diff --git a/tests/tools/test_computer_use.py b/tests/tools/test_computer_use.py
index 4a140c36be3..49d5bbdfbf9 100644
--- a/tests/tools/test_computer_use.py
+++ b/tests/tools/test_computer_use.py
@@ -226,6 +226,21 @@ class TestDispatch:
         parsed = json.loads(out)
         assert "error" in parsed
 
+    def test_set_value_routes_to_backend(self, noop_backend):
+        """set_value must reach the backend — regression for missing _NoopBackend stub."""
+        from tools.computer_use.tool import handle_computer_use
+        out = handle_computer_use({"action": "set_value", "value": "Option A", "element": 5})
+        parsed = json.loads(out)
+        assert parsed.get("ok") is True
+        assert parsed.get("action") == "set_value"
+        assert any(c[0] == "set_value" for c in noop_backend.calls)
+
+    def test_set_value_missing_value_returns_error(self, noop_backend):
+        from tools.computer_use.tool import handle_computer_use
+        out = handle_computer_use({"action": "set_value"})
+        parsed = json.loads(out)
+        assert "error" in parsed
+
 
 # ---------------------------------------------------------------------------
 # Safety guards (type / key block lists)
diff --git a/tools/computer_use/backend.py b/tools/computer_use/backend.py
index 9952510e9cc..c9686e41b04 100644
--- a/tools/computer_use/backend.py
+++ b/tools/computer_use/backend.py
@@ -142,6 +142,14 @@ class ComputerUseBackend(ABC):
     def focus_app(self, app: str, raise_window: bool = False) -> ActionResult:
         """Route input to `app` (by name or bundle ID). Default: focus without raise."""
 
+    # ── Native-value mutation ────────────────────────────────────────
+    @abstractmethod
+    def set_value(self, value: str, element: Optional[int] = None) -> ActionResult:
+        """Set a native value on an element (e.g. AXPopUpButton selection).
+
+        `element` is the 1-based SOM index returned by a prior capture call.
+        """
+
     # ── Timing ──────────────────────────────────────────────────────
     def wait(self, seconds: float) -> ActionResult:
         """Default implementation: time.sleep."""
diff --git a/tools/computer_use/tool.py b/tools/computer_use/tool.py
index 82c6792eeb9..41308733df1 100644
--- a/tools/computer_use/tool.py
+++ b/tools/computer_use/tool.py
@@ -200,6 +200,10 @@ class _NoopBackend(ComputerUseBackend):  # pragma: no cover
         self.calls.append(("focus_app", {"app": app, "raise": raise_window}))
         return ActionResult(ok=True, action="focus_app")
 
+    def set_value(self, value: str, element: Optional[int] = None) -> ActionResult:
+        self.calls.append(("set_value", {"value": value, "element": element}))
+        return ActionResult(ok=True, action="set_value")
+
 
 # ---------------------------------------------------------------------------
 # Dispatch