diff --git a/e2e/browser-mode/config.test.ts b/e2e/browser-mode/config.test.ts
index 5935bdd59..f4ae03294 100644
--- a/e2e/browser-mode/config.test.ts
+++ b/e2e/browser-mode/config.test.ts
@@ -7,4 +7,13 @@ describe('browser mode - config options', () => {
     await expectExecSuccess();
     expect(cli.stdout).toMatch(/Tests.*passed/);
   });
+
+  it('should fail early when browser provider is invalid', async () => {
+    const { expectExecFailed, expectStderrLog, cli } =
+      await runBrowserCli('invalid-provider');
+
+    await expectExecFailed();
+    expectStderrLog(/browser\.provider must be one of: playwright\./);
+    expect(cli.stdout).not.toMatch(/Browser mode opened at/);
+  });
 });
diff --git a/e2e/browser-mode/fixtures/invalid-provider/rstest.config.ts b/e2e/browser-mode/fixtures/invalid-provider/rstest.config.ts
new file mode 100644
index 000000000..9ed10f981
--- /dev/null
+++ b/e2e/browser-mode/fixtures/invalid-provider/rstest.config.ts
@@ -0,0 +1,10 @@
+import { defineConfig } from '@rstest/core';
+
+export default defineConfig({
+  browser: {
+    enabled: true,
+    provider: 'invalid' as unknown as 'playwright',
+    headless: true,
+  },
+  include: ['tests/**/*.test.ts'],
+});
diff --git a/e2e/browser-mode/fixtures/invalid-provider/tests/smoke.test.ts b/e2e/browser-mode/fixtures/invalid-provider/tests/smoke.test.ts
new file mode 100644
index 000000000..316ca0399
--- /dev/null
+++ b/e2e/browser-mode/fixtures/invalid-provider/tests/smoke.test.ts
@@ -0,0 +1,7 @@
+import { describe, expect, it } from '@rstest/core';
+
+describe('smoke', () => {
+  it('should not run with invalid provider', () => {
+    expect(true).toBe(true);
+  });
+});
diff --git a/packages/browser/src/configValidation.ts b/packages/browser/src/configValidation.ts
new file mode 100644
index 000000000..be34ab30b
--- /dev/null
+++ b/packages/browser/src/configValidation.ts
@@ -0,0 +1,66 @@
+import type { Rstest } from '@rstest/core/browser';
+import { resolveBrowserViewportPreset } from './viewportPresets';
+
+const SUPPORTED_PROVIDERS = ['playwright'] as const;
+
+const isPlainObject = (value: unknown): value is Record<string, unknown> => {
+  return Object.prototype.toString.call(value) === '[object Object]';
+};
+
+const validateViewport = (viewport: unknown): void => {
+  if (viewport == null) {
+    return;
+  }
+
+  if (typeof viewport === 'string') {
+    const presetId = viewport.trim();
+    if (!presetId) {
+      throw new Error('browser.viewport must be a non-empty preset id.');
+    }
+    if (!resolveBrowserViewportPreset(presetId)) {
+      throw new Error(
+        `browser.viewport must be a valid preset id. Received: ${viewport}`,
+      );
+    }
+    return;
+  }
+
+  if (isPlainObject(viewport)) {
+    const width = (viewport as any).width;
+    const height = (viewport as any).height;
+    if (!Number.isFinite(width) || width <= 0) {
+      throw new Error('browser.viewport.width must be a positive number.');
+    }
+    if (!Number.isFinite(height) || height <= 0) {
+      throw new Error('browser.viewport.height must be a positive number.');
+    }
+    return;
+  }
+
+  throw new Error(
+    'browser.viewport must be either a preset id or { width, height }.',
+  );
+};
+
+export const validateBrowserConfig = (context: Rstest): void => {
+  for (const project of context.projects) {
+    const browser = project.normalizedConfig.browser;
+    if (!browser.enabled) {
+      continue;
+    }
+
+    if (!browser.provider) {
+      throw new Error(
+        'browser.provider is required when browser.enabled is true.',
+      );
+    }
+
+    if (!SUPPORTED_PROVIDERS.includes(browser.provider)) {
+      throw new Error(
+        `browser.provider must be one of: ${SUPPORTED_PROVIDERS.join(', ')}.`,
+      );
+    }
+
+    validateViewport(browser.viewport);
+  }
+};
diff --git a/packages/browser/src/index.ts b/packages/browser/src/index.ts
index a78b4ba71..b11c3e228 100644
--- a/packages/browser/src/index.ts
+++ b/packages/browser/src/index.ts
@@ -9,6 +9,14 @@ import {
   runBrowserController,
 } from './hostController';
 
