Fix SSE client handling of nested path URLs

src/mcp/client/sse.py

@@ -1,4 +1,5 @@
 import logging
+import re
 from contextlib import asynccontextmanager
 from typing import Any
 from urllib.parse import urljoin, urlparse
@@ -61,7 +62,15 @@ async def sse_reader(
                                 logger.debug(f"Received SSE event: {sse.event}")
                                 match sse.event:
                                     case "endpoint":
-                                        endpoint_url = urljoin(url, sse.data)
+                                        base_path = re.search(
+                                            r"https?://[^/]+/(.+)/sse$", url
+                                        )
+                                        base_path = (
+                                            base_path.group(1) if base_path else ""
+                                        )
+                                        endpoint_url = urljoin(
+                                            url, base_path + sse.data
+                                        )
                                         logger.info(
                                             f"Received endpoint URL: {endpoint_url}"
                                         )