fix(cli): handle tmux false positive background detection (#27572)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Gal Zahavi <38544478+galz10@users.noreply.github.com>

packages/cli/src/ui/utils/terminalCapabilityManager.test.ts

@@ -117,6 +117,51 @@ describe('TerminalCapabilityManager', () => {
     expect(manager.getTerminalBackgroundColor()).toBe('#00ff00');
   });
 
+  it('should ignore #ffffff in tmux as it is a common false positive', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.spyOn(manager, 'isTmux').mockReturnValue(true);
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for white
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:ffff/ffff/ffff\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBeUndefined();
+  });
+
+  it('should not ignore #ffffff when NOT in tmux', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.spyOn(manager, 'isTmux').mockReturnValue(false);
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for white
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:ffff/ffff/ffff\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBe('#ffffff');
+  });
+
+  it('should NOT ignore other colors in tmux', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.stubEnv('TMUX', '1');
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for grey
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:8888/8888/8888\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBe('#888888');
+  });
+
   it('should detect Terminal Name', async () => {
     const manager = TerminalCapabilityManager.getInstance();
     const promise = manager.detectCapabilities();

packages/cli/src/ui/utils/terminalCapabilityManager.ts

@@ -161,9 +161,21 @@ export class TerminalCapabilityManager {
               match[2],
               match[3],
             );
-            debugLogger.log(
-              `Detected terminal background color: ${this.terminalBackgroundColor}`,
-            );
+
+            // Heuristic: tmux 3.5+ may report #ffffff when it doesn't know the
+            // actual host terminal color (e.g. over mosh). We ignore this specific
+            // fallback value to prevent blinding the user with a light theme in a
+            // likely dark terminal.
+            if (this.terminalBackgroundColor === '#ffffff' && this.isTmux()) {
+              debugLogger.log(
+                'Ignored #ffffff background in tmux (common false positive over mosh).',
+              );
+              this.terminalBackgroundColor = undefined;
+            } else {
+              debugLogger.log(
+                `Detected terminal background color: ${this.terminalBackgroundColor}`,
+              );
+            }
           }
         }
 

packages/cli/src/utils/terminalTheme.test.ts

@@ -0,0 +1,115 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { setupTerminalAndTheme } from './terminalTheme.js';
+import { terminalCapabilityManager } from '../ui/utils/terminalCapabilityManager.js';
+import { themeManager } from '../ui/themes/theme-manager.js';
+import { coreEvents, type Config } from '@google/gemini-cli-core';
+import type { LoadedSettings } from '../config/settings.js';
+import type { Theme } from '../ui/themes/theme.js';
+
+vi.mock('../ui/utils/terminalCapabilityManager.js', () => ({
+  terminalCapabilityManager: {
+    detectCapabilities: vi.fn(),
+    getTerminalBackgroundColor: vi.fn(),
+  },
+}));
+
+vi.mock('../ui/themes/theme-manager.js', () => ({
+  themeManager: {
+    loadCustomThemes: vi.fn(),
+    setActiveTheme: vi.fn(),
+    getActiveTheme: vi.fn(),
+    setTerminalBackground: vi.fn(),
+    isThemeCompatible: vi.fn(),
+    getAllThemes: vi.fn().mockReturnValue([]),
+  },
+  DEFAULT_THEME: { name: 'Default Dark' },
+}));
+
+vi.mock('@google/gemini-cli-core', () => ({
+  coreEvents: {
+    emitFeedback: vi.fn(),
+  },
+  debugLogger: {
+    warn: vi.fn(),
+  },
+}));
+
+describe('setupTerminalAndTheme', () => {
+  let mockConfig: Config;
+  let mockSettings: LoadedSettings;
+  const originalIsTTY = process.stdin.isTTY;
+
+  beforeEach(() => {
+    vi.resetAllMocks();
+    mockConfig = {
+      isInteractive: vi.fn().mockReturnValue(true),
+      setTerminalBackground: vi.fn(),
+    } as Partial<Config> as Config;
+    mockSettings = {
+      merged: {
+        ui: {
+          customThemes: {},
+          theme: 'Dracula',
+          autoThemeSwitching: true,
+        },
+      },
+    } as Partial<LoadedSettings> as LoadedSettings;
+
+    // Mock process.stdin.isTTY
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: true,
+      configurable: true,
+    });
+  });
+
+  afterEach(() => {
+    Object.defineProperty(process.stdin, 'isTTY', {
+      value: originalIsTTY,
+      configurable: true,
+    });
+  });
+
+  it('should emit warning when theme is incompatible and autoThemeSwitching is enabled', async () => {
+    vi.mocked(
+      terminalCapabilityManager.getTerminalBackgroundColor,
+    ).mockReturnValue('#ffffff'); // Light
+    vi.mocked(themeManager.setActiveTheme).mockReturnValue(true);
+    vi.mocked(themeManager.getActiveTheme).mockReturnValue({
+      name: 'Dracula',
+      type: 'dark',
+    } as Theme);
+    vi.mocked(themeManager.isThemeCompatible).mockReturnValue(false);
+
+    await setupTerminalAndTheme(mockConfig, mockSettings);
+
+    expect(coreEvents.emitFeedback).toHaveBeenCalledWith(
+      'warning',
+      expect.stringContaining(
+        "Theme 'Dracula' (dark) might look incorrect on your light terminal background",
+      ),
+    );
+  });
+
+  it('should NOT emit warning when theme is incompatible but autoThemeSwitching is DISABLED', async () => {
+    mockSettings.merged.ui.autoThemeSwitching = false;
+    vi.mocked(
+      terminalCapabilityManager.getTerminalBackgroundColor,
+    ).mockReturnValue('#ffffff'); // Light
+    vi.mocked(themeManager.setActiveTheme).mockReturnValue(true);
+    vi.mocked(themeManager.getActiveTheme).mockReturnValue({
+      name: 'Dracula',
+      type: 'dark',
+    } as Theme);
+    vi.mocked(themeManager.isThemeCompatible).mockReturnValue(false);
+
+    await setupTerminalAndTheme(mockConfig, mockSettings);
+
+    expect(coreEvents.emitFeedback).not.toHaveBeenCalled();
+  });
+});

packages/cli/src/utils/terminalTheme.ts

@@ -56,7 +56,10 @@ export async function setupTerminalAndTheme(
   config.setTerminalBackground(terminalBackground);
   themeManager.setTerminalBackground(terminalBackground);
 
-  if (terminalBackground !== undefined) {
+  if (
+    terminalBackground !== undefined &&
+    (settings.merged.ui.autoThemeSwitching ?? true)
+  ) {
     const currentTheme = themeManager.getActiveTheme();
     if (!themeManager.isThemeCompatible(currentTheme, terminalBackground)) {
       const backgroundType =