+export { validateBrowserConfig } from './configValidation';
+
+export {
+  BROWSER_VIEWPORT_PRESET_DIMENSIONS,
+  BROWSER_VIEWPORT_PRESET_IDS,
+  resolveBrowserViewportPreset,
+} from './viewportPresets';
+
 export async function runBrowserTests(
   context: Rstest,
   options?: BrowserTestRunOptions,
diff --git a/packages/browser/src/viewportPresets.ts b/packages/browser/src/viewportPresets.ts
new file mode 100644
index 000000000..2f228254b
--- /dev/null
+++ b/packages/browser/src/viewportPresets.ts
@@ -0,0 +1,64 @@
+/**
+ * Runtime source of truth for browser viewport presets.
+ *
+ * IMPORTANT: Keep this list/map in sync with `DevicePreset` typing in
+ * `@rstest/core` (`packages/core/src/types/config.ts`) so `defineConfig`
+ * autocomplete and runtime validation stay consistent.
+ */
+export const BROWSER_VIEWPORT_PRESET_IDS = [
+  'iPhoneSE',
+  'iPhoneXR',
+  'iPhone12Pro',
+  'iPhone14ProMax',
+  'Pixel7',
+  'SamsungGalaxyS8Plus',
+  'SamsungGalaxyS20Ultra',
+  'iPadMini',
+  'iPadAir',
+  'iPadPro',
+  'SurfacePro7',
+  'SurfaceDuo',
+  'GalaxyZFold5',
+  'AsusZenbookFold',
+  'SamsungGalaxyA51A71',
+  'NestHub',
+  'NestHubMax',
+] as const;
+
+type BrowserViewportPresetId = (typeof BROWSER_VIEWPORT_PRESET_IDS)[number];
+
+type BrowserViewportSize = {
+  width: number;
+  height: number;
+};
+
+export const BROWSER_VIEWPORT_PRESET_DIMENSIONS: Record<
+  BrowserViewportPresetId,
+  BrowserViewportSize
+> = {
+  iPhoneSE: { width: 375, height: 667 },
+  iPhoneXR: { width: 414, height: 896 },
+  iPhone12Pro: { width: 390, height: 844 },
+  iPhone14ProMax: { width: 430, height: 932 },
+  Pixel7: { width: 412, height: 915 },
+  SamsungGalaxyS8Plus: { width: 360, height: 740 },
+  SamsungGalaxyS20Ultra: { width: 412, height: 915 },
+  iPadMini: { width: 768, height: 1024 },
+  iPadAir: { width: 820, height: 1180 },
+  iPadPro: { width: 1024, height: 1366 },
+  SurfacePro7: { width: 912, height: 1368 },
+  SurfaceDuo: { width: 540, height: 720 },
+  GalaxyZFold5: { width: 344, height: 882 },
+  AsusZenbookFold: { width: 853, height: 1280 },
+  SamsungGalaxyA51A71: { width: 412, height: 914 },
+  NestHub: { width: 1024, height: 600 },
+  NestHubMax: { width: 1280, height: 800 },
+};
+
+export const resolveBrowserViewportPreset = (
+  presetId: string,
+): BrowserViewportSize | null => {
+  const size =
+    BROWSER_VIEWPORT_PRESET_DIMENSIONS[presetId as BrowserViewportPresetId];
+  return size ?? null;
+};
diff --git a/packages/core/src/config.ts b/packages/core/src/config.ts
index a6e21670b..ac0b22dfd 100644
--- a/packages/core/src/config.ts
+++ b/packages/core/src/config.ts
@@ -8,56 +8,15 @@ import { dirname, isAbsolute, join, resolve } from 'pathe';
 import { isCI } from 'std-env';
 import type { NormalizedConfig, ProjectConfig, RstestConfig } from './types';
 import {
-  BROWSER_VIEWPORT_PRESET_IDS,
   castArray,
   color,
   DEFAULT_CONFIG_EXTENSIONS,
   DEFAULT_CONFIG_NAME,
   formatRootStr,
-  isPlainObject,
   logger,
   TEMP_RSTEST_OUTPUT_DIR_GLOB,
 } from './utils';
 
