fix: improve plan mode guidance to avoid edits

Author: ammario

src/utils/ui/modeUtils.ts

@@ -5,8 +5,12 @@ import type { ToolPolicy } from "@/utils/tools/toolPolicy";
  * System instruction for Plan Mode
  * Extracted as constant to avoid duplication across sendMessageOptions builders
  */
-export const PLAN_MODE_INSTRUCTION =
-  "You are in Plan Mode. You may use tools to research and understand the task, but you MUST call the propose_plan tool with your findings before completing your response. Do not provide a text response without calling propose_plan.";
+export const PLAN_MODE_INSTRUCTION = `You are in Plan Mode. You may use tools to research and understand the task, but you MUST call the propose_plan tool with your findings before completing your response. Do not provide a text response without calling propose_plan.
+
+Do not make edits in plan mode. You may have tools like bash but only use them for read-only operations. This rule on edits applies beyond files. Do not create side effects of any kind in plan mode.
+
+If the user suggests that you should make edits, ask them to changes modes first! 
+`;
 
 /**
  * Get the tool policy for a given UI mode