Fix `observation?.includes is not a function` (#2696)

packages/components/nodes/tools/CustomTool/core.ts

@@ -96,6 +96,9 @@ export class DynamicStructuredTool<
             await runManager?.handleToolError(e)
             throw e
         }
+        if (result && typeof result !== 'string') {
+            result = JSON.stringify(result)
+        }
         await runManager?.handleToolEnd(result)
         return result
     }