Merge pull request #1635 from jlowin/claude/issue-1625-20250826-0138

Reuse session for `StdioTransport` in `Client.new`

src/fastmcp/client/client.py

@@ -54,6 +54,7 @@ from .transports import (
     PythonStdioTransport,
     SessionKwargs,
     SSETransport,
+    StdioTransport,
     StreamableHttpTransport,
     infer_transport,
 )
@@ -337,11 +338,11 @@ class Client(Generic[ClientTransportT]):
                 await fresh_client.call_tool("some_tool", {})
             ```
         """
-
         new_client = copy.copy(self)
 
-        # Reset session state to fresh state
-        new_client._session_state = ClientSessionState()
+        if not isinstance(self.transport, StdioTransport):
+            # Reset session state to fresh state
+            new_client._session_state = ClientSessionState()
 
         new_client.name += f":{secrets.token_hex(2)}"
 
@@ -394,6 +395,7 @@ class Client(Generic[ClientTransportT]):
                 self._session_state.session_task is None
                 or self._session_state.session_task.done()
             )
+
             if need_to_start:
                 if self._session_state.nesting_counter != 0:
                     raise RuntimeError(
@@ -419,6 +421,7 @@ class Client(Generic[ClientTransportT]):
                     ) from exception
 
             self._session_state.nesting_counter += 1
+
         return self
 
     async def _disconnect(self, force: bool = False):

src/fastmcp/utilities/mcp_config.py

@@ -51,6 +51,7 @@ def mcp_server_type_to_servers_and_transports(
         client: ProxyClient[StreamableHttpTransport | SSETransport | StdioTransport] = (
             ProxyClient(transport=transport, name=client_name)
         )
+
         server = FastMCP.as_proxy(name=server_name, backend=client)
 
     return name, server, transport

tests/client/test_stdio.py

@@ -7,7 +7,7 @@ import weakref
 import psutil
 import pytest
 
-from fastmcp import Client
+from fastmcp import Client, FastMCP
 from fastmcp.client.transports import PythonStdioTransport, StdioTransport
 
 
@@ -24,6 +24,44 @@ def gc_collect_harder():
     gc.collect()
 
 
+class TestParallelCalls:
+    @pytest.fixture
+    def stdio_script(self, tmp_path):
+        script = inspect.cleandoc('''
+            import os
+            from fastmcp import FastMCP
+
+            mcp = FastMCP()
+
+            @mcp.tool
+            def pid() -> int:
+                """Gets PID of server"""
+                return os.getpid()
+
+            if __name__ == "__main__":
+                mcp.run()
+            ''')
+        script_file = tmp_path / "stdio.py"
+        script_file.write_text(script)
+        return script_file
+
+    async def test_parallel_calls(self, stdio_script):
+        backend_transport = PythonStdioTransport(script_path=stdio_script)
+        backend_client = Client(transport=backend_transport)
+
+        proxy = FastMCP.as_proxy(backend=backend_client, name="PROXY")
+
+        count = 10
+
+        tasks = [proxy.get_tools() for _ in range(count)]
+
+        results = await asyncio.gather(*tasks, return_exceptions=True)
+
+        assert len(results) == count
+        errors = [result for result in results if isinstance(result, Exception)]
+        assert len(errors) == 0
+
+
 class TestKeepAlive:
     # https://github.com/jlowin/fastmcp/issues/581
 

tests/test_mcp_config.py

@@ -243,6 +243,50 @@ async def test_multi_client(tmp_path: Path):
         assert result_2.data == 3
 
 
+async def test_multi_client_parallel_calls(tmp_path: Path):
+    server_script = inspect.cleandoc("""
+        from fastmcp import FastMCP
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        def add(a: int, b: int) -> int:
+            return a + b
+
+        if __name__ == '__main__':
+            mcp.run()
+        """)
+
+    script_path = tmp_path / "test.py"
+    script_path.write_text(server_script)
+
+    config = {
+        "mcpServers": {
+            "test_1": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+            "test_2": {
+                "command": "python",
+                "args": [str(script_path)],
+            },
+        }
+    }
+
+    client = Client(config)
+
+    async with client:
+        _ = await client.list_tools()
+
+        tasks = [client.list_tools() for _ in range(40)]
+
+        results = await asyncio.gather(*tasks, return_exceptions=True)
+        exceptions = [result for result in results if isinstance(result, Exception)]
+        assert len(exceptions) == 0
+        assert len(results) == 40
+        assert all(len(result) == 2 for result in results)
+
+
 @pytest.mark.skipif(
     running_under_debugger() or sys.platform.startswith("win32"),
     reason="Debugger holds a reference to the transport; Windows has process lifecycle issues",