-const VALID_BROWSER_VIEWPORT_PRESETS = new Set<string>(
-  BROWSER_VIEWPORT_PRESET_IDS,
-);
-
-const validateBrowserViewport = (viewport: unknown): void => {
-  if (viewport == null) {
-    return;
-  }
-
-  if (typeof viewport === 'string') {
-    const presetId = viewport.trim();
-    if (!presetId) {
-      throw new Error('browser.viewport must be a non-empty preset id.');
-    }
-    if (!VALID_BROWSER_VIEWPORT_PRESETS.has(presetId)) {
-      throw new Error(
-        `browser.viewport must be a valid preset id. Received: ${viewport}`,
-      );
-    }
-    return;
-  }
-
-  if (isPlainObject(viewport)) {
-    const width = (viewport as any).width;
-    const height = (viewport as any).height;
-    if (!Number.isFinite(width) || width <= 0) {
-      throw new Error('browser.viewport.width must be a positive number.');
-    }
-    if (!Number.isFinite(height) || height <= 0) {
-      throw new Error('browser.viewport.height must be a positive number.');
-    }
-    return;
-  }
-
-  throw new Error(
-    'browser.viewport must be either a preset id or { width, height }.',
-  );
-};
-
 const findConfig = (basePath: string): string | undefined => {
   return DEFAULT_CONFIG_EXTENSIONS.map((ext) => basePath + ext).find(
     fs.existsSync,
@@ -254,27 +213,6 @@ const createDefaultConfig = (): NormalizedConfig => ({
 });
 
 export const withDefaultConfig = (config: RstestConfig): NormalizedConfig => {
-  // Validate browser config when browser mode is enabled.
-  if (config.browser?.enabled === true) {
-    if (!config.browser.provider) {
-      throw new Error(
-        'browser.provider is required when browser.enabled is true.',
-      );
-    }
-
-    // Keep runtime validation even though TypeScript narrows the type, since
-    // config can be loaded from JS or forced via `as unknown as RstestConfig`.
-    const supportedProviders = ['playwright'] as const;
-    if (!supportedProviders.includes(config.browser.provider)) {
-      throw new Error(
-        `browser.provider must be one of: ${supportedProviders.join(', ')}.`,
-      );
-    }
-
-    // Validate viewport (optional)
-    validateBrowserViewport((config.browser as any).viewport);
-  }
-
   const merged = mergeRstestConfig(
     createDefaultConfig(),
     config,
diff --git a/packages/core/src/core/browserLoader.ts b/packages/core/src/core/browserLoader.ts
index 51a4ed131..18b52462e 100644
--- a/packages/core/src/core/browserLoader.ts
+++ b/packages/core/src/core/browserLoader.ts
@@ -13,6 +13,7 @@ export type { BrowserTestRunOptions, BrowserTestRunResult } from '../types';
  * Type definition for the @rstest/browser package exports.
  */
 export interface BrowserModule {
+  validateBrowserConfig: (context: unknown) => void;
   runBrowserTests: (
     context: unknown,
     options?: BrowserTestRunOptions,
diff --git a/packages/core/src/core/listTests.ts b/packages/core/src/core/listTests.ts
index 32858e3b5..ff0eb0056 100644
--- a/packages/core/src/core/listTests.ts
+++ b/packages/core/src/core/listTests.ts
@@ -190,10 +190,11 @@ const collectBrowserTests = async ({
   const { loadBrowserModule } = await import('./browserLoader');
   // Pass project roots to resolve @rstest/browser from project-specific node_modules
   const projectRoots = browserProjects.map((p) => p.rootPath);
-  const { listBrowserTests } = await loadBrowserModule({ projectRoots });
-  // Cast to any because listBrowserTests expects Rstest but we have RstestContext
-  // In practice, context is always a Rstest instance
-  return listBrowserTests(context as any, { shardedEntries });
+  const { validateBrowserConfig, listBrowserTests } = await loadBrowserModule({
+    projectRoots,
+  });
+  validateBrowserConfig(context);
+  return listBrowserTests(context, { shardedEntries });
 };
 
 const collectTestFiles = async ({
diff --git a/packages/core/src/core/runTests.ts b/packages/core/src/core/runTests.ts
index cb51a8c82..7ca4ceb8e 100644
--- a/packages/core/src/core/runTests.ts
+++ b/packages/core/src/core/runTests.ts
@@ -28,7 +28,10 @@ async function runBrowserModeTests(
   options: BrowserTestRunOptions,
 ): Promise<BrowserTestRunResult | void> {
   const projectRoots = browserProjects.map((p) => p.rootPath);
-  const { runBrowserTests } = await loadBrowserModule({ projectRoots });
+  const { validateBrowserConfig, runBrowserTests } = await loadBrowserModule({
+    projectRoots,
+  });
+  validateBrowserConfig(context);
   return runBrowserTests(context, options);
 }
 
@@ -166,6 +169,10 @@ export async function runTests(context: Rstest): Promise<void> {
       skipOnTestRunEnd: shouldUnifyReporter,
       shardedEntries: shard ? browserEntries : undefined,
     });
+
+    // Prevent an unhandled rejection window in mixed node+browser runs.
+    // We still await the original promise later to surface the error.
+    browserResultPromise.catch(() => {});
   }
 
   // If there are no node tests to run, we can potentially exit early.
diff --git a/packages/core/src/env.d.ts b/packages/core/src/env.d.ts
index c6ead8ed5..ec1bd7da3 100644
--- a/packages/core/src/env.d.ts
+++ b/packages/core/src/env.d.ts
@@ -11,6 +11,7 @@ declare const PLAYWRIGHT_VERSION: string;
 declare module '@rstest/browser' {
   import type { ListCommandResult, RstestContext } from './types';
 
+  export function validateBrowserConfig(context: RstestContext): void;
   export function runBrowserTests(context: RstestContext): Promise<void>;
   export function listBrowserTests(context: RstestContext): Promise<{
     list: ListCommandResult[];
diff --git a/packages/core/src/types/config.ts b/packages/core/src/types/config.ts
index 2e4f38906..df59030bc 100644
--- a/packages/core/src/types/config.ts
+++ b/packages/core/src/types/config.ts
@@ -1,7 +1,6 @@
 import type { RsbuildConfig } from '@rsbuild/core';
 import type { SnapshotStateOptions } from '@vitest/snapshot';
 import type { config } from 'chai';
-import type { BROWSER_VIEWPORT_PRESET_IDS } from '../utils/constants';
 import type { CoverageOptions, NormalizedCoverageOptions } from './coverage';
 import type {
   BuiltInReporterNames,
@@ -54,8 +53,32 @@ export type BrowserName = 'chromium' | 'firefox' | 'webkit';
  * Device presets aligned with Chrome DevTools device toolbar.
  *
  * These values are stable identifiers (not user-facing labels).
+ *
+ * IMPORTANT: Keep this union in sync with
+ * `@rstest/browser` preset runtime source:
+ * `packages/browser/src/viewportPresets.ts`.
+ *
+ * `@rstest/core` owns `defineConfig` typing, while `@rstest/browser` owns
+ * runtime validation and resolution for preset ids.
  */
-export type DevicePreset = (typeof BROWSER_VIEWPORT_PRESET_IDS)[number];
+export type DevicePreset =
+  | 'iPhoneSE'
+  | 'iPhoneXR'
+  | 'iPhone12Pro'
+  | 'iPhone14ProMax'
+  | 'Pixel7'
+  | 'SamsungGalaxyS8Plus'
+  | 'SamsungGalaxyS20Ultra'
+  | 'iPadMini'
+  | 'iPadAir'
+  | 'iPadPro'
+  | 'SurfacePro7'
+  | 'SurfaceDuo'
+  | 'GalaxyZFold5'
+  | 'AsusZenbookFold'
+  | 'SamsungGalaxyA51A71'
+  | 'NestHub'
+  | 'NestHubMax';
 
 export type BrowserViewport =
   | {
diff --git a/packages/core/src/utils/constants.ts b/packages/core/src/utils/constants.ts
index 9873fc215..e36496ec8 100644
--- a/packages/core/src/utils/constants.ts
+