diff --git a/packages/cli/src/utils/sandbox.ts b/packages/cli/src/utils/sandbox.ts
index 95f23d82075..9e4b4a22664 100644
--- a/packages/cli/src/utils/sandbox.ts
+++ b/packages/cli/src/utils/sandbox.ts
@@ -296,9 +296,15 @@ export async function start_sandbox(
           sandboxEnv['NO_PROXY'] = noProxy;
           sandboxEnv['no_proxy'] = noProxy;
         }
-        proxyProcess = spawn(proxyCommand, {
+
+        // Parse proxyCommand to split arguments correctly, filtering out non-string args
+        const parsedProxyCommand = parse(proxyCommand).filter(
+          (arg): arg is string => typeof arg === 'string',
+        );
+        const [proxyCmd, ...proxyArgs] = parsedProxyCommand;
+
+        proxyProcess = spawn(proxyCmd, proxyArgs, {
           stdio: ['ignore', 'pipe', 'pipe'],
-          shell: true,
           detached: true,
         });
         // install handlers to stop proxy on exit/signal
diff --git a/packages/cli/src/utils/sandbox_repro.security.test.ts b/packages/cli/src/utils/sandbox_repro.security.test.ts
new file mode 100644
index 00000000000..4724ca0e8ea
--- /dev/null
+++ b/packages/cli/src/utils/sandbox_repro.security.test.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import * as cp from 'node:child_process';
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import path from 'node:path';
+
+// Mock factories
+vi.mock('node:child_process', () => {
+    const mockCp = {
+        spawn: vi.fn(),
+        exec: vi.fn(),
+        execSync: vi.fn(),
+    };
+    return {
+        ...mockCp,
+        default: mockCp,
+    };
+});
+
+vi.mock('node:fs', () => {
+    const mockFs = {
+        realpathSync: vi.fn((p) => p),
+        existsSync: vi.fn(() => true),
+        readFileSync: vi.fn(() => ''),
+        mkdirSync: vi.fn(),
+        writeFileSync: vi.fn(),
+        resolve: vi.fn(),
+        promises: {
+            readFile: vi.fn(),
+        },
+        constants: {},
+    };
+    return {
+        ...mockFs,
+        default: mockFs,
+    };
+});
+
+vi.mock('node:os', () => {
+    const mockOs = {
+        platform: vi.fn(() => 'darwin'), // sandbox-exec is macOS only
+        tmpdir: vi.fn(() => '/tmp'),
+        homedir: vi.fn(() => '/home/user'),
+        userInfo: vi.fn(),
+        release: vi.fn(() => '1.0.0'),
+        type: vi.fn(() => 'Darwin'),
+    };
+    return {
+        ...mockOs,
+        default: mockOs,
+    };
+});
+
+vi.mock('@google/gemini-cli-core', () => ({
+  Config: class {
+      getDebugMode() { return false; }
+      getTargetDir() { return '/target'; }
+      getWorkspaceContext() {
+          return {
+              getDirectories() { return ['/dir1']; }
+          };
+      }
+  },
+}));
+
+// Import subject under test AFTER mocks
+import { start_sandbox } from './sandbox.js';
+
+describe('sandbox security repro', () => {
+  const originalEnv = process.env;
+
+  beforeEach(() => {
+    vi.resetAllMocks();
+    process.env = { ...originalEnv };
+    process.env.GEMINI_SANDBOX_PROXY_COMMAND = 'echo proxy';
+    process.env.SEATBELT_PROFILE = 'permissive-open';
+
+    vi.mocked(os.platform).mockReturnValue('darwin');
+    vi.mocked(os.tmpdir).mockReturnValue('/tmp');
+    vi.mocked(os.homedir).mockReturnValue('/home/user');
+
+    vi.mocked(fs.realpathSync).mockImplementation((p) => p.toString());
+    vi.mocked(fs.existsSync).mockReturnValue(true);
+
+    // Mock exec for curl check
+    // @ts-ignore
+    vi.mocked(cp.exec).mockImplementation(((cmd: string, options: any, cb: any) => {
+        if (typeof options === 'function') {
+            cb = options;
+            options = {};
+        }
+        if (cb) cb(null, 'ok', '');
+        return { unref: vi.fn(), on: vi.fn() } as any;
+    }) as any);
+
+    vi.mocked(cp.execSync).mockReturnValue('mock-output');
+
+    // Mock spawn process logic
+    const baseMockProcess = {
+        on: vi.fn(),
+        stdout: { on: vi.fn(), removeListener: vi.fn() },
+        stderr: { on: vi.fn(), removeListener: vi.fn() },
+        pid: 123,
+        kill: vi.fn(),
+    };
+
+    // @ts-ignore
+    vi.mocked(cp.spawn).mockImplementation((cmd, args, opts) => {
+        const mockP = { ...baseMockProcess };
+        mockP.on = vi.fn().mockImplementation((event, cb) => {
+            if (event === 'close') {
+                setImmediate(() => cb(0, null));
+            }
+            return mockP;
+        });
+        return mockP as any;
+    });
+  });
+
+  afterEach(() => {
+    process.env = originalEnv;
+    vi.restoreAllMocks();
+  });
+
+  it('no longer uses shell: true for GEMINI_SANDBOX_PROXY_COMMAND in sandbox-exec path', async () => {
+    const config = {
+      command: 'sandbox-exec',
+      image: 'test-image',
+    };
+
+    await start_sandbox(config as any);
+
+    const spawnCalls = vi.mocked(cp.spawn).mock.calls;
+
+    // In the fixed version, the proxy command is called as spawn(cmd, args, options)
+    // For 'echo proxy', it should be spawn('echo', ['proxy'], options)
+    const proxyCall = spawnCalls.find(call => {
+        return call[0] === 'echo';
+    });
+
+    expect(proxyCall).toBeDefined();
+    const args = proxyCall![1] as string[];
+    expect(args).toEqual(['proxy']);
+
+    const options = proxyCall![2] as any;
+    expect(options.shell).toBeUndefined();
+  });
+});