feat(plan): support automatic model switching for Plan Mode (#20240)

docs/cli/plan-mode.md

@@ -27,6 +27,7 @@ implementation. It allows you to:
     - [Example: Allow git commands in Plan Mode](#example-allow-git-commands-in-plan-mode)
     - [Example: Enable research subagents in Plan Mode](#example-enable-research-subagents-in-plan-mode)
   - [Custom Plan Directory and Policies](#custom-plan-directory-and-policies)
+- [Automatic Model Routing](#automatic-model-routing)
 
 ## Enabling Plan Mode
 
@@ -242,6 +243,32 @@ modes = ["plan"]
 argsPattern = "\"file_path\":\"[^\"]+[\\\\/]+\\.gemini[\\\\/]+plans[\\\\/]+[\\w-]+\\.md\""
 ```
 
+## Automatic Model Routing
+
+When using an [**auto model**], Gemini CLI automatically optimizes [**model
+routing**] based on the current phase of your task:
+
+1.  **Planning Phase:** While in Plan Mode, the CLI routes requests to a
+    high-reasoning **Pro** model to ensure robust architectural decisions and
+    high-quality plans.
+2.  **Implementation Phase:** Once a plan is approved and you exit Plan Mode,
+    the CLI detects the existence of the approved plan and automatically
+    switches to a high-speed **Flash** model. This provides a faster, more
+    responsive experience during the implementation of the plan.
+
+This behavior is enabled by default to provide the best balance of quality and
+performance. You can disable this automatic switching in your settings:
+
+```json
+{
+  "general": {
+    "plan": {
+      "modelRouting": false
+    }
+  }
+}
+```
+
 [`list_directory`]: /docs/tools/file-system.md#1-list_directory-readfolder
 [`read_file`]: /docs/tools/file-system.md#2-read_file-readfile
 [`grep_search`]: /docs/tools/file-system.md#5-grep_search-searchtext
@@ -259,3 +286,5 @@ argsPattern = "\"file_path\":\"[^\"]+[\\\\/]+\\.gemini[\\\\/]+plans[\\\\/]+[\\w-
 [YOLO mode]: /docs/reference/configuration.md#command-line-arguments
 [`plan.toml`]:
   https://github.com/google-gemini/gemini-cli/blob/main/packages/core/src/policy/policies/plan.toml
+[auto model]: /docs/reference/configuration.md#model-settings
+[model routing]: /docs/cli/telemetry.md#model-routing