fix(core): remove proactive subagent system-reminder injection

packages/cli/src/acp-integration/session/Session.test.ts

@@ -215,9 +215,6 @@ describe('Session', () => {
 
     mockToolRegistry = {
       getTool: vi.fn(),
-      // #executePrompt → #buildInitialSystemReminders calls
-      // getToolRegistry().ensureTool(ToolNames.AGENT) on every session.prompt(),
-      // so the default mock must provide it (#1151 / #3479).
       ensureTool: vi.fn().mockResolvedValue(true),
     };
     const fileService = { shouldGitIgnoreFile: vi.fn().mockReturnValue(false) };
@@ -239,12 +236,6 @@ describe('Session', () => {
         .fn()
         .mockReturnValue(mockChatRecordingService),
       getToolRegistry: vi.fn().mockReturnValue(mockToolRegistry),
-      // #buildInitialSystemReminders iterates listSubagents() on every
-      // session.prompt(). Default to an empty list so tests that don't
-      // exercise subagent reminders don't need to stub it (#1151 / #3479).
-      getSubagentManager: vi.fn().mockReturnValue({
-        listSubagents: vi.fn().mockResolvedValue([]),
-      }),
       getFileService: vi.fn().mockReturnValue(fileService),
       getFileFilteringRespectGitIgnore: vi.fn().mockReturnValue(true),
       getEnableRecursiveFileSearch: vi.fn().mockReturnValue(false),
@@ -2874,20 +2865,7 @@ describe('Session', () => {
         return capture;
       };
 
-      const stubEmptySubagents = () => {
-        (mockConfig as unknown as Record<string, unknown>)[
-          'getSubagentManager'
-        ] = vi.fn().mockReturnValue({
-          listSubagents: vi.fn().mockResolvedValue([]),
-        });
-        // ensureTool is called on the result of getToolRegistry(); add it.
-        (
-          mockToolRegistry as unknown as { ensureTool: () => Promise<boolean> }
-        ).ensureTool = vi.fn().mockResolvedValue(true);
-      };
-
       it('prepends plan-mode reminder when approval mode is PLAN (#1151)', async () => {
-        stubEmptySubagents();
         mockConfig.getApprovalMode = vi.fn().mockReturnValue(ApprovalMode.PLAN);
         const capture = captureFirstTurnMessage();
 
@@ -2910,7 +2888,6 @@ describe('Session', () => {
       });
 
       it('does not prepend plan-mode reminder in default approval mode', async () => {
-        stubEmptySubagents();
         mockConfig.getApprovalMode = vi
           .fn()
           .mockReturnValue(ApprovalMode.DEFAULT);
@@ -2926,40 +2903,6 @@ describe('Session', () => {
         );
         expect(hasPlanReminder).toBe(false);
       });
-
-      it('prepends subagent reminder when user-level subagents exist', async () => {
-        (mockConfig as unknown as Record<string, unknown>)[
-          'getSubagentManager'
-        ] = vi.fn().mockReturnValue({
-          listSubagents: vi.fn().mockResolvedValue([
-            { name: 'researcher', level: 'user' },
-            { name: 'planner', level: 'project' },
-            // builtin entries are filtered out, matching client.ts:853.
-            { name: 'builtin-helper', level: 'builtin' },
-          ]),
-        });
-        (
-          mockToolRegistry as unknown as { ensureTool: () => Promise<boolean> }
-        ).ensureTool = vi.fn().mockResolvedValue(true);
-        mockConfig.getApprovalMode = vi
-          .fn()
-          .mockReturnValue(ApprovalMode.DEFAULT);
-        const capture = captureFirstTurnMessage();
-
-        await session.prompt({
-          sessionId: 'test-session-id',
-          prompt: [{ type: 'text', text: 'hi' }],
-        });
-
-        const reminder = capture.parts.find(
-          (p) =>
-            p.text &&
-            p.text.includes('researcher') &&
-            p.text.includes('planner'),
-        );
-        expect(reminder).toBeTruthy();
-        expect(reminder!.text).not.toContain('builtin-helper');
-      });
     });
   });
 });

packages/cli/src/acp-integration/session/Session.ts

@@ -52,7 +52,6 @@ import {
   generateToolUseId,
   MessageBusType,
   getPlanModeSystemReminder,
-  getSubagentSystemReminder,
   getArenaSystemReminder,
   STARTUP_CONTEXT_MODEL_ACK,
   evaluatePermissionFlow,
@@ -1738,16 +1737,6 @@ export class Session implements SessionContext {
   async #buildInitialSystemReminders(): Promise<Part[]> {
     const reminders: Part[] = [];
 
