fix(computer_use): a capped accessibility walk says so in the capture hint

`_capture_view`'s hint promised a "full element tree saved to elements_file",
but once `computer_use.ax_max_elements` bounds the driver's walk the spill
holds only the first N nodes. When the capture returned at least the bound
(bound > 0), drop "full" and add "accessibility walk capped at N elements;
pass app= to narrow" so the model knows how to reach the rest.

tests/tools/test_computer_use_ax_walk_bound.py

@@ -75,3 +75,31 @@ class TestAxWalkBound:
         args = _StubCapture()._gws_args()
         assert args["pid"] == 607 and args["window_id"] == 382
         assert args["max_elements"] == 250
+
+
+class TestCappedWalkHint:
+    """When the driver's walk stopped at the bound, the spill file is not the full tree — say so."""
+
+    @staticmethod
+    def _summary(monkeypatch, n_elements: int, bound: int) -> str:
+        from tools.computer_use import tool
+        from tools.computer_use.backend import CaptureResult, UIElement
+        monkeypatch.setattr(cua_backend, "_cua_configured_ax_max_elements", lambda: bound)
+        monkeypatch.setattr(tool, "_spill_elements_to_file", lambda cap: "/tmp/elements.json")
+        cap = CaptureResult(mode="ax", width=800, height=600,
+                            elements=[UIElement(index=i + 1, role="AXButton", label=f"b{i}") for i in range(n_elements)])
+        return "\n".join(tool._capture_summary_lines(tool._capture_view(cap, max_elements=5)))
+
+    def test_capped_walk_is_named_and_full_is_dropped(self, monkeypatch):
+        text = self._summary(monkeypatch, n_elements=8, bound=8)
+        assert "accessibility walk capped at 8 elements; pass app= to narrow" in text
+        assert "full element tree" not in text
+        assert "element tree with untruncated labels saved to" in text
+
+    def test_uncapped_walk_still_promises_the_full_tree(self, monkeypatch):
+        text = self._summary(monkeypatch, n_elements=8, bound=50)
+        assert "accessibility walk capped" not in text
+        assert "full element tree with untruncated labels saved to" in text
+
+    def test_zero_bound_never_claims_a_cap(self, monkeypatch):
+        assert "accessibility walk capped" not in self._summary(monkeypatch, n_elements=8, bound=0)

tools/computer_use/tool.py

@@ -551,12 +551,24 @@ def _capture_view(cap: CaptureResult, max_elements: int) -> SimpleNamespace:
     lost_detail = len(cap.elements) > len(visible) or any(len(e.label) > _MAX_ELEMENT_LABEL_CHARS for e in visible)
     too_small = bool(dims) and min(dims) < _MIN_PROVIDER_IMAGE_DIMENSION
     has_image = bool(cap.png_b64) and cap.mode != "ax" and not too_small
+    # The driver's own AX walk may have stopped at ``computer_use.ax_max_elements``: then the spill file is NOT
+    # the full tree, and the hint must not promise one.
+    ax_bound = _ax_walk_bound()
+    ax_capped = ax_bound > 0 and len(cap.elements) >= ax_bound
     return SimpleNamespace(cap=cap, visible=visible, total=len(cap.elements), width=width, height=height,
                            truncated=len(cap.elements) - len(visible), bounds_scale=scale, bounds_note=note,
+                           ax_capped=ax_bound if ax_capped else 0,
                            elements_file=_spill_elements_to_file(cap) if lost_detail else None,
                            screenshot_path=_persist_capture_image(cap) if has_image else None,
                            dims_omitted=dims if too_small else None, has_image=has_image)
 
+def _ax_walk_bound() -> int:
+    """``computer_use.ax_max_elements`` as the driver saw it (0 = unbounded / unknown)."""
+    with contextlib.suppress(Exception):
+        from tools.computer_use.cua_backend import _cua_configured_ax_max_elements
+        return int(_cua_configured_ax_max_elements())
+    return 0
+
 def _capture_summary_lines(v: SimpleNamespace) -> List[str]:
     """Human-readable capture summary; line ORDER is contract. Lists only what `elements` surfaces, otherwise the
     summary names indices the model can't find."""
@@ -565,8 +577,10 @@ def _capture_summary_lines(v: SimpleNamespace) -> List[str]:
                                            f"{v.bounds_scale} ≈ native coordinate)" if v.bounds_scale else ""),
         v.screenshot_path and f"shareable screenshot saved to {v.screenshot_path}",
         v.cap.note,
-        v.elements_file and (f"full element tree with untruncated labels saved to {v.elements_file} — "
-                             "read_file/search_files it if you need dropped label text or elements beyond the cap"),
+        v.elements_file and (f"{'' if getattr(v, 'ax_capped', 0) else 'full '}element tree with untruncated labels "
+                             f"saved to {v.elements_file} — read_file/search_files it if you need dropped label "
+                             "text or elements beyond the cap"),
+        getattr(v, "ax_capped", 0) and (f"accessibility walk capped at {v.ax_capped} elements; pass app= to narrow"),
     )
     return [
         f"capture mode={v.cap.mode} {v.width}x{v.height}"