python
@@ -623,6 +623,10 @@ Some options are read from the :mod:`sys` attributes. For example, the option
 
    .. versionadded:: 3.14
 
+   .. versionchanged:: next
+      The function now replaces :data:`sys.flags` (create a new object),
+      instead of modifying :data:`sys.flags` in-place.
+
 
 .. _pyconfig_api:
 

diff --git a/Lib/test/test_capi/test_config.py b/Lib/test/test_capi/test_config.py
@@ -356,9 +356,11 @@ def expect_bool_not(value):
                     for value in new_values:
                         expected, expect_flag = expect_func(value)
 
+                        old_flags = sys.flags
                         config_set(name, value)
                         self.assertEqual(config_get(name), expected)
                         self.assertEqual(getattr(sys.flags, sys_flag), expect_flag)
+                        self.assertIsNot(sys.flags, old_flags)
                         if name == "write_bytecode":
                             self.assertEqual(getattr(sys, "dont_write_bytecode"),
                                              expect_flag)

diff --git a/Lib/test/test_free_threading/test_sys.py b/Lib/test/test_free_threading/test_sys.py
@@ -0,0 +1,28 @@
+import sys
+import unittest
+from test.support import threading_helper
+
+
+class SysModuleTest(unittest.TestCase):
+    def test_int_max_str_digits_thread(self):
+        # gh-151218: Check that it's safe to call get_int_max_str_digits() and
+        # set_int_max_str_digits() in parallel. Previously, this test triggered
+        # warnings in TSan on a free threaded build.
+
+        old_limit = sys.get_int_max_str_digits()
+        self.addCleanup(sys.set_int_max_str_digits, old_limit)
+
+        def worker(worker_id):
+            if not worker_id:
+                for i in range (20_000):
+                    sys.get_int_max_str_digits()
+            else:
+                for i in range (20_000):
+                    sys.set_int_max_str_digits(4300 + (i & 7))
+
+        workers = [lambda: worker(i) for i in range(5)]
+        threading_helper.run_concurrently(workers)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -1349,6 +1349,26 @@ def test_pystats(self):
     def test_disable_gil_abi(self):
         self.assertEqual('t' in sys.abiflags, support.Py_GIL_DISABLED)
 
+    def test_int_max_str_digits(self):
+        old_limit = sys.get_int_max_str_digits()
+        self.assertIsInstance(old_limit, int)
+        self.assertGreaterEqual(old_limit, 0)
+        self.addCleanup(sys.set_int_max_str_digits, old_limit)
+
+        sys.set_int_max_str_digits(0)
+        self.assertEqual(sys.get_int_max_str_digits(), 0)
+
+        sys.set_int_max_str_digits(2_048)
+        self.assertEqual(sys.get_int_max_str_digits(), 2_048)
+
+        with self.assertRaises(ValueError):
+            # the minimum is 640 digits
+            sys.set_int_max_str_digits(5)
+        with self.assertRaises(ValueError):
+            sys.set_int_max_str_digits(-2)
+        with self.assertRaises(TypeError):
+            sys.set_int_max_str_digits(2_048.0)
+
 
 @test.support.cpython_only
 @test.support.force_not_colorized_test_class

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-15-30-25.gh-issue-151218.5M_nv8.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-15-30-25.gh-issue-151218.5M_nv8.rst
@@ -0,0 +1,3 @@
+:c:func:`PyConfig_Set` and :func:`sys.set_int_max_str_digits` now replace
+:data:`sys.flags` (create a new object), instead of modifying :data:`sys.flags`
+in-place. Patch by Victor Stinner.
diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -2125,7 +2125,7 @@ long_to_decimal_string_internal(PyObject *aa,
     if (size_a >= 10 * _PY_LONG_MAX_STR_DIGITS_THRESHOLD
                   / (3 * PyLong_SHIFT) + 2) {
         PyInterpreterState *interp = _PyInterpreterState_GET();
-        int max_str_digits = interp->long_state.max_str_digits;
+        int max_str_digits = _Py_atomic_load_int(&interp->long_state.max_str_digits);
         if ((max_str_digits > 0) &&
             (max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10)) {
             PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_STR,
@@ -2206,7 +2206,7 @@ long_to_decimal_string_internal(PyObject *aa,
     }
     if (strlen > _PY_LONG_MAX_STR_DIGITS_THRESHOLD) {
         PyInterpreterState *interp = _PyInterpreterState_GET();
-        int max_str_digits = interp->long_state.max_str_digits;
+        int max_str_digits = _Py_atomic_load_int(&interp->long_state.max_str_digits);
         Py_ssize_t strlen_nosign = strlen - negative;
         if ((max_str_digits > 0) && (strlen_nosign > max_str_digits)) {
             Py_DECREF(scratch);
@@ -3021,7 +3021,7 @@ long_from_string_base(const char **str, int base, PyLongObject **res)
          * quadratic algorithm. */
         if (digits > _PY_LONG_MAX_STR_DIGITS_THRESHOLD) {
             PyInterpreterState *interp = _PyInterpreterState_GET();
-            int max_str_digits = interp->long_state.max_str_digits;
+            int max_str_digits = _Py_atomic_load_int(&interp->long_state.max_str_digits);
             if ((max_str_digits > 0) && (digits > max_str_digits)) {
                 PyErr_Format(PyExc_ValueError, _MAX_STR_DIGITS_ERROR_FMT_TO_INT,
                              max_str_digits, digits);

