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diff --git a/packages/core/src/services/ideContext.ts b/packages/core/src/services/ideContext.ts
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+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { z } from 'zod';
+
+/**
+ * The reserved server name for the IDE's MCP server.
+ */
+export const IDE_SERVER_NAME = '_ide_server';
+
+/**
+ * Zod schema for validating a cursor position.
+ */
+export const CursorSchema = z.object({
+  line: z.number(),
+  character: z.number(),
+});
+export type Cursor = z.infer<typeof CursorSchema>;
+
+/**
+ * Zod schema for validating an active file context from the IDE.
+ */
+export const ActiveFileSchema = z.object({
+  filePath: z.string(),
+  cursor: CursorSchema.optional(),
+});
+export type ActiveFile = z.infer<typeof ActiveFileSchema>;
+
+/**
+ * Zod schema for validating the 'ide/activeFileChanged' notification from the IDE.
+ */
+export const ActiveFileNotificationSchema = z.object({
+  method: z.literal('ide/activeFileChanged'),
+  params: ActiveFileSchema,
+});
+
+type ActiveFileSubscriber = (activeFile: ActiveFile | undefined) => void;
+
+/**
+ * Creates a new store for managing the IDE's active file context.
+ * This factory function encapsulates the state and logic, allowing for the creation
+ * of isolated instances, which is particularly useful for testing.
+ *
+ * @returns An object with methods to interact with the active file context.
+ */
+export function createIdeContextStore() {
+  let activeFileContext: ActiveFile | undefined = undefined;
+  const subscribers = new Set<ActiveFileSubscriber>();
+
+  /**
+   * Notifies all registered subscribers about the current active file context.
+   */
+  function notifySubscribers(): void {
+    for (const subscriber of subscribers) {
+      subscriber(activeFileContext);
+    }
+  }
+
+  /**
+   * Sets the active file context and notifies all registered subscribers of the change.
+   * @param newActiveFile The new active file context from the IDE.
+   */
+  function setActiveFileContext(newActiveFile: ActiveFile): void {
+    activeFileContext = newActiveFile;
+    notifySubscribers();
+  }
+
+  /**
+   * Retrieves the current active file context.
+   * @returns The `ActiveFile` object if a file is active, otherwise `undefined`.
+   */
+  function getActiveFileContext(): ActiveFile | undefined {
+    return activeFileContext;
+  }
+
+  /**
+   * Subscribes to changes in the active file context.
+   *
+   * When the active file context changes, the provided `subscriber` function will be called.
+   * Note: The subscriber is not called with the current value upon subscription.
+   *
+   * @param subscriber The function to be called when the active file context changes.
+   * @returns A function that, when called, will unsubscribe the provided subscriber.
+   */
+  function subscribeToActiveFile(subscriber: ActiveFileSubscriber): () => void {
+    subscribers.add(subscriber);
+    return () => {
+      subscribers.delete(subscriber);
+    };
+  }
+
+  return {
+    setActiveFileContext,
+    getActiveFileContext,
+    subscribeToActiveFile,
+  };
+}
+
+/**
+ * The default, shared instance of the IDE context store for the application.
+ */
+export const ideContext = createIdeContextStore();