Avoid stale context leakage when proxying with an already‑connected ProxyClient

src/fastmcp/server/providers/proxy.py

@@ -642,6 +642,17 @@ def _create_client_factory(
     """
     if isinstance(target, Client):
         client = target
+        if client.is_connected() and type(client) is ProxyClient:
+            logger.info(
+                "Proxy detected connected ProxyClient - creating fresh sessions for each "
+                "request to avoid request context leakage."
+            )
+
+            def fresh_client_factory() -> Client:
+                return client.new()
+
+            return fresh_client_factory
+
         if client.is_connected():
             logger.info(
                 "Proxy detected connected client - reusing existing session for all requests. "
@@ -652,12 +663,11 @@ def reuse_client_factory() -> Client:
                 return client
 
             return reuse_client_factory
-        else:
 
-            def fresh_client_factory() -> Client:
-                return client.new()
+        def fresh_client_factory() -> Client:
+            return client.new()
 
-            return fresh_client_factory
+        return fresh_client_factory
     else:
         # target is not a Client, so it's compatible with ProxyClient.__init__
         base_client = ProxyClient(cast(Any, target))

tests/server/providers/proxy/test_proxy_client.py

@@ -17,7 +17,7 @@
 from fastmcp.client.sampling import RequestContext, SamplingMessage, SamplingParams
 from fastmcp.exceptions import ToolError
 from fastmcp.server.elicitation import AcceptedElicitation
-from fastmcp.server.providers.proxy import ProxyClient
+from fastmcp.server.providers.proxy import ProxyClient, _create_client_factory
 
 
 @pytest.fixture
@@ -419,12 +419,20 @@ async def test_client_factory_creates_fresh_sessions(self, fastmcp_server: FastM
         assert hasattr(proxy_via_as_proxy, "_local_provider")
         assert hasattr(proxy_via_factory, "_local_provider")
 
-    async def test_connected_client_reuses_sessions(self, fastmcp_server: FastMCP):
-        """Test that connected clients passed to as_proxy reuse sessions (preserves #959 behavior)."""
-        # Create a connected client (should reuse sessions)
-        async with Client(fastmcp_server) as connected_client:
-            proxy = FastMCP.as_proxy(connected_client)
-
-            # Verify the proxy is created successfully and uses session reuse
-            assert proxy is not None
-            assert hasattr(proxy, "_local_provider")
+    async def test_connected_proxy_client_uses_fresh_sessions(
+        self, fastmcp_server: FastMCP
+    ):
+        """Connected ProxyClient targets should create fresh sessions to avoid stale context."""
+        async with ProxyClient(fastmcp_server) as connected_client:
+            factory = _create_client_factory(connected_client)
+
+            client_a = factory()
+            client_b = factory()
+
+            assert isinstance(client_a, Client)
+            assert isinstance(client_b, Client)
+            assert client_a is not connected_client
+            assert client_b is not connected_client
+            assert client_a is not client_b
+            assert not client_a.is_connected()
+            assert not client_b.is_connected()