@@ -4641,7 +4641,8 @@ config_get(const PyConfig *config, const PyConfigSpec *spec,
 
         if (strcmp(spec->name, "int_max_str_digits") == 0) {
             PyInterpreterState *interp = _PyInterpreterState_GET();
-            return PyLong_FromLong(interp->long_state.max_str_digits);
+            int maxdigits = _Py_atomic_load_int(&interp->long_state.max_str_digits);
+            return PyLong_FromLong(maxdigits);
         }
     }
 

@@ -429,7 +429,8 @@ interpreter_update_config(PyThreadState *tstate, int only_update_path_config)
         }
     }
 
-    tstate->interp->long_state.max_str_digits = config->int_max_str_digits;
+    _Py_atomic_store_int(&tstate->interp->long_state.max_str_digits,
+                         config->int_max_str_digits);
 
     // Update the sys module for the new configuration
     if (_PySys_UpdateConfig(tstate) < 0) {

@@ -1874,7 +1874,8 @@ sys_get_int_max_str_digits_impl(PyObject *module)
 /*[clinic end generated code: output=0042f5e8ae0e8631 input=77fb74e987ba7ecb]*/
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    return PyLong_FromLong(interp->long_state.max_str_digits);
+    int maxdigits = _Py_atomic_load_int(&interp->long_state.max_str_digits);
+    return PyLong_FromLong(maxdigits);
 }
 
 
@@ -3490,14 +3491,39 @@ sys_set_flag(PyObject *flags, Py_ssize_t pos, PyObject *value)
 int
 _PySys_SetFlagObj(Py_ssize_t pos, PyObject *value)
 {
-    PyObject *flags = PySys_GetAttrString("flags");
-    if (flags == NULL) {
-        return -1;
+    PyObject *new_flags = NULL;
+    PyObject *flags_str = &_Py_ID(flags);  // immortal ref
+
+    PyObject *old_flags = PySys_GetAttr(flags_str);
+    if (old_flags == NULL) {
+        goto error;
     }
 
-    sys_set_flag(flags, pos, value);
-    Py_DECREF(flags);
-    return 0;
+    new_flags = PyStructSequence_New(&FlagsType);
+    if (new_flags == NULL) {
+        goto error;
+    }
+
+    for (Py_ssize_t i = 0; i < (Py_ssize_t)(Py_ARRAY_LENGTH(flags_fields) - 1); i++) {
+        if (i != pos) {
+            PyObject *old_value;
+            old_value = PyStructSequence_GET_ITEM(old_flags, i);  // borrowed ref
+            sys_set_flag(new_flags, i, old_value);
+        }
+        else {
+            sys_set_flag(new_flags, pos, value);
+        }
+    }
+
+    int res = _PySys_SetAttr(flags_str, new_flags);
+    Py_DECREF(old_flags);
+    Py_DECREF(new_flags);
+    return res;
+
+error:
+    Py_XDECREF(old_flags);
+    Py_XDECREF(new_flags);
+    return -1;
 }
 
 
@@ -3521,8 +3547,6 @@ set_flags_from_config(PyInterpreterState *interp, PyObject *flags)
     const PyPreConfig *preconfig = &interp->runtime->preconfig;
     const PyConfig *config = _PyInterpreterState_GetConfig(interp);
 
-    // _PySys_UpdateConfig() modifies sys.flags in-place:
-    // Py_XDECREF() is needed in this case.
     Py_ssize_t pos = 0;
 #define SetFlagObj(expr) \
     do { \
@@ -4033,7 +4057,7 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
     /* implementation */
     SET_SYS("implementation", make_impl_info(version_info));
 
-    // sys.flags: updated in-place later by _PySys_UpdateConfig()
+    // sys.flags: updated later by _PySys_UpdateConfig()
     ENSURE_INFO_TYPE(FlagsType, flags_desc);
     SET_SYS("flags", make_flags(tstate->interp));
 
@@ -4153,16 +4177,21 @@ _PySys_UpdateConfig(PyThreadState *tstate)
 #undef COPY_LIST
 #undef COPY_WSTR
 
-    // sys.flags
-    PyObject *flags = PySys_GetAttrString("flags");
-    if (flags == NULL) {
+    // replace sys.flags
+    PyObject *new_flags = PyStructSequence_New(&FlagsType);
+    if (new_flags == NULL) {
         return -1;
     }
-    if (set_flags_from_config(interp, flags) < 0) {
-        Py_DECREF(flags);
+    if (set_flags_from_config(interp, new_flags) < 0) {
+        Py_DECREF(new_flags);
+        return -1;
+    }
+
+    res = _PySys_SetAttr(&_Py_ID(flags), new_flags);
+    Py_DECREF(new_flags);
+    if (res < 0) {
         return -1;
     }
-    Py_DECREF(flags);
 
     SET_SYS("dont_write_bytecode", PyBool_FromLong(!config->write_bytecode));
 
@@ -4675,7 +4704,7 @@ _PySys_SetIntMaxStrDigits(int maxdigits)
     // Set PyInterpreterState.long_state.max_str_digits
     // and PyInterpreterState.config.int_max_str_digits.
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    interp->long_state.max_str_digits = maxdigits;
-    interp->config.int_max_str_digits = maxdigits;
+    _Py_atomic_store_int(&interp->long_state.max_str_digits, maxdigits);
+    _Py_atomic_store_int(&interp->config.int_max_str_digits, maxdigits);
     return 0;
 }