docs(plan): add documentation for plan mode tools (#18827)

docs/tools/index.md

@@ -86,6 +86,7 @@ Gemini CLI's built-in tools can be broadly categorized as follows:
   information across sessions.
 - **[Todo Tool](./todos.md) (`write_todos`):** For managing subtasks of complex
   requests.
+- **[Planning Tools](./planning.md):** For entering and exiting Plan Mode.
 
 Additionally, these tools incorporate:
 

docs/tools/planning.md

@@ -0,0 +1,55 @@
+# Gemini CLI planning tools
+
+Planning tools allow the Gemini model to switch into a safe, read-only "Plan
+Mode" for researching and planning complex changes, and to signal the
+finalization of a plan to the user.
+
+## 1. `enter_plan_mode` (EnterPlanMode)
+
+`enter_plan_mode` switches the CLI to Plan Mode. This tool is typically called
+by the agent when you ask it to "start a plan" using natural language. In this
+mode, the agent is restricted to read-only tools to allow for safe exploration
+and planning.
+
+- **Tool name:** `enter_plan_mode`
+- **Display name:** Enter Plan Mode
+- **File:** `enter-plan-mode.ts`
+- **Parameters:**
+  - `reason` (string, optional): A short reason explaining why the agent is
+    entering plan mode (e.g., "Starting a complex feature implementation").
+- **Behavior:**
+  - Switches the CLI's approval mode to `PLAN`.
+  - Notifies the user that the agent has entered Plan Mode.
+- **Output (`llmContent`):** A message indicating the switch, e.g.,
+  `Switching to Plan mode.`
+- **Confirmation:** Yes. The user is prompted to confirm entering Plan Mode.
+
+## 2. `exit_plan_mode` (ExitPlanMode)
+
+`exit_plan_mode` signals that the planning phase is complete. It presents the
+finalized plan to the user and requests approval to start the implementation.
+
+- **Tool name:** `exit_plan_mode`
+- **Display name:** Exit Plan Mode
+- **File:** `exit-plan-mode.ts`
+- **Parameters:**
+  - `plan_path` (string, required): The path to the finalized Markdown plan
+    file. This file MUST be located within the project's temporary plans
+    directory (e.g., `~/.gemini/tmp/<project>/plans/`).
+- **Behavior:**
+  - Validates that the `plan_path` is within the allowed directory and that the
+    file exists and has content.
+  - Presents the plan to the user for review.
+  - If the user approves the plan:
+    - Switches the CLI's approval mode to the user's chosen approval mode (
+      `DEFAULT` or `AUTO_EDIT`).
+    - Marks the plan as approved for implementation.
+  - If the user rejects the plan:
+    - Stays in Plan Mode.
+    - Returns user feedback to the model to refine the plan.
+- **Output (`llmContent`):**
+  - On approval: A message indicating the plan was approved and the new approval
+    mode.
+  - On rejection: A message containing the user's feedback.
+- **Confirmation:** Yes. Shows the finalized plan and asks for user approval to
+  proceed with implementation.