fix: use normalized path for sse message endpoint

Author: adamsaimi

src/mcp/server/fastmcp/server.py

@@ -742,7 +742,7 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):
             )
             routes.append(
                 Mount(
-                    self.settings.message_path,
+                    normalized_message_endpoint,
                     app=RequireAuthMiddleware(sse.handle_post_message, required_scopes),
                 )
             )
@@ -762,7 +762,7 @@ async def sse_endpoint(request: Request) -> Response:
             )
             routes.append(
                 Mount(
-                    self.settings.message_path,
+                    normalized_message_endpoint,
                     app=sse.handle_post_message,
                 )
             )

tests/issues/test_783_mount_path_sse_message.py

@@ -0,0 +1,14 @@
+import pytest
+
+from mcp.server.fastmcp.server import FastMCP
+
+
+@pytest.mark.parametrize("mount_path", ["/sse", "/custom/mount/path", None])
+async def test_mount_path_included_in_sse_message_path(mount_path):
+    """Test that the mount path is included in the SSE message path when specified."""
+    app = FastMCP("Test App")
+    starlette_app = app.sse_app(mount_path)
+    
+    message_mount = starlette_app.routes[1]
+    
+    assert message_mount.path == f"{mount_path}/message" if mount_path else "/message" # type: ignore