microsoft · moonbox3 · Jun 4, 2026 · May 12, 2026 · Jun 4, 2026
diff --git a/python/packages/core/agent_framework/_harness/_background_agents.py b/python/packages/core/agent_framework/_harness/_background_agents.py
@@ -349,6 +349,8 @@ def background_agents_start_task(agent_name: str, input: str, description: str)
             _save_provider_state(session, provider_state, source_id=source_id)
             return f"Background task {task_id} started on agent '{agent_name}'."
 
+        background_agents_start_task._invoke_sync_on_event_loop = True  # pyright: ignore[reportPrivateUsage]
+
         @tool(name="background_agents_wait_for_first_completion", approval_mode="never_require")
         async def background_agents_wait_for_first_completion(task_ids: list[int]) -> str:
             """Block until the first of the specified background tasks completes. Returns the completed task's ID."""
@@ -471,6 +473,8 @@ def background_agents_continue_task(task_id: int, text: str) -> str:
             _save_provider_state(session, provider_state, source_id=source_id)
             return f"Task {task_id} continued with new input."
 
+        background_agents_continue_task._invoke_sync_on_event_loop = True  # pyright: ignore[reportPrivateUsage]
+
         @tool(name="background_agents_clear_completed_task", approval_mode="never_require")
         def background_agents_clear_completed_task(task_id: int) -> str:
             """Remove a completed or failed task and release its session to free memory."""

diff --git a/python/packages/core/agent_framework/_tools.py b/python/packages/core/agent_framework/_tools.py
@@ -292,6 +292,7 @@ class WeatherArgs(BaseModel):
         "_cached_parameters",
         "_input_schema",
         "_schema_supplied",
+        "_invoke_sync_on_event_loop",
     }
 
     def __init__(
@@ -366,6 +367,7 @@ def __init__(
         self.description = description
         self.kind = kind
         self.additional_properties = additional_properties
+        self._invoke_sync_on_event_loop = False
         for key, value in kwargs.items():
             setattr(self, key, value)
 
@@ -537,6 +539,16 @@ def __call__(self, *args: Any, **kwargs: Any) -> Any:
             self.invocation_exception_count += 1
             raise
 
+    async def _invoke_function(self, call_kwargs: Mapping[str, Any]) -> Any:
+        """Run sync tools off the event loop during async invocation."""
+        func = self.func.func if isinstance(self.func, FunctionTool) else self.func
+        if inspect.iscoroutinefunction(func) or getattr(self, "_invoke_sync_on_event_loop", False):
+            res = self.__call__(**call_kwargs)
+            return await res if inspect.isawaitable(res) else res
+
+        res = await asyncio.to_thread(self.__call__, **call_kwargs)
+        return await res if inspect.isawaitable(res) else res
+
     @overload
     async def invoke(
         self,
@@ -679,8 +691,7 @@ async def invoke(
         if not OBSERVABILITY_SETTINGS.ENABLED:  # type: ignore[name-defined]
             logger.info(f"Function name: {self.name}")
             logger.debug(f"Function arguments: {observable_kwargs}")
-            res = self.__call__(**call_kwargs)
-            result = await res if inspect.isawaitable(res) else res
+            result = await self._invoke_function(call_kwargs)
             if skip_parsing:
                 logger.info(f"Function {self.name} succeeded.")
                 logger.debug(f"Function result: {type(result).__name__}")
@@ -730,8 +741,7 @@ async def invoke(
             start_time_stamp = perf_counter()
             end_time_stamp: float | None = None
             try:
-                res = self.__call__(**call_kwargs)
-                result = await res if inspect.isawaitable(res) else res
+                result = await self._invoke_function(call_kwargs)
                 end_time_stamp = perf_counter()
             except Exception as exception:
                 end_time_stamp = perf_counter()

diff --git a/python/packages/core/tests/core/test_tools.py b/python/packages/core/tests/core/test_tools.py
@@ -1,4 +1,6 @@
 # Copyright (c) Microsoft. All rights reserved.
+import asyncio
+import threading
 from typing import Annotated, Any, Literal, get_args, get_origin
 from unittest.mock import Mock
 
@@ -1346,6 +1348,45 @@ async def slow(x: int) -> int:
     assert raw == 42
 
 
+async def test_invoke_sync_tool_does_not_block_event_loop() -> None:
+    release_tool = threading.Event()
+    tool_thread_ids: list[int] = []
+    event_loop_thread_id = threading.get_ident()
+
+    @tool
+    def wait_for_release() -> str:
+        tool_thread_ids.append(threading.get_ident())
+        return "released" if release_tool.wait(timeout=0.2) else "timed out"
+
+    async def release_soon() -> None:
+        await asyncio.sleep(0.01)
+        release_tool.set()
+
+    tool_task = asyncio.create_task(wait_for_release.invoke(skip_parsing=True))
+    release_task = asyncio.create_task(release_soon())
+
+    assert await asyncio.wait_for(tool_task, timeout=1) == "released"
+    await release_task
+    assert tool_thread_ids
+    assert tool_thread_ids[0] != event_loop_thread_id
+
+
+async def test_invoke_sync_tool_can_stay_on_event_loop() -> None:
+    event_loop_thread_id = threading.get_ident()
+    tool_thread_ids: list[int] = []
+
+    @tool
+    def needs_event_loop() -> str:
+        tool_thread_ids.append(threading.get_ident())
+        asyncio.get_running_loop()
+        return "ok"
+
+    needs_event_loop._invoke_sync_on_event_loop = True
+
+    assert await needs_event_loop.invoke(skip_parsing=True) == "ok"
+    assert tool_thread_ids == [event_loop_thread_id]
+
+
 async def test_invoke_skip_parsing_bypasses_configured_result_parser() -> None:
     """The tool's own result_parser is bypassed when skip_parsing=True is requested."""
     parser_calls: list[Any] = []