fix: patch fastmcp lowlevel session method

We need this until jlowin/fastmcp#2531 is
accepted.

Author: wzxxing

mcp_proxy_for_aws/__init__.py

@@ -16,6 +16,8 @@
 
 from importlib.metadata import version as _metadata_version
 
+import mcp_proxy_for_aws.fastmcp_patch as _fastmcp_patch
+
 
 __all__ = ['__version__']
 __version__ = _metadata_version('mcp-proxy-for-aws')

mcp_proxy_for_aws/fastmcp_patch.py

@@ -0,0 +1,31 @@
+import fastmcp.server.low_level as low_level_module
+import mcp.types
+from functools import wraps
+from mcp import McpError
+from mcp.server.stdio import stdio_server as stdio_server
+from mcp.shared.session import RequestResponder
+
+
+original_receive_request = low_level_module.MiddlewareServerSession._received_request
+
+
+@wraps(original_receive_request)
+async def _received_request(
+    self,
+    responder: RequestResponder[mcp.types.ClientRequest, mcp.types.ServerResult],
+):
+    """Monkey patch fastmcp so that the initialize error from the middleware can be send back to the client."""
+    if isinstance(responder.request.root, mcp.types.InitializeRequest):
+        try:
+            return await original_receive_request(self, responder)
+        except McpError as e:
+            if not responder._completed:
+                with responder:
+                    return await responder.respond(e.error)
+
+            raise e
+    else:
+        return await original_receive_request(self, responder)
+
+
+low_level_module.MiddlewareServerSession._received_request = _received_request