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/host_agent/assignment.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from collections.abc import Callable
 from dataclasses import dataclass, field
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 from cloud.internal_api.models import AssignmentModel
 from core.models import Task
@@ -11,6 +11,9 @@ from host_agent.planner_context import bind_planner_execution_context
 from host_agent.progress import TaskProgressHolder, TaskProgressSnapshot
 from runtime.task import is_cancellation_reason
 
+if TYPE_CHECKING:
+    from host_agent.mcp_lock import McpBusyTracker
+
 
 @dataclass(frozen=True)
 class AssignmentExecutionResult:
@@ -20,9 +23,15 @@ class AssignmentExecutionResult:
 
 
 class AssignmentExecutor:
-    def __init__(self, factories: ExecutionFactories) -> None:
+    def __init__(
+        self,
+        factories: ExecutionFactories,
+        *,
+        mcp_busy_tracker: McpBusyTracker | None = None,
+    ) -> None:
         self.factories = factories
         self._progress = TaskProgressHolder()
+        self._mcp_busy_tracker = mcp_busy_tracker
 
     def latest_progress(self) -> TaskProgressSnapshot | None:
         """Latest step progress reported by the currently-running assignment."""
@@ -36,6 +45,16 @@ class AssignmentExecutor:
         stop_reason: Callable[[], str | None] | None = None,
     ) -> AssignmentExecutionResult:
         self._progress.clear()
+        if self._mcp_busy_tracker is not None and (
+            assignment.device_id in self._mcp_busy_tracker.busy_device_ids()
+        ):
+            return AssignmentExecutionResult(
+                status="failed",
+                failure_reason=(
+                    f"device {assignment.device_id} is held by an active "
+                    "MCP session"
+                ),
+            )
         with bind_planner_execution_context(assignment):
             if should_stop is not None and should_stop():
                 reason = stop_reason() if stop_reason is not None else None