diff --git a/src/mcp/server/fastmcp/server.py b/src/mcp/server/fastmcp/server.py
index 3282baae6..e1a1e85f0 100644
--- a/src/mcp/server/fastmcp/server.py
+++ b/src/mcp/server/fastmcp/server.py
@@ -790,6 +790,12 @@ async def handle_streamable_http(
         middleware: list[Middleware] = []
         required_scopes = []
 
+        # Always mount both /mcp and /mcp/ for full compatibility, regardless of default
+        # Verify that _main_path has root format -> /mcp
+        _main_path = self.settings.streamable_http_path.removesuffix("/")
+        # Format _alt_path so it ends with '/' -> /mcp/
+        _alt_path  = _main_path + "/"
+
         # Add auth endpoints if auth provider is configured
         if self._auth_server_provider:
             assert self.settings.auth
@@ -815,19 +821,27 @@ async def handle_streamable_http(
                     revocation_options=self.settings.auth.revocation_options,
                 )
             )
-            routes.append(
+            routes.extend([
                 Mount(
-                    self.settings.streamable_http_path,
+                    _main_path,
                     app=RequireAuthMiddleware(handle_streamable_http, required_scopes),
-                )
+                ),
+                Mount(
+                    _alt_path,
+                    app=RequireAuthMiddleware(handle_streamable_http, required_scopes),
+                )]
             )
         else:
             # Auth is disabled, no wrapper needed
-            routes.append(
+            routes.extend([
                 Mount(
-                    self.settings.streamable_http_path,
+                    _main_path,
                     app=handle_streamable_http,
-                )
+                ),
+                Mount(
+                    _alt_path,
+                    app=handle_streamable_http,
+                )]
             )
 
         routes.extend(self._custom_starlette_routes)
@@ -1051,4 +1065,4 @@ async def warning(self, message: str, **extra: Any) -> None:
 
     async def error(self, message: str, **extra: Any) -> None:
         """Send an error log message."""
-        await self.log("error", message, **extra)
+        await self.log("error", message, **extra)
\ No newline at end of file