL;
+        }
+        assert(_PyUnicodeWriter_CanWrite(writer));
 
         if (spacepad && !(flags & F_LJUST)) {
             if (PyUnicode_Fill(writer->buffer, writer->pos, spacepad, ' ') == -1)
@@ -5371,6 +5380,7 @@ _PyUnicode_DecodeUTF8Writer(_PyUnicodeWriter *writer,
     if (_PyUnicodeWriter_Prepare(writer, size, 127) < 0) {
         return -1;
     }
+    assert(_PyUnicodeWriter_CanWrite(writer));
 
     const char *starts = s;
     const char *end = s + size;