diff --git a/Misc/NEWS.d/next/C_API/2026-06-17-08-12-29.gh-issue-151515.0rJRKv.rst b/Misc/NEWS.d/next/C_API/2026-06-17-08-12-29.gh-issue-151515.0rJRKv.rst
new file mode 100644
index 000000000000000..16bbf07af7e2b98
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2026-06-17-08-12-29.gh-issue-151515.0rJRKv.rst
@@ -0,0 +1,12 @@
+Summary
+
+This PR fixes an undefined behavior bug in the C code generated by Parser/asdl_c.py for get_ast_state().
+The Problem
+
+In the generated code, _PyOnceFlag_CallOnce is invoked like this:
+_PyOnceFlag_CallOnce(&state->once, (_Py_once_fn_t *)&init_types, state)
+
+Using &init_types passes a pointer to a function pointer (effectively a double pointer), rather than the function pointer itself. by forcing this with an explicit cast (_Py_once_fn_t *) silences the compiler but triggers a strict aliasing violation and undefined behavior at runtime when the pointer is dereferenced. This can lead to compiler-optimization-driven segmentation faults, particularly in free-threaded/GIL-disabled builds where one-time initialization flags are heavily relied upon.
+The Fix
+
+Updated Parser/asdl_c.py to pass init_types directly with a standard function pointer cast (_Py_once_fn_t)init_types (removing the extra & and *). Ran make regen-ast to cleanly update the generated files.
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index e2a57177d20afb7..14fe70000b829ab 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -2274,7 +2274,7 @@ def generate_module_def(mod, metadata, f, internal_h):
             PyInterpreterState *interp = _PyInterpreterState_GET();
             struct ast_state *state = &interp->ast;
             assert(!state->finalized);
-            if (_PyOnceFlag_CallOnce(&state->once, (_Py_once_fn_t *)&init_types, state) < 0) {
+            if (_PyOnceFlag_CallOnce(&state->once, &init_types, state) < 0) {
                 return NULL;
             }
             return state;
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 49b6bf1d12b6fab..a8cc3847b3154ff 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -23,7 +23,7 @@ get_ast_state(void)
     PyInterpreterState *interp = _PyInterpreterState_GET();
     struct ast_state *state = &interp->ast;
     assert(!state->finalized);
-    if (_PyOnceFlag_CallOnce(&state->once, (_Py_once_fn_t *)&init_types, state) < 0) {
+    if (_PyOnceFlag_CallOnce(&state->once, &init_types, state) < 0) {
         return NULL;
     }
     return state;