fix(watcher): enable and robustify watcher sub-agent loop

packages/core/src/config/config.ts

@@ -44,6 +44,7 @@ import { AskUserTool } from '../tools/ask-user.js';
 import { UpdateTopicTool } from '../tools/topicTool.js';
 import { TopicState } from './topicState.js';
 import { AgentTool } from '../agents/agent-tool.js';
+import { WatcherTool } from '../tools/watcherTool.js';
 import { ExitPlanModeTool } from '../tools/exit-plan-mode.js';
 import { EnterPlanModeTool } from '../tools/enter-plan-mode.js';
 import {
@@ -3650,6 +3651,12 @@ export class Config implements McpContext, AgentLoopContext {
       registry.registerTool(new AgentTool(this, this.messageBus)),
     );
 
+    if (this.isExperimentalWatcherEnabled()) {
+      maybeRegister(WatcherTool, () =>
+        registry.registerTool(new WatcherTool(this, this.messageBus)),
+      );
+    }
+
     await registry.discoverAllTools();
     registry.sortTools();
     return registry;

packages/core/src/tools/definitions/base-declarations.ts

@@ -137,3 +137,8 @@ export const TOPIC_PARAM_STRATEGIC_INTENT = 'strategic_intent';
 // -- complete_task --
 export const COMPLETE_TASK_TOOL_NAME = 'complete_task';
 export const COMPLETE_TASK_DISPLAY_NAME = 'Complete Task';
+
+// -- watcher --
+export const WATCHER_TOOL_NAME = 'watcher';
+export const WATCHER_DISPLAY_NAME = 'Watcher';
+export const WATCHER_PARAM_RECENT_HISTORY = 'recentHistory';

packages/core/src/tools/watcherTool.ts

@@ -0,0 +1,69 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  BaseDeclarativeTool,
+  type ToolResult,
+  Kind,
+  type ToolInvocation,
+} from './tools.js';
+import { type AgentLoopContext } from '../config/agent-loop-context.js';
+import type { MessageBus } from '../confirmation-bus/message-bus.js';
+import {
+  WATCHER_TOOL_NAME,
+  WATCHER_DISPLAY_NAME,
+} from './definitions/base-declarations.js';
+import { LocalSubagentInvocation } from '../agents/local-invocation.js';
+import { WatcherAgent } from '../agents/watcher-agent.js';
+import type { AgentInputs } from '../agents/types.js';
+
+/**
+ * A specialized tool for the internal Watcher agent loop.
+ *
+ * This tool wraps the Watcher sub-agent to allow it to be discovered and
+ * executed by the GeminiClient's internal monitoring loop.
+ */
+export class WatcherTool extends BaseDeclarativeTool<
+  AgentInputs,
+  ToolResult
+> {
+  static readonly Name = WATCHER_TOOL_NAME;
+
+  constructor(
+    private readonly context: AgentLoopContext,
+    messageBus: MessageBus,
+  ) {
+    const definition = WatcherAgent(context.config);
+    super(
+      WATCHER_TOOL_NAME,
+      WATCHER_DISPLAY_NAME,
+      definition.description,
+      Kind.Agent,
+      definition.inputConfig.inputSchema,
+      messageBus,
+      /* isOutputMarkdown */ true,
+      /* canUpdateOutput */ true,
+    );
+  }
+
+  protected createInvocation(
+    params: AgentInputs,
+    messageBus: MessageBus,
+    _toolName?: string,
+    _toolDisplayName?: string,
+  ): ToolInvocation<AgentInputs, ToolResult> {
+    const definition = WatcherAgent(this.context.config);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any, @typescript-eslint/no-unsafe-type-assertion
+    return new LocalSubagentInvocation(
+      definition as any,
+      this.context,
+      params,
+      messageBus,
+      _toolName,
+      _toolDisplayName,
+    ) as any;
+  }
+}