feat(host-agent): fail-fast cloud assignment when MCP holds device

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/device-host-agent/tests/test_assignment.py

@@ -180,6 +180,63 @@ def test_workflow_assignment_maps_cancellation_stop_to_cancelled_status() -> Non
     }
 
 
+def test_execute_fails_fast_when_mcp_session_holds_device() -> None:
+    """Cloud assignment arriving for a device currently held by an MCP
+    session must fail immediately rather than fight for the device."""
+    from host_agent.mcp_lock import McpBusyTracker
+
+    tracker = McpBusyTracker()
+    tracker.acquire("phone-1", "sess-mcp")
+    executor = AssignmentExecutor(
+        _build_factories(),
+        mcp_busy_tracker=tracker,
+    )
+    assignment = _assignment(device_id="phone-1")
+    result = executor.execute(assignment)
+    assert result.status == "failed"
+    assert "MCP" in (result.failure_reason or "")
+
+
+def test_execute_skips_check_when_tracker_is_none() -> None:
+    """Default backward-compat: no tracker → no fail-fast."""
+    executor = AssignmentExecutor(_build_factories())
+    # Without a real workflow store / task runner this test verifies the
+    # entry-point path doesn't raise on the mcp_busy check.
+    # We use a goal + a mock runner factory so execute() runs through.
+    assignment = _assignment()
+    result = executor.execute(assignment)
+    # Should run through normally (not fail on MCP check)
+    assert result.status == "done"
+
+
+def _build_factories() -> ExecutionFactories:
+    """Shared factory fixture used by MCP-hold tests."""
+    received: list[Task] = []
+
+    class FakeTaskRunner:
+        def run(self, task: Task) -> Task:
+            received.append(task)
+            task.status = "completed"
+            return task
+
+    class FakeMetadataStore:
+        def create_task(
+            self,
+            task: Task,
+            *,
+            source_task_id: str | None = None,
+            source_attempt: int | None = None,
+        ) -> None:
+            pass
+
+    return ExecutionFactories(
+        task_runner_factory=lambda: FakeTaskRunner(),  # type: ignore[arg-type,return-value]
+        workflow_runner_factory=lambda: object(),  # type: ignore[arg-type,return-value]
+        workflow_store=object(),  # type: ignore[arg-type]
+        metadata_store=FakeMetadataStore(),  # type: ignore[arg-type]
+    )
+
+
 def test_unknown_workflow_fails_without_running() -> None:
     class FakeWorkflowStore:
         def get_definition(self, definition_id: str):