fix for SSE URL handling when a server name is specified

src/mcp/client/sse.py

@@ -1,7 +1,7 @@
 import logging
 from contextlib import asynccontextmanager
 from typing import Any
-from urllib.parse import urljoin, urlparse
+from urllib.parse import urljoin, urlparse, urlunparse
 
 import anyio
 import httpx
@@ -19,6 +19,31 @@ def remove_request_params(url: str) -> str:
     return urljoin(url, urlparse(url).path)
 
 
+def custom_url_join(base_url: str, endpoint: str) -> str:
+    """
+    Custom URL join function to handle the case where the endpoint is relative
+    to a base URL. This function ensures that the base URL and endpoint are
+    combined correctly, even if the endpoint is not a full URL.
+    """
+    # Parse the base URL
+    parsed_base = urlparse(base_url)
+
+    # Get the path prefix (e.g., '/weather')
+    path_prefix = "/".join(parsed_base.path.split("/")[:-1])
+
+    # Remove any leading slash from the endpoint
+    clean_endpoint = endpoint.lstrip("/")
+
+    # Create the new path by joining prefix and endpoint
+    new_path = f"{path_prefix}/{clean_endpoint}"
+
+    # Create a new parsed URL with the updated path
+    parsed_new = parsed_base._replace(path=new_path)
+
+    # Convert back to a string URL
+    return urlunparse(parsed_new)
+
+
 @asynccontextmanager
 async def sse_client(
     url: str,
@@ -44,6 +69,15 @@ async def sse_client(
     async with anyio.create_task_group() as tg:
         try:
             logger.info(f"Connecting to SSE endpoint: {remove_request_params(url)}")
+
+            # extract MCP server name from URL, for example: https://mcp.example.com/weather/sse
+            # will extract 'weather'
+            path_tokens = urlparse(url).path.split("/")
+            optional_mcp_server_name = (
+                path_tokens[1:-1] if len(path_tokens) > 2 else None
+            )
+            logger.debug(f"MCP Server name (optional): {optional_mcp_server_name}")
+
             async with httpx.AsyncClient(headers=headers) as client:
                 async with aconnect_sse(
                     client,
@@ -62,7 +96,12 @@ async def sse_reader(
                                 logger.debug(f"Received SSE event: {sse.event}")
                                 match sse.event:
                                     case "endpoint":
-                                        endpoint_url = urljoin(url, sse.data)
+                                        if optional_mcp_server_name:
+                                            endpoint_url = custom_url_join(
+                                                url, sse.data
+                                            )
+                                        else:
+                                            endpoint_url = urljoin(url, sse.data)
                                         logger.info(
                                             f"Received endpoint URL: {endpoint_url}"
                                         )

tests/client/test_sse.py

@@ -0,0 +1,30 @@
+import pytest
+
+from mcp.client.sse import custom_url_join
+
+
+@pytest.mark.parametrize(
+    "base_url,endpoint,expected",
+    [
+        # Additional test cases to verify behavior with different URL structures
+        (
+            "https://mcp.example.com/weather/sse",
+            "/messages/?session_id=616df71373444d76bd566df4377c9629",
+            "https://mcp.example.com/weather/messages/?session_id=616df71373444d76bd566df4377c9629",
+        ),
+        (
+            "https://mcp.example.com/weather/clarksburg/sse",
+            "/messages/?session_id=616df71373444d76bd566df4377c9629",
+            "https://mcp.example.com/weather/clarksburg/messages/?session_id=616df71373444d76bd566df4377c9629",
+        ),
+        (
+            "https://mcp.example.com/sse",
+            "/messages/?session_id=616df71373444d76bd566df4377c9629",
+            "https://mcp.example.com/messages/?session_id=616df71373444d76bd566df4377c9629",
+        ),
+    ],
+)
+def test_custom_url_join(base_url, endpoint, expected):
+    """Test the custom_url_join function with messages endpoint and session ID."""
+    result = custom_url_join(base_url, endpoint)
+    assert result == expected