-    const hasAgentTool = await this.config
-      .getToolRegistry()
-      .ensureTool(ToolNames.AGENT);
-    const subagents = (await this.config.getSubagentManager().listSubagents())
-      .filter((subagent) => subagent.level !== 'builtin')
-      .map((subagent) => subagent.name);
-    if (hasAgentTool && subagents.length > 0) {
-      reminders.push({ text: getSubagentSystemReminder(subagents) });
-    }
-
     if (this.config.getApprovalMode() === ApprovalMode.PLAN) {
       reminders.push({
         text: getPlanModeSystemReminder(this.config.getSdkMode?.()),

packages/cli/src/acp-integration/session/Session.worktree.test.ts

@@ -116,13 +116,8 @@ describe('Session.pendingWorktreeNotice', () => {
       }),
       getToolRegistry: vi.fn().mockReturnValue({
         getTool: vi.fn(),
-        // Called on every prompt() via #buildInitialSystemReminders
         ensureTool: vi.fn().mockResolvedValue(true),
       }),
-      // Called on every prompt() to check subagent system reminders
-      getSubagentManager: vi.fn().mockReturnValue({
-        listSubagents: vi.fn().mockResolvedValue([]),
-      }),
       getFileService: vi.fn().mockReturnValue({
         shouldGitIgnoreFile: vi.fn().mockReturnValue(false),
       }),

packages/core/src/core/client.test.ts

@@ -418,10 +418,6 @@ describe('Gemini Client (client.ts)', () => {
       vertexai: false,
       authType: AuthType.USE_GEMINI,
     };
-    const mockSubagentManager = {
-      listSubagents: vi.fn().mockResolvedValue([]),
-      addChangeListener: vi.fn().mockReturnValue(() => {}),
-    };
     mockConfig = {
       getContentGeneratorConfig: vi
         .fn()
@@ -473,7 +469,6 @@ describe('Gemini Client (client.ts)', () => {
       },
       getContentGenerator: vi.fn().mockReturnValue(mockContentGenerator),
       getBaseLlmClient: vi.fn(),
-      getSubagentManager: vi.fn().mockReturnValue(mockSubagentManager),
       getSkipLoopDetection: vi.fn().mockReturnValue(false),
       getChatRecordingService: vi.fn().mockReturnValue(undefined),
       getResumedSessionData: vi.fn().mockReturnValue(undefined),

packages/core/src/core/client.ts

@@ -36,7 +36,6 @@ import {
   getCoreSystemPrompt,
   getCustomSystemPrompt,
   getPlanModeSystemReminder,
-  getSubagentSystemReminder,
 } from './prompts.js';
 import {
   CompressionStatus,
@@ -1610,20 +1609,6 @@ export class GeminiClient {
       ) {
         const systemReminders = [];
 
-        // add subagent system reminder if there are subagents
-        const hasAgentTool = await this.config
-          .getToolRegistry()
-          .ensureTool(ToolNames.AGENT);
-        const subagents = (
-          await this.config.getSubagentManager().listSubagents()
-        )
-          .filter((subagent) => subagent.level !== 'builtin')
-          .map((subagent) => subagent.name);
-
-        if (hasAgentTool && subagents.length > 0) {
-          systemReminders.push(getSubagentSystemReminder(subagents));
-        }
-
         // add plan mode system reminder if approval mode is plan
         if (this.config.getApprovalMode() === ApprovalMode.PLAN) {
           systemReminders.push(

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

@@ -9,7 +9,6 @@ import {
   buildDeferredToolsSection,
   getCoreSystemPrompt,
   getCustomSystemPrompt,
-  getSubagentSystemReminder,
   getPlanModeSystemReminder,
   resolvePathFromEnv,
 } from './prompts.js';
@@ -454,31 +453,6 @@ describe('getCustomSystemPrompt', () => {
   });
 });
 
-describe('getSubagentSystemReminder', () => {
-  it('should format single agent type correctly', () => {
-    const result = getSubagentSystemReminder(['python']);
-
-    expect(result).toMatch(/^<system-reminder>.*<\/system-reminder>$/);
-    expect(result).toContain('available agent types are: python');
-    expect(result).toContain('PROACTIVELY use the');
-  });
-
-  it('should join multiple agent types with commas', () => {
-    const result = getSubagentSystemReminder(['python', 'web', 'analysis']);
-
-    expect(result).toContain(
-      'available agent types are: python, web, analysis',
-    );
-  });
-
-  it('should handle empty array', () => {
-    const result = getSubagentSystemReminder([]);
-
-    expect(result).toContain('available agent types are: ');
-    expect(result).toContain('<system-reminder>');
-  });
-});
-
 describe('buildDeferredToolsSection', () => {
   it('returns an empty string when no deferred tools are passed', () => {
     expect(buildDeferredToolsSection([])).toBe('');

packages/core/src/core/prompts.ts

@@ -896,26 +896,6 @@ function getToolCallExamples(model?: string): string {
   return generalToolCallExamples;
 }
 
-/**
- * Generates a system reminder message about available subagents for the AI assistant.
- *
- * This function creates an internal system message that informs the AI about specialized
- * agents it can delegate tasks to. The reminder encourages proactive use of the TASK tool
- * when user requests match agent capabilities.
- *
- * @param agentTypes - Array of available agent type names (e.g., ['python', 'web', 'analysis'])
- * @returns A formatted system reminder string wrapped in XML tags for internal AI processing
- *
- * @example
- * ```typescript
- * const reminder = getSubagentSystemReminder(['python', 'web']);
- * // Returns: "<system-reminder>You have powerful specialized agents..."
- * ```
- */
-export function getSubagentSystemReminder(agentTypes: string[]): string {
-  return `<system-reminder>You have powerful specialized agents at your disposal, available agent types are: ${agentTypes.join(', ')}. PROACTIVELY use the ${ToolNames.AGENT} tool to delegate user's task to appropriate agent when user's task matches agent capabilities. Ignore this message if user's task is not relevant to any agent. This message is for internal use only. Do not mention this to user in your response.</system-reminder>`;
-}
-
 /**
  * Generates a system reminder message for plan mode operation.
  *