diff --git a/mcp_browser/proxy.py b/mcp_browser/proxy.py
index 7e43288..79e2ba6 100644
--- a/mcp_browser/proxy.py
+++ b/mcp_browser/proxy.py
@@ -116,9 +116,30 @@ class MCPBrowser:
         
         # Determine which server to use
         server_name = self._server_name or self.config.default_server
-        if not server_name or server_name not in self.config.servers:
+        if not server_name:
+            raise ValueError("No default MCP server configured")
+        
+        if server_name not in self.config.servers:
             raise ValueError(f"Server '{server_name}' not found in configuration")
         
+        # If built-ins are disabled but the selected server is builtin-only,
+        # fall back to the first enabled non-builtin server if available.
+        if (not self._enable_builtin_servers) and server_name == "builtin-only":
+            fallback_name = next(
+                (
+                    name for name, cfg in self.config.servers.items()
+                    if name != "builtin-only" and cfg.enabled
+                ),
+                None
+            )
+            if fallback_name:
+                self.logger.info(
+                    "Built-in servers disabled; falling back to '%s' instead of 'builtin-only'",
+                    fallback_name,
+                )
+                server_name = fallback_name
+                self._server_name = fallback_name
+        
         server_config = self.config.servers[server_name]
         if self._transport_override:
             self._apply_transport_override(server_config)
@@ -418,12 +439,16 @@ class MCPBrowser:
         if self.server:
             tools_response = await self.server.send_request("tools/list", {})
             
-            if "error" in tools_response:
-                raise RuntimeError(f"Failed to list tools: {tools_response['error']}")
-                
-            # Update registry with tools
-            if "result" in tools_response and "tools" in tools_response["result"]:
-                self.registry.update_tools(tools_response["result"]["tools"])
+            # Normalize response structure
+            tools_data = None
+            if isinstance(tools_response, dict):
+                if "tools" in tools_response:
+                    tools_data = tools_response["tools"]
+                elif "result" in tools_response and isinstance(tools_response["result"], dict):
+                    tools_data = tools_response["result"].get("tools")
+            
+            if tools_data:
+                self.registry.update_tools(tools_data)
         
         # Also get tools from multi-server if enabled
         if self.multi_server: