fix: Retry ClosedResourceError on isolated session (#2711)

Author: elainegan-openai

src/agents/mcp/server.py

@@ -14,6 +14,7 @@
 
 if sys.version_info < (3, 11):
     from exceptiongroup import BaseExceptionGroup  # pyright: ignore[reportMissingImports]
+from anyio import ClosedResourceError
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from mcp import ClientSession, StdioServerParameters, Tool as MCPTool, stdio_client
 from mcp.client.session import MessageHandlerFnT
@@ -1165,7 +1166,15 @@ async def _call_tool_with_session(
         return await session.call_tool(tool_name, arguments, meta=meta)
 
     def _should_retry_in_isolated_session(self, exc: BaseException) -> bool:
-        if isinstance(exc, (asyncio.CancelledError, httpx.ConnectError, httpx.TimeoutException)):
+        if isinstance(
+            exc,
+            (
+                asyncio.CancelledError,
+                ClosedResourceError,
+                httpx.ConnectError,
+                httpx.TimeoutException,
+            ),
+        ):
             return True
         if isinstance(exc, httpx.HTTPStatusError):
             return exc.response.status_code >= 500

tests/mcp/test_client_session_retries.py

@@ -5,6 +5,7 @@
 
 import httpx
 import pytest
+from anyio import ClosedResourceError
 from mcp import ClientSession, Tool as MCPTool
 from mcp.types import CallToolResult, GetPromptResult, ListPromptsResult, ListToolsResult
 
@@ -218,6 +219,15 @@ async def call_tool(self, tool_name, arguments, meta=None):
         raise httpx.TimeoutException(self.message)
 
 
+class ClosedResourceSession:
+    def __init__(self):
+        self.call_tool_attempts = 0
+
+    async def call_tool(self, tool_name, arguments, meta=None):
+        self.call_tool_attempts += 1
+        raise ClosedResourceError()
+
+
 class IsolatedRetrySession:
     def __init__(self):
         self.call_tool_attempts = 0
@@ -304,6 +314,18 @@ async def test_streamable_http_retries_5xx_on_isolated_session():
     assert isolated_session.call_tool_attempts == 1
 
 
+@pytest.mark.asyncio
+async def test_streamable_http_retries_closed_resource_on_isolated_session():
+    isolated_session = IsolatedRetrySession()
+    server = DummyStreamableHttpServer(ClosedResourceSession(), isolated_session)
+    server.max_retry_attempts = 1
+
+    result = await server.call_tool("tool", None)
+
+    assert isinstance(result, CallToolResult)
+    assert isolated_session.call_tool_attempts == 1
+
+
 @pytest.mark.asyncio
 async def test_streamable_http_does_not_retry_4xx_on_isolated_session():
     isolated_session = IsolatedRetrySession()