diff --git a/src/main/index.ts b/src/main/index.ts
index cdebf9abc..c969d302e 100644
--- a/src/main/index.ts
+++ b/src/main/index.ts
@@ -27,7 +27,7 @@ import { registerAltJsRuntimeHandlers } from './altJsRuntimes';
 import { registerAppInfoHandlers } from './ipc/appInfo';
 import { registerFileSystemHandlers } from './ipc/fileSystem';
 import { registerLocaleHandlers } from './ipc/locale';
-import { registerDesktopSmokeHandlers } from './ipc/desktopSmoke';
+import { desktopSmokeRendererArguments, registerDesktopSmokeHandlers } from './ipc/desktopSmoke';
 import { registerEnvHandlers } from './ipc/env';
 import { registerLspHandlers, disposeLspBridge } from './ipc/lsp';
 import { registerProfileHandlers } from './ipc/profile';
@@ -171,6 +171,7 @@ const createWindow = async () => {
       contextIsolation: true,
       nodeIntegration: false,
       sandbox: true,
+      additionalArguments: desktopSmokeRendererArguments(),
       // Pin the same-origin policy explicitly. This is the
       // Electron default, but pinning it ensures a future edit cannot
       // silently disable web security (which would let a compromised
diff --git a/src/main/ipc/desktopSmoke.ts b/src/main/ipc/desktopSmoke.ts
index c7e5ef59c..4a77e84ed 100644
--- a/src/main/ipc/desktopSmoke.ts
+++ b/src/main/ipc/desktopSmoke.ts
@@ -6,8 +6,8 @@ import {
   getBlockedOfflineSmokeUrls,
   isOfflineSmokeRequested,
 } from '../offlineSmoke';
+import { DESKTOP_SMOKE_FLAG } from '../../shared/desktopSmoke';
 
-const DESKTOP_SMOKE_FLAG = '--lingua-desktop-smoke';
 const SMOKE_ARTIFACT_DIR_PREFIX = '--lingua-smoke-artifact-dir=';
 const SMOKE_LICENSE_TOKEN_PREFIX = '--lingua-smoke-license-token=';
 const SMOKE_LAUNCHED_AT_ENV = 'LINGUA_SMOKE_LAUNCHED_AT_MS';
@@ -27,6 +27,14 @@ function isDesktopSmokeEnabled(): boolean {
   );
 }
 
+/**
+ * Renderer arguments that let the sandboxed preload see smoke mode. A packaged
+ * renderer does not inherit the launcher's environment or main's argv.
+ */
+export function desktopSmokeRendererArguments(): string[] {
+  return isDesktopSmokeEnabled() ? [DESKTOP_SMOKE_FLAG] : [];
+}
+
 /**
  * packaged-subset gate. When the smoke harness is
  * launched against a release artifact (the `.app` bundle on macOS),
diff --git a/src/preload/index.ts b/src/preload/index.ts
index 41c80097b..d50d26059 100644
--- a/src/preload/index.ts
+++ b/src/preload/index.ts
@@ -12,6 +12,7 @@ import type { PythonDebuggerBridge } from '../shared/pythonDebugger';
 import type { GoDebuggerBridge } from '../shared/goDebugger';
 import type { RustDebuggerBridge } from '../shared/rustDebugger';
 import { typedInvoke, typedOn, typedSend } from './ipcTyped';
+import { DESKTOP_SMOKE_FLAG } from '../shared/desktopSmoke';
 
 const lsp: LspBridge = {
   resolveTarget: (rootId: RootId, uri: string) => typedInvoke('lsp:resolve-target', rootId, uri),
@@ -48,7 +49,7 @@ const lsp: LspBridge = {
 
 const desktopSmokeEnabled =
   process.env.LINGUA_DESKTOP_SMOKE === '1' ||
-  process.argv.includes('--lingua-desktop-smoke');
+  process.argv.includes(DESKTOP_SMOKE_FLAG);
 
 const http: HttpDesktopAPI = {
   execute: (runId, request, options) =>
diff --git a/src/shared/desktopSmoke.ts b/src/shared/desktopSmoke.ts
new file mode 100644
index 000000000..5bd87b0c0
--- /dev/null
+++ b/src/shared/desktopSmoke.ts
@@ -0,0 +1,2 @@
+/** Read by main and by the sandboxed preload, which receives it via additionalArguments. */
+export const DESKTOP_SMOKE_FLAG = '--lingua-desktop-smoke';
diff --git a/tests/ipc/desktopSmoke.test.ts b/tests/ipc/desktopSmoke.test.ts
index 7080e350f..e64ec13eb 100644
--- a/tests/ipc/desktopSmoke.test.ts
+++ b/tests/ipc/desktopSmoke.test.ts
@@ -1,4 +1,7 @@
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { DESKTOP_SMOKE_FLAG } from '#src/shared/desktopSmoke';
 
 const handlers = new Map<string, (...args: unknown[]) => unknown>();
 const listeners = new Map<string, (...args: unknown[]) => unknown>();
@@ -283,4 +286,29 @@ describe('desktop smoke IPC handlers', () => {
       reason: 'smoke-disabled',
     });
   });
+
+  it('forwards smoke mode to the sandboxed renderer only when main is in smoke mode', async () => {
+    const { desktopSmokeRendererArguments } = await import('#src/main/ipc/desktopSmoke');
+    expect(desktopSmokeRendererArguments()).toEqual(['--lingua-desktop-smoke']);
+
+    delete process.env.LINGUA_DESKTOP_SMOKE;
+    process.argv = ['electron', 'main.js'];
+    expect(desktopSmokeRendererArguments()).toEqual([]);
+
+    process.argv = ['electron', 'main.js', '--lingua-desktop-smoke'];
+    expect(desktopSmokeRendererArguments()).toEqual(['--lingua-desktop-smoke']);
+  });
+
+  it('wires the forwarded flag into the main window and reads the same flag in the preload', () => {
+    const root = path.resolve(__dirname, '../..');
+    const main = readFileSync(path.join(root, 'src/main/index.ts'), 'utf8');
+    const webPreferences = main.slice(main.indexOf('webPreferences: {'));
+    expect(webPreferences.slice(0, webPreferences.indexOf('}'))).toContain(
+      'additionalArguments: desktopSmokeRendererArguments()'
+    );
+
+    const preload = readFileSync(path.join(root, 'src/preload/index.ts'), 'utf8');
+    expect(preload).toContain('process.argv.includes(DESKTOP_SMOKE_FLAG)');
+    expect(preload).not.toContain(`'${DESKTOP_SMOKE_FLAG}'`);
+  });
 });