feat(plan): enable AskUser tool in Plan mode for clarifying questions (#17694)

Author: jerop

packages/core/src/core/__snapshots__/prompts.test.ts.snap

@@ -185,7 +185,8 @@ The following read-only tools are available in Plan Mode:
 
 ### Phase 1: Requirements Understanding
 - Analyze the user's request to identify core requirements and constraints
-- If critical information is missing or ambiguous, ask ONE clarifying question at a time
+- If critical information is missing or ambiguous, ask clarifying questions using the \`ask_user\` tool
+- When using \`ask_user\`, prefer providing multiple-choice options for the user to select from when possible
 - Do NOT explore the project or create a plan yet
 
 ### Phase 2: Project Exploration

packages/core/src/core/prompts.test.ts

@@ -274,17 +274,19 @@ describe('Core System Prompt (prompts.ts)', () => {
 
     it('should only list available tools in PLAN mode', () => {
       vi.mocked(mockConfig.getApprovalMode).mockReturnValue(ApprovalMode.PLAN);
-      // Only enable glob and read_file, disable others (like web search)
+      // Only enable a subset of tools, including ask_user
       vi.mocked(mockConfig.getToolRegistry().getAllToolNames).mockReturnValue([
         'glob',
         'read_file',
+        'ask_user',
       ]);
 
       const prompt = getCoreSystemPrompt(mockConfig);
 
       // Should include enabled tools
       expect(prompt).toContain('`glob`');
       expect(prompt).toContain('`read_file`');
+      expect(prompt).toContain('`ask_user`');
 
       // Should NOT include disabled tools
       expect(prompt).not.toContain('`google_web_search`');

packages/core/src/policy/policies/plan.toml

@@ -64,6 +64,12 @@ decision = "allow"
 priority = 50
 modes = ["plan"]
 
+[[rule]]
+toolName = "ask_user"
+decision = "allow"
+priority = 50
+modes = ["plan"]
+
 # Allow write_file for .md files in plans directory
 [[rule]]
 toolName = "write_file"

packages/core/src/prompts/snippets.ts

@@ -6,6 +6,7 @@
 
 import {
   ACTIVATE_SKILL_TOOL_NAME,
+  ASK_USER_TOOL_NAME,
   EDIT_TOOL_NAME,
   GLOB_TOOL_NAME,
   GREP_TOOL_NAME,
@@ -312,7 +313,8 @@ ${options.planModeToolsList}
 
 ### Phase 1: Requirements Understanding
 - Analyze the user's request to identify core requirements and constraints
-- If critical information is missing or ambiguous, ask ONE clarifying question at a time
+- If critical information is missing or ambiguous, ask clarifying questions using the \`${ASK_USER_TOOL_NAME}\` tool
+- When using \`${ASK_USER_TOOL_NAME}\`, prefer providing multiple-choice options for the user to select from when possible
 - Do NOT explore the project or create a plan yet
 
 ### Phase 2: Project Exploration

packages/core/src/tools/tool-names.ts

@@ -60,6 +60,7 @@ export const PLAN_MODE_TOOLS = [
   READ_FILE_TOOL_NAME,
   LS_TOOL_NAME,
   WEB_SEARCH_TOOL_NAME,
+  ASK_USER_TOOL_NAME,
 ] as const;
 
 /**