python
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -1236,6 +1236,16 @@ always available.
    .. versionadded:: 3.13
 
 
+.. function:: _jit_enabled()
+
+   Return :const:`True` if the :ref:`JIT compiler <whatsnew313-jit-compiler>` is active,
+   and :const:`False` otherwise.
+
+   Available only in JIT builds; tier 2 interpreter does not count here.
+
+   .. versionadded:: next
+
+
 .. function:: is_finalizing()
 
    Return :const:`True` if the main Python interpreter is

@@ -443,6 +443,9 @@ sys
   which only exists in specialized builds of Python, may now return objects
   from other interpreters than the one it's called in.
 
+* Add :func:`sys._jit_enabled` to determine at runtime if the
+  :ref:`JIT compiler <whatsnew313-jit-compiler>` is active.
+
 
 unicodedata
 -----------

@@ -22,6 +22,10 @@ extern int _PySys_ClearAttrString(PyInterpreterState *interp,
 extern int _PySys_SetFlagObj(Py_ssize_t pos, PyObject *new_value);
 extern int _PySys_SetIntMaxStrDigits(int maxdigits);
 
+#if defined(_Py_TIER2) && (_Py_TIER2 % 2 != 0)
+extern int _PySys_JITEnabled(void);
+#endif
+
 #ifdef __cplusplus
 }
 #endif

diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
@@ -1782,10 +1782,8 @@ def test_initconfig_api(self):
             'perf_profiling': 2,
         }
         config_dev_mode(preconfig, config)
-        # Temporarily enable ignore_stderr=True to ignore warnings on JIT builds
-        # See gh-126255 for more information
         self.check_all_configs("test_initconfig_api", config, preconfig,
-                               api=API_ISOLATED, ignore_stderr=True)
+                               api=API_ISOLATED, env={'PYTHON_JIT': '0'})
 
     def test_initconfig_get_api(self):
         self.run_embedded_interpreter("test_initconfig_get_api")

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-11-10-04-59-43.gh-issue-126204.ASKDL1.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-11-10-04-59-43.gh-issue-126204.ASKDL1.rst
@@ -0,0 +1 @@
+Allow to determine whether the JIT is enabled at runtime via :func:`sys._jit_enabled`.
diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
@@ -1296,43 +1296,34 @@ init_interp_main(PyThreadState *tstate)
 #endif
     }
 
-    // Turn on experimental tier 2 (uops-based) optimizer
-    // This is also needed when the JIT is enabled
+// Turn on experimental tier 2 (uops-based) optimizer
+// This is also needed when the JIT is enabled
 #ifdef _Py_TIER2
     if (is_main_interp) {
-        int enabled = 1;
-#if _Py_TIER2 & 2
-        enabled = 0;
-#endif
-        char *env = Py_GETENV("PYTHON_JIT");
-        if (env && *env != '\0') {
-            // PYTHON_JIT=0|1 overrides the default
-            enabled = *env != '0';
+// perf profiler works fine with tier 2 interpreter, so
+// only checking for a "real JIT".
+#if _Py_TIER % 2 != 0
+        if (_PySys_JITEnabled() && config->perf_profiling > 0) {
+            (void)PyErr_WarnEx(
+                PyExc_RuntimeWarning,
+	            "JIT deactivated as perf profiling support is active",
+	            0);
         }
-        if (enabled) {
-#ifdef _Py_JIT
-            // perf profiler works fine with tier 2 interpreter, so
-            // only checking for a "real JIT".
-            if (config->perf_profiling > 0) {
-                (void)PyErr_WarnEx(
-                    PyExc_RuntimeWarning,
-                    "JIT deactivated as perf profiling support is active",
-                    0);
-            } else
+        else
 #endif
-            {
-                PyObject *opt = _PyOptimizer_NewUOpOptimizer();
-                if (opt == NULL) {
-                    return _PyStatus_ERR("can't initialize optimizer");
-                }
-                if (_Py_SetTier2Optimizer((_PyOptimizerObject *)opt)) {
-                    return _PyStatus_ERR("can't install optimizer");
-                }
+        {
+            PyObject *opt = _PyOptimizer_NewUOpOptimizer();
+            if (opt == NULL) {
+                return _PyStatus_ERR("can't initialize optimizer");
+            }
+            if (_Py_SetTier2Optimizer((_PyOptimizerObject *)opt)) {
                 Py_DECREF(opt);
+                return _PyStatus_ERR("can't install optimizer");
             }
+            Py_DECREF(opt);
         }
     }
-#endif
+#endif // Py_TIER2
 
     if (!is_main_interp) {
         // The main interpreter is handled in Py_Main(), for now.

@@ -2264,12 +2264,14 @@ sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
 /*[clinic end generated code: output=5783cdeb51874b43 input=a12df928758a82b4]*/
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
-#ifdef _Py_JIT
-    _PyOptimizerObject* optimizer = _Py_GetOptimizer();
-    if (optimizer != NULL) {
-        Py_DECREF(optimizer);
-        PyErr_SetString(PyExc_ValueError, "Cannot activate the perf trampoline if the JIT is active");
-        return NULL;
+#if defined(_Py_TIER2) && (_Py_TIER2 % 2 != 0)
+    if (_PySys_JITEnabled()) {
+        _PyOptimizerObject *optimizer = _Py_GetOptimizer();
+        if (optimizer != NULL) {
+            Py_DECREF(optimizer);
+            PyErr_SetString(PyExc_ValueError, "Cannot activate the perf trampoline if the JIT is active");
+            return NULL;
+        }
     }
 #endif
 
@@ -2533,6 +2535,46 @@ PyAPI_FUNC(int) PyUnstable_CopyPerfMapFile(const char* parent_filename) {
 }
 
 
+#ifdef _Py_TIER2
+// _Py_TIER2 is set to 4 or 6 in tier 2 builds.
+#if _Py_TIER2 % 2 != 0
+int
+_PySys_JITEnabled(void)
+{
+    const char *env = Py_GETENV("PYTHON_JIT");
+    if (_Py_TIER2 == 1) {
+        // Interpreter was built with enabled jit, but it can be disabled via PYTHON_JIT=0
+        if (env && *env != '\0') {
+            return *env != '0';
+        }
+        return 1;
+    }
+    else if (_Py_TIER2 == 3) {
+        // Interpreter was built with disabled jit, but it can be enabled via PYTHON_JIT=1
+        if (env && *env != '\0') {
+            return *env == '1';
+        }
+        return 0;
+    }
+    Py_UNREACHABLE();
+}
+
+/*[clinic input]
+sys._jit_enabled -> bool
+
+Returns True if JIT is enabled, False otherwise.
+[clinic start generated code]*/
+
+static int
+sys__jit_enabled_impl(PyObject *module)
+/*[clinic end generated code: output=e8adca3fa5011880 input=f2bdfa820c11f65b]*/
+{
+    return _PySys_JITEnabled();
+}
+#endif
+#endif // _Py_TIER2
+
+
 static PyMethodDef sys_methods[] = {
     /* Might as well keep this in alphabetic order */
     SYS_ADDAUDITHOOK_METHODDEF
@@ -2603,6 +2645,7 @@ static PyMethodDef sys_methods[] = {
 #endif
     SYS__GET_CPU_COUNT_CONFIG_METHODDEF
     SYS__IS_GIL_ENABLED_METHODDEF
+    SYS__JIT_ENABLED_METHODDEF
     {NULL, NULL}  // sentinel
 };
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Allow to determine whether the JIT is enabled at runtime via :func:`sys._jit_enabled`.