feat(cli): add ACP host input requests

Author: benjaminwestern

docs/cli/acp-mode.md

@@ -52,6 +52,58 @@ and Gemini CLI (the server).
 The core of the ACP implementation can be found in
 `packages/cli/src/acp/acpClient.ts`.
 
+## Host input extensions
+
+ACP covers prompts, cancellations, permissions, and session control. Gemini CLI
+also exposes a Gemini-specific ACP extension for structured host input so an ACP
+client can surface questions in its own UI instead of handing control back to
+the terminal.
+
+During `initialize`, Gemini CLI advertises the extension in
+`agentCapabilities._meta.geminiCli.hostInput`:
+
+```json
+{
+  "version": 1,
+  "requestUserInput": true,
+  "method": "gemini/requestUserInput",
+  "supportedKinds": ["ask_user", "exit_plan_mode"]
+}
+```
+
+To enable host input, the ACP client must advertise
+`clientCapabilities._meta.geminiCli.hostInput` with `requestUserInput: true`.
+The client can also narrow support by setting `supportedKinds`.
+
+- Include `ask_user` to let Gemini CLI surface `ask_user` tool requests over
+  ACP.
+- Include `exit_plan_mode` to let Gemini CLI surface plan-approval questions
+  over ACP.
+- Omit `ask_user` if you want to keep `ask_user` disabled in ACP mode while
+  still supporting other host-input request kinds.
+
+If the client doesn't opt in, Gemini CLI keeps `ask_user` excluded in ACP mode.
+This preserves the default ACP behavior and avoids opening host-driven dialogs
+unless the client has explicitly implemented them.
+
+When Gemini CLI needs host input, it calls `gemini/requestUserInput` as an ACP
+extension request. The request includes:
+
+- `sessionId` for the active ACP session
+- `requestId` for the host-input interaction
+- `kind`, such as `ask_user` or `exit_plan_mode`
+- `title` and `questions`
+- optional `extraParts`
+- optional `toolCall` metadata
+
+The client responds with either:
+
+- `{"outcome":"submitted","answers":{"0":"..."}}`
+- `{"outcome":"cancelled"}`
+
+This extension is separate from MCP. Use it when you want Gemini CLI to keep
+owning the tool flow while your ACP host owns the user-facing input surface.
+
 ### Extending with MCP
 
 ACP can be used with the Model Context Protocol (MCP). This lets an ACP client
@@ -78,7 +130,7 @@ control Gemini CLI.
 ### Core methods
 
 - `initialize`: Establishes the initial connection and lets the client to
-  register its MCP server.
+  register its MCP server and negotiate Gemini-specific ACP extensions.
 - `authenticate`: Authenticates the user.
 - `newSession`: Starts a new chat session.
 - `loadSession`: Loads a previous session.

docs/tools/ask-user.md

@@ -33,13 +33,81 @@ confirmation.
   - Presents an interactive dialog to the user with the specified questions.
   - Pauses execution until the user provides answers or dismisses the dialog.
   - Returns the user's answers to the model.
+  - In ACP mode, Gemini CLI keeps `ask_user` disabled unless the ACP client
+    explicitly opts in to Gemini CLI host-input requests.
 
 - **Output (`llmContent`):** A JSON string containing the user's answers,
   indexed by question position (e.g.,
   `{"answers":{"0": "Option A", "1": "Some text"}}`).
 
 - **Confirmation:** Yes. The tool inherently involves user interaction.
 
+## ACP mode
+
+In ACP mode, Gemini CLI doesn't assume that the host client can handle
+interactive user questions. To preserve existing ACP behavior, Gemini CLI
+excludes `ask_user` unless the host explicitly advertises support.
+
+To enable `ask_user` over ACP, the host client must do all of the following:
+
+1. Set `clientCapabilities._meta.geminiCli.hostInput.requestUserInput` to
+   `true`.
+2. Include `ask_user` in
+   `clientCapabilities._meta.geminiCli.hostInput.supportedKinds`, or omit
+   `supportedKinds` entirely.
+3. Handle the `gemini/requestUserInput` ACP extension request and return either
+   submitted answers or cancellation.
+
+If the host omits `ask_user` from `supportedKinds`, Gemini CLI keeps the tool
+disabled in ACP mode. This lets a client support other host-input request kinds
+without taking on `ask_user`.
+
+When enabled, Gemini CLI sends the same question payload that the terminal UI
+uses. The ACP extension request looks like this:
+
+```json
+{
+  "sessionId": "session-123",
+  "requestId": "ask_user-456",
+  "kind": "ask_user",
+  "title": "Ask User",
+  "questions": [
+    {
+      "header": "Database",
+      "question": "Which database would you like to use?",
+      "type": "choice",
+      "options": [
+        {
+          "label": "PostgreSQL",
+          "description": "Powerful, open source object-relational database system."
+        },
+        {
+          "label": "SQLite",
+          "description": "C-library that implements a SQL database engine."
+        }
+      ]
+    }
+  ]
+}
+```
+
+The ACP client responds with one of these payloads:
+
+```json
+{
+  "outcome": "submitted",
+  "answers": {
+    "0": "PostgreSQL"
+  }
+}
+```
+
+```json
+{
+  "outcome": "cancelled"
+}
+```
+
 ## Usage Examples
 
 ### Multiple Choice Question