Fix SSE client handling of nested path URLs

src/mcp/client/sse.py

@@ -65,8 +65,9 @@ async def sse_reader(
                                         logger.info(
                                             f"Received endpoint URL: {endpoint_url}"
                                         )
-
+                                        
                                         url_parsed = urlparse(url)
+
                                         endpoint_parsed = urlparse(endpoint_url)
                                         if (
                                             url_parsed.netloc != endpoint_parsed.netloc

src/mcp/server/sse.py

@@ -32,6 +32,7 @@ async def handle_sse(request):
 """
 
 import logging
+import re
 from contextlib import asynccontextmanager
 from typing import Any
 from urllib.parse import quote
@@ -95,7 +96,14 @@ async def connect_sse(self, scope: Scope, receive: Receive, send: Send):
         write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 
         session_id = uuid4()
-        session_uri = f"{quote(self._endpoint)}?session_id={session_id.hex}"
+        request_path = scope["path"]
+
+        match = re.match(r"^/([^/]+(?:/mcp)?)/sse$", request_path)
+        mount_prefix = match.group(1) if match else ""
+
+        session_uri = f"/{quote(mount_prefix)}{quote(self._endpoint)}"
+        session_uri += f"?session_id={session_id.hex}"
+
         self._read_stream_writers[session_id] = read_stream_writer
         logger.debug(f"Created new session with ID: {session_id}")