fix(mcp): reset stdio session stack state on cleanup

src/agents/mcp/server.py

@@ -638,6 +638,7 @@ async def get_prompt(
     async def cleanup(self):
         """Cleanup the server."""
         async with self._cleanup_lock:
+            cleanup_cancelled = False
             # Only raise HTTP errors if we're cleaning up after a failed connection.
             # During normal teardown (via __aexit__), log but don't raise to avoid
             # masking the original exception.
@@ -646,6 +647,7 @@ async def cleanup(self):
             try:
                 await self.exit_stack.aclose()
             except asyncio.CancelledError as e:
+                cleanup_cancelled = True
                 logger.debug(f"Cleanup cancelled for MCP server '{self.name}': {e}")
                 raise
             except BaseExceptionGroup as eg:
@@ -709,7 +711,12 @@ async def cleanup(self):
                 else:
                     logger.error(f"Error cleaning up server: {e}")
             finally:
-                self.session = None
+                if not cleanup_cancelled:
+                    # Reset stack state only after a completed cleanup. If cleanup is cancelled,
+                    # keep the existing stack so a follow-up cleanup can finish unwinding it.
+                    self.exit_stack = AsyncExitStack()
+                    self.session = None
+                    self.server_initialize_result = None
 
 
 class MCPServerStdioParams(TypedDict):

src/agents/result.py

@@ -8,6 +8,9 @@
 from dataclasses import InitVar, dataclass, field
 from typing import Any, Literal, TypeVar, cast
 
+from pydantic import GetCoreSchemaHandler
+from pydantic_core import core_schema
+
 from .agent import Agent
 from .agent_output import AgentOutputSchemaBase
 from .exceptions import (
@@ -124,6 +127,16 @@ class RunResultBase(abc.ABC):
     _trace_state: TraceState | None = field(default=None, init=False, repr=False)
     """Serialized trace metadata captured during the run."""
 
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls,
+        _source_type: Any,
+        _handler: GetCoreSchemaHandler,
+    ) -> core_schema.CoreSchema:
+        # RunResult objects are runtime values; schema generation should treat them as instances
+        # instead of recursively traversing internal dataclass annotations.
+        return core_schema.is_instance_schema(cls)
+
     @property
     @abc.abstractmethod
     def last_agent(self) -> Agent[Any]:

tests/mcp/test_connect_disconnect.py

@@ -1,3 +1,4 @@
+import asyncio
 from unittest.mock import AsyncMock, patch
 
 import pytest
@@ -8,6 +9,28 @@
 from .helpers import DummyStreamsContextManager, tee
 
 
+class CountingStreamsContextManager:
+    def __init__(self, counter: dict[str, int]):
+        self.counter = counter
+
+    async def __aenter__(self):
+        self.counter["enter"] += 1
+        return (object(), object())
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb):
+        self.counter["exit"] += 1
+
+
+class CancelThenCloseExitStack:
+    def __init__(self):
+        self.close_calls = 0
+
+    async def aclose(self):
+        self.close_calls += 1
+        if self.close_calls == 1:
+            raise asyncio.CancelledError("first cleanup interrupted")
+
+
 @pytest.mark.asyncio
 @patch("mcp.client.stdio.stdio_client", return_value=DummyStreamsContextManager())
 @patch("mcp.client.session.ClientSession.initialize", new_callable=AsyncMock, return_value=None)
@@ -67,3 +90,62 @@ async def test_manual_connect_disconnect_works(
 
     await server.cleanup()
     assert server.session is None, "Server should be disconnected"
+
+
+@pytest.mark.asyncio
+@patch("agents.mcp.server.ClientSession.initialize", new_callable=AsyncMock, return_value=None)
+@patch("agents.mcp.server.stdio_client")
+async def test_cleanup_resets_exit_stack_and_reconnects(
+    mock_stdio_client: AsyncMock, mock_initialize: AsyncMock
+):
+    counter = {"enter": 0, "exit": 0}
+    mock_stdio_client.side_effect = lambda params: CountingStreamsContextManager(counter)
+
+    server = MCPServerStdio(
+        params={
+            "command": tee,
+        },
+        cache_tools_list=True,
+    )
+
+    await server.connect()
+    original_exit_stack = server.exit_stack
+
+    await server.cleanup()
+    assert server.session is None
+    assert server.exit_stack is not original_exit_stack
+    assert server.server_initialize_result is None
+    assert counter == {"enter": 1, "exit": 1}
+
+    await server.connect()
+    await server.cleanup()
+    assert counter == {"enter": 2, "exit": 2}
+
+
+@pytest.mark.asyncio
+async def test_cleanup_cancellation_preserves_exit_stack_for_retry():
+    server = MCPServerStdio(
+        params={
+            "command": tee,
+        },
+        cache_tools_list=True,
+    )
+    cancelled_exit_stack = CancelThenCloseExitStack()
+
+    server.exit_stack = cancelled_exit_stack  # type: ignore[assignment]
+    server.session = object()  # type: ignore[assignment]
+    server.server_initialize_result = object()  # type: ignore[assignment]
+
+    with pytest.raises(asyncio.CancelledError):
+        await server.cleanup()
+
+    assert id(server.exit_stack) == id(cancelled_exit_stack)
+    assert server.session is not None
+    assert server.server_initialize_result is not None
+
+    await server.cleanup()
+
+    assert cancelled_exit_stack.close_calls == 2
+    assert id(server.exit_stack) != id(cancelled_exit_stack)
+    assert server.session is None
+    assert server.server_initialize_result is None

tests/test_result_cast.py

@@ -7,7 +7,7 @@
 
 import pytest
 from openai.types.responses import ResponseOutputMessage, ResponseOutputText
-from pydantic import BaseModel
+from pydantic import BaseModel, ConfigDict
 
 from agents import (
     Agent,
@@ -45,6 +45,16 @@ class Foo(BaseModel):
     bar: int
 
 
+def test_run_result_streaming_supports_pydantic_model_rebuild() -> None:
+    class StreamingRunContainer(BaseModel):
+        query_id: str
+        run_stream: RunResultStreaming | None
+
+        model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    StreamingRunContainer.model_rebuild()
+
+
 def _create_message(text: str) -> ResponseOutputMessage:
     return ResponseOutputMessage(
         id="msg",