diff --git a/packages/vscode-ide-companion/src/ide-server.test.ts b/packages/vscode-ide-companion/src/ide-server.test.ts
index 2367ac1551..614c338063 100644
--- a/packages/vscode-ide-companion/src/ide-server.test.ts
+++ b/packages/vscode-ide-companion/src/ide-server.test.ts
@@ -9,6 +9,7 @@ import type * as vscode from 'vscode';
 import * as fs from 'node:fs/promises';
 import type * as os from 'node:os';
 import * as path from 'node:path';
+import * as http from 'node:http';
 import { IDEServer } from './ide-server.js';
 import type { DiffManager } from './diff-manager.js';
 
@@ -62,26 +63,26 @@ vi.mock('./open-files-manager', () => {
   return { OpenFilesManager };
 });
 
+const getPortFromMock = (
+  replaceMock: ReturnType<
+    () => vscode.ExtensionContext['environmentVariableCollection']['replace']
+  >,
+) => {
+  const port = vi
+    .mocked(replaceMock)
+    .mock.calls.find((call) => call[0] === 'GEMINI_CLI_IDE_SERVER_PORT')?.[1];
+
+  if (port === undefined) {
+    expect.fail('Port was not set');
+  }
+  return port;
+};
+
 describe('IDEServer', () => {
   let ideServer: IDEServer;
   let mockContext: vscode.ExtensionContext;
   let mockLog: (message: string) => void;
 
-  const getPortFromMock = (
-    replaceMock: ReturnType<
-      () => vscode.ExtensionContext['environmentVariableCollection']['replace']
-    >,
-  ) => {
-    const port = vi
-      .mocked(replaceMock)
-      .mock.calls.find((call) => call[0] === 'GEMINI_CLI_IDE_SERVER_PORT')?.[1];
-
-    if (port === undefined) {
-      expect.fail('Port was not set');
-    }
-    return port;
-  };
-
   beforeEach(() => {
     mockLog = vi.fn();
     ideServer = new IDEServer(mockLog, mocks.diffManager);
@@ -456,3 +457,105 @@ describe('IDEServer', () => {
     });
   });
 });
+
+const request = (
+  port: string,
+  options: http.RequestOptions,
+  body?: string,
+): Promise<http.IncomingMessage> =>
+  new Promise((resolve, reject) => {
+    const req = http.request(
+      {
+        hostname: '127.0.0.1',
+        port,
+        ...options,
+      },
+      (res) => {
+        res.resume(); // Consume response data to free up memory
+        resolve(res);
+      },
+    );
+    req.on('error', reject);
+    if (body) {
+      req.write(body);
+    }
+    req.end();
+  });
+
+describe('IDEServer HTTP endpoints', () => {
+  let ideServer: IDEServer;
+  let mockContext: vscode.ExtensionContext;
+  let mockLog: (message: string) => void;
+  let port: string;
+
+  beforeEach(async () => {
+    mockLog = vi.fn();
+    ideServer = new IDEServer(mockLog, mocks.diffManager);
+    mockContext = {
+      subscriptions: [],
+      environmentVariableCollection: {
+        replace: vi.fn(),
+        clear: vi.fn(),
+      },
+    } as unknown as vscode.ExtensionContext;
+    await ideServer.start(mockContext);
+    const replaceMock = mockContext.environmentVariableCollection.replace;
+    port = getPortFromMock(replaceMock);
+  });
+
+  afterEach(async () => {
+    await ideServer.stop();
+    vi.restoreAllMocks();
+  });
+
+  it('should deny requests with an origin header', async () => {
+    const response = await request(
+      port,
+      {
+        path: '/mcp',
+        method: 'POST',
+        headers: {
+          Host: `localhost:${port}`,
+          Origin: 'https://evil.com',
+          'Content-Type': 'application/json',
+        },
+      },
+      JSON.stringify({ jsonrpc: '2.0', method: 'initialize' }),
+    );
+    expect(response.statusCode).toBe(403);
+  });
+
+  it('should deny requests with an invalid host header', async () => {
+    const response = await request(
+      port,
+      {
+        path: '/mcp',
+        method: 'POST',
+        headers: {
+          Host: 'evil.com',
+          'Content-Type': 'application/json',
+        },
+      },
+      JSON.stringify({ jsonrpc: '2.0', method: 'initialize' }),
+    );
+    expect(response.statusCode).toBe(403);
+  });
+
+  it('should allow requests with a valid host header', async () => {
+    const response = await request(
+      port,
+      {
+        path: '/mcp',
+        method: 'POST',
+        headers: {
+          Host: `localhost:${port}`,
+          'Content-Type': 'application/json',
+        },
+      },
+      JSON.stringify({ jsonrpc: '2.0', method: 'initialize' }),
+    );
+    // We expect a 400 here because we are not sending a valid MCP request,
+    // but it's not a host error, which is what we are testing.
+    expect(response.statusCode).toBe(400);
+  });
+});
diff --git a/packages/vscode-ide-companion/src/ide-server.ts b/packages/vscode-ide-companion/src/ide-server.ts
index c1bb85d3fe..12dbaac6bd 100644
--- a/packages/vscode-ide-companion/src/ide-server.ts
+++ b/packages/vscode-ide-companion/src/ide-server.ts
@@ -13,7 +13,12 @@ import {
 import { isInitializeRequest } from '@modelcontextprotocol/sdk/types.js';
 import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
 import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
-import express, { type Request, type Response } from 'express';
+import express, {
+  type Request,
+  type Response,
+  type NextFunction,
+} from 'express';
+import cors from 'cors';
 import { randomUUID } from 'node:crypto';
 import { type Server as HTTPServer } from 'node:http';
 import * as path from 'node:path';
@@ -23,6 +28,13 @@ import type { z } from 'zod';
 import type { DiffManager } from './diff-manager.js';
 import { OpenFilesManager } from './open-files-manager.js';
 
+class CORSError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'CORSError';
+  }
+}
+
 const MCP_SESSION_ID_HEADER = 'mcp-session-id';
 const IDE_SERVER_PORT_ENV_VAR = 'GEMINI_CLI_IDE_SERVER_PORT';
 const IDE_WORKSPACE_PATH_ENV_VAR = 'GEMINI_CLI_IDE_WORKSPACE_PATH';
@@ -131,6 +143,34 @@ export class IDEServer {
 
       const app = express();
       app.use(express.json({ limit: '10mb' }));
+
+      app.use(
+        cors({
+          origin: (origin, callback) => {
+            // Only allow non-browser requests with no origin.
+            if (!origin) {
+              return callback(null, true);
+            }
+            return callback(
+              new CORSError('Request denied by CORS policy.'),
+              false,
+            );
+          },
+        }),
+      );
+
+      app.use((req, res, next) => {
+        const host = req.headers.host || '';
+        const allowedHosts = [
+          `localhost:${this.port}`,
+          `127.0.0.1:${this.port}`,
+        ];
+        if (!allowedHosts.includes(host)) {
+          return res.status(403).json({ error: 'Invalid Host header' });
+        }
+        next();
+      });
+
       app.use((req, res, next) => {
         const authHeader = req.headers.authorization;
         if (authHeader) {
@@ -274,7 +314,15 @@ export class IDEServer {
 
       app.get('/mcp', handleSessionRequest);
 
-      this.server = app.listen(0, async () => {
+      app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
+        if (err instanceof CORSError) {
+          res.status(403).json({ error: 'Request denied by CORS policy.' });
+        } else {
+          next(err);
+        }
+      });
+
+      this.server = app.listen(0, '127.0.0.1', async () => {
         const address = (this.server as HTTPServer).address();
         if (address && typeof address !== 'string') {
           this.port = address.port;
@@ -286,7 +334,7 @@ export class IDEServer {
             os.tmpdir(),
             `gemini-ide-server-${process.ppid}.json`,
           );
-          this.log(`IDE server listening on port ${this.port}`);
+          this.log(`IDE server listening on http://127.0.0.1:${this.port}`);
 
           if (this.authToken) {
             await writePortAndWorkspace({