Fix SSE client handling of nested path URLs

src/mcp/client/sse.py

@@ -1,5 +1,4 @@
 import logging
-import re
 from contextlib import asynccontextmanager
 from typing import Any
 from urllib.parse import urljoin, urlparse
@@ -62,21 +61,12 @@ async def sse_reader(
                                 logger.debug(f"Received SSE event: {sse.event}")
                                 match sse.event:
                                     case "endpoint":
-                                        url_parsed = urlparse(url)
-
-                                        base_path = re.search(
-                                            r"https?://[^/]+/(.+?)(?:/mcp)?/sse$", url
-                                        )
-                                        base_path = (
-                                            base_path.group(1) if base_path else ""
-                                        )
-                                        endpoint_url = urljoin(
-                                            url_parsed.scheme + "://" + url_parsed.netloc,  # noqa: E501
-                                            base_path + sse.data
-                                        )
+                                        endpoint_url = urljoin(url, sse.data)
                                         logger.info(
                                             f"Received endpoint URL: {endpoint_url}"
                                         )
+
+                                        url_parsed = urlparse(url)
 
                                         endpoint_parsed = urlparse(endpoint_url)
                                         if (

src/mcp/server/sse.py

@@ -37,6 +37,7 @@ async def handle_sse(request):
 from urllib.parse import quote
 from uuid import UUID, uuid4
 
+import re
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import ValidationError
@@ -95,7 +96,11 @@ 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_id={session_id.hex}"
+
         self._read_stream_writers[session_id] = read_stream_writer
         logger.debug(f"Created new session with ID: {session_id}")