diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-01-04-00-04-53.gh-issue-143401.m8lb4b.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-04-00-04-53.gh-issue-143401.m8lb4b.rst
new file mode 100644
index 00000000000000..39060b52c4dc06
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-04-00-04-53.gh-issue-143401.m8lb4b.rst
@@ -0,0 +1 @@
+Fix an incorrect optimization regarding string concatenation.
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 366bca44f83f16..e7220dc3469c8f 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -788,17 +788,17 @@ dummy_func(
             _PyStackRef *target_local = &GETLOCAL(next_oparg);
             assert(PyUnicode_CheckExact(left_o));
             DEOPT_IF(PyStackRef_AsPyObjectBorrow(*target_local) != left_o);
+            /* gh-143401: The local should be uniquely referenced to modify this in-place.
+             * This check is required as 3.14 no longer creates new references when pushing
+             * values to the stack in some cases.
+             */
+            DEOPT_IF(!_PyObject_IsUniquelyReferenced(PyStackRef_AsPyObjectBorrow(*target_local)));
             STAT_INC(BINARY_OP, hit);
             /* Handle `left = left + right` or `left += right` for str.
              *
              * When possible, extend `left` in place rather than
              * allocating a new PyUnicodeObject. This attempts to avoid
              * quadratic behavior when one neglects to use str.join().
-             *
-             * If `left` has only two references remaining (one from
-             * the stack, one in the locals), DECREFing `left` leaves
-             * only the locals reference, so PyUnicode_Append knows
-             * that the string is safe to mutate.
              */
             assert(Py_REFCNT(left_o) >= 2 || !PyStackRef_IsHeapSafe(left));
             PyObject *temp = PyStackRef_AsPyObjectSteal(*target_local);
@@ -808,9 +808,9 @@ dummy_func(
             DEAD(right);
             PyStackRef_CLOSE_SPECIALIZED(left, _PyUnicode_ExactDealloc);
             DEAD(left);
+            *target_local = PyStackRef_NULL;
             ERROR_IF(temp == NULL);
             res = PyStackRef_FromPyObjectSteal(temp);
-            *target_local = PyStackRef_NULL;
         }
 
        op(_GUARD_BINARY_OP_EXTEND, (descr/4, left, right -- left, right)) {
diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
index 3fe7a3200269c3..f9d3f9b64090ad 100644
--- a/Python/executor_cases.c.h
+++ b/Python/executor_cases.c.h
@@ -4954,6 +4954,13 @@
                 SET_CURRENT_CACHED_VALUES(2);
                 JUMP_TO_JUMP_TARGET();
             }
+            if (!_PyObject_IsUniquelyReferenced(PyStackRef_AsPyObjectBorrow(*target_local))) {
+                UOP_STAT_INC(uopcode, miss);
+                _tos_cache1 = right;
+                _tos_cache0 = left;
+                SET_CURRENT_CACHED_VALUES(2);
+                JUMP_TO_JUMP_TARGET();
+            }
             STAT_INC(BINARY_OP, hit);
             assert(Py_REFCNT(left_o) >= 2 || !PyStackRef_IsHeapSafe(left));
             PyObject *temp = PyStackRef_AsPyObjectSteal(*target_local);
@@ -4967,6 +4974,7 @@
             stack_pointer = _PyFrame_GetStackPointer(frame);
             PyStackRef_CLOSE_SPECIALIZED(right, _PyUnicode_ExactDealloc);
             PyStackRef_CLOSE_SPECIALIZED(left, _PyUnicode_ExactDealloc);
+            *target_local = PyStackRef_NULL;
             if (temp == NULL) {
                 stack_pointer += -2;
                 ASSERT_WITHIN_STACK_BOUNDS(__FILE__, __LINE__);
@@ -4974,7 +4982,6 @@
                 JUMP_TO_ERROR();
             }
             res = PyStackRef_FromPyObjectSteal(temp);
-            *target_local = PyStackRef_NULL;
             _tos_cache0 = res;
             _tos_cache1 = PyStackRef_ZERO_BITS;
             _tos_cache2 = PyStackRef_ZERO_BITS;
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h
index a1d5a98255c405..20c19a4f86d297 100644
--- a/Python/generated_cases.c.h
+++ b/Python/generated_cases.c.h
@@ -425,6 +425,11 @@
                     assert(_PyOpcode_Deopt[opcode] == (BINARY_OP));
                     JUMP_TO_PREDICTED(BINARY_OP);
                 }
+                if (!_PyObject_IsUniquelyReferenced(PyStackRef_AsPyObjectBorrow(*target_local))) {
+                    UPDATE_MISS_STATS(BINARY_OP);
+                    assert(_PyOpcode_Deopt[opcode] == (BINARY_OP));
+                    JUMP_TO_PREDICTED(BINARY_OP);
+                }
                 STAT_INC(BINARY_OP, hit);
                 assert(Py_REFCNT(left_o) >= 2 || !PyStackRef_IsHeapSafe(left));
                 PyObject *temp = PyStackRef_AsPyObjectSteal(*target_local);
@@ -434,11 +439,11 @@
                 stack_pointer = _PyFrame_GetStackPointer(frame);
                 PyStackRef_CLOSE_SPECIALIZED(right, _PyUnicode_ExactDealloc);
                 PyStackRef_CLOSE_SPECIALIZED(left, _PyUnicode_ExactDealloc);
+                *target_local = PyStackRef_NULL;
                 if (temp == NULL) {
                     JUMP_TO_LABEL(pop_2_error);
                 }
                 res = PyStackRef_FromPyObjectSteal(temp);
-                *target_local = PyStackRef_NULL;
             }
             stack_pointer[-2] = res;
             stack_pointer += -1;