4 files changed, 245 insertions, 0 deletions
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index df7db12c..5f1dc3e7 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -37,6 +37,7 @@ export * from './utils/quotaErrorDetection.js';
 // Export services
 export * from './services/fileDiscoveryService.js';
 export * from './services/gitService.js';
+export * from './services/ideContext.js';
 
 // Export base tool definitions
 export * from './tools/tools.js';
diff --git a/packages/core/src/services/ideContext.test.ts b/packages/core/src/services/ideContext.test.ts
new file mode 100644
index 00000000..0e6ff045
--- /dev/null
+++ b/packages/core/src/services/ideContext.test.ts
@@ -0,0 +1,125 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { createIdeContextStore } from './ideContext.js';
+
+describe('ideContext - Active File', () => {
+  let ideContext: ReturnType<typeof createIdeContextStore>;
+
+  beforeEach(() => {
+    // Create a fresh, isolated instance for each test
+    ideContext = createIdeContextStore();
+  });
+
+  it('should return undefined initially for active file context', () => {
+    expect(ideContext.getActiveFileContext()).toBeUndefined();
+  });
+
+  it('should set and retrieve the active file context', () => {
+    const testFile = {
+      filePath: '/path/to/test/file.ts',
+      cursor: { line: 5, character: 10 },
+    };
+
+    ideContext.setActiveFileContext(testFile);
+
+    const activeFile = ideContext.getActiveFileContext();
+    expect(activeFile).toEqual(testFile);
+  });
+
+  it('should update the active file context when called multiple times', () => {
+    const firstFile = {
+      filePath: '/path/to/first.js',
+      cursor: { line: 1, character: 1 },
+    };
+    ideContext.setActiveFileContext(firstFile);
+
+    const secondFile = {
+      filePath: '/path/to/second.py',
+      cursor: { line: 20, character: 30 },
+    };
+    ideContext.setActiveFileContext(secondFile);
+
+    const activeFile = ideContext.getActiveFileContext();
+    expect(activeFile).toEqual(secondFile);
+  });
+
+  it('should handle empty string for file path', () => {
+    const testFile = {
+      filePath: '',
+      cursor: { line: 0, character: 0 },
+    };
+    ideContext.setActiveFileContext(testFile);
+    expect(ideContext.getActiveFileContext()).toEqual(testFile);
+  });
+
+  it('should notify subscribers when active file context changes', () => {
+    const subscriber1 = vi.fn();
+    const subscriber2 = vi.fn();
+
+    ideContext.subscribeToActiveFile(subscriber1);
+    ideContext.subscribeToActiveFile(subscriber2);
+
+    const testFile = {
+      filePath: '/path/to/subscribed.ts',
+      cursor: { line: 15, character: 25 },
+    };
+    ideContext.setActiveFileContext(testFile);
+
+    expect(subscriber1).toHaveBeenCalledTimes(1);
+    expect(subscriber1).toHaveBeenCalledWith(testFile);
+    expect(subscriber2).toHaveBeenCalledTimes(1);
+    expect(subscriber2).toHaveBeenCalledWith(testFile);
+
+    // Test with another update
+    const newFile = {
+      filePath: '/path/to/new.js',
+      cursor: { line: 1, character: 1 },
+    };
+    ideContext.setActiveFileContext(newFile);
+
+    expect(subscriber1).toHaveBeenCalledTimes(2);
+    expect(subscriber1).toHaveBeenCalledWith(newFile);
+    expect(subscriber2).toHaveBeenCalledTimes(2);
+    expect(subscriber2).toHaveBeenCalledWith(newFile);
+  });
+
+  it('should stop notifying a subscriber after unsubscribe', () => {
+    const subscriber1 = vi.fn();
+    const subscriber2 = vi.fn();
+
+    const unsubscribe1 = ideContext.subscribeToActiveFile(subscriber1);
+    ideContext.subscribeToActiveFile(subscriber2);
+
+    ideContext.setActiveFileContext({
+      filePath: '/path/to/file1.txt',
+      cursor: { line: 1, character: 1 },
+    });
+    expect(subscriber1).toHaveBeenCalledTimes(1);
+    expect(subscriber2).toHaveBeenCalledTimes(1);
+
+    unsubscribe1();
+
+    ideContext.setActiveFileContext({
+      filePath: '/path/to/file2.txt',
+      cursor: { line: 2, character: 2 },
+    });
+    expect(subscriber1).toHaveBeenCalledTimes(1); // Should not be called again
+    expect(subscriber2).toHaveBeenCalledTimes(2);
+  });
+
+  it('should allow the cursor to be optional', () => {
+    const testFile = {
+      filePath: '/path/to/test/file.ts',
+    };
+
+    ideContext.setActiveFileContext(testFile);
+
+    const activeFile = ideContext.getActiveFileContext();
+    expect(activeFile).toEqual(testFile);
+  });
+});
diff --git a/packages/core/src/services/ideContext.ts b/packages/core/src/services/ideContext.ts
new file mode 100644
index 00000000..6bbe8cb9
--- /dev/null
+++ b/packages/core/src/services/ideContext.ts
@@ -0,0 +1,105 @@
+/**
+ * @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();
diff --git a/packages/core/src/tools/mcp-client.ts b/packages/core/src/tools/mcp-client.ts
index eb82190b..beb70549 100644
--- a/packages/core/src/tools/mcp-client.ts
+++ b/packages/core/src/tools/mcp-client.ts
@@ -20,6 +20,11 @@ import { MCPServerConfig } from '../config/config.js';
 import { DiscoveredMCPTool } from './mcp-tool.js';
 import { FunctionDeclaration, Type, mcpToTool } from '@google/genai';
 import { sanitizeParameters, ToolRegistry } from './tool-registry.js';
+import {
+  ActiveFileNotificationSchema,
+  IDE_SERVER_NAME,
+  ideContext,
+} from '../services/ideContext.js';
 
 export const MCP_DEFAULT_TIMEOUT_MSEC = 10 * 60 * 1000; // default to 10 minutes
 
@@ -211,6 +216,15 @@ export async function connectAndDiscover(
         updateMCPServerStatus(mcpServerName, MCPServerStatus.DISCONNECTED);
       };
 
+      if (mcpServerName === IDE_SERVER_NAME) {
+        mcpClient.setNotificationHandler(
+          ActiveFileNotificationSchema,
+          (notification) => {
+            ideContext.setActiveFileContext(notification.params);
+          },
+        );
+      }
+
       const tools = await discoverTools(
         mcpServerName,
         mcpServerConfig,