fix: incorrect resolution of the /messages endpoint URL in the SSE client when the FastAPI app is mounted under a base path (e.g., /mcp).

Author: rkondra-eightfold

src/mcp/client/sse.py

@@ -14,8 +14,31 @@
 logger = logging.getLogger(__name__)
 
 
+# TODO: move these to utils/url_utils.py
+def get_origin(url: str) -> str:
+    parsed_url = urlparse(url)
+    return f"{parsed_url.scheme}://{parsed_url.netloc}"
+
+
+def get_path(url: str) -> str:
+    parsed_url = urlparse(url)
+    return parsed_url.path
+
+
+def get_endpoint_url(
+    base_url: str, sse_relative_url: str, server_mount_path: str = ""
+) -> str:
+    endpoint_url = urljoin(base_url, sse_relative_url)
+    if server_mount_path:
+        origin, path = get_origin(endpoint_url), get_path(endpoint_url)
+        endpoint_url = urljoin(
+            f"{origin}/{server_mount_path.strip('/')}/", path.lstrip("/")
+        )
+    return endpoint_url
+
+
 def remove_request_params(url: str) -> str:
-    return urljoin(url, urlparse(url).path)
+    return urljoin(url, get_path(url))
 
 
 @asynccontextmanager
@@ -24,12 +47,16 @@ async def sse_client(
     headers: dict[str, Any] | None = None,
     timeout: float = 5,
     sse_read_timeout: float = 60 * 5,
+    server_mount_path: str = "",
 ):
     """
     Client transport for SSE.
 
     `sse_read_timeout` determines how long (in seconds) the client will wait for a new
     event before disconnecting. All other HTTP operations are controlled by `timeout`.
+
+    `server_mount_path` provides the relative mount path of the MCP server
+    (used if it is mounted relatively on another ASGI server).
     """
     read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
     read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]
@@ -61,18 +88,15 @@ async def sse_reader(
                                 logger.debug(f"Received SSE event: {sse.event}")
                                 match sse.event:
                                     case "endpoint":
-                                        endpoint_url = urljoin(url, sse.data)
+                                        endpoint_url = get_endpoint_url(
+                                            base_url=url,
+                                            sse_relative_url=sse.data,
+                                            server_mount_path=server_mount_path,
+                                        )
                                         logger.info(
                                             f"Received endpoint URL: {endpoint_url}"
                                         )
-
-                                        url_parsed = urlparse(url)
-                                        endpoint_parsed = urlparse(endpoint_url)
-                                        if (
-                                            url_parsed.netloc != endpoint_parsed.netloc
-                                            or url_parsed.scheme
-                                            != endpoint_parsed.scheme
-                                        ):
+                                        if get_origin(url) != get_origin(endpoint_url):
                                             error_msg = (
                                                 "Endpoint origin does not match "
                                                 f"connection origin: {endpoint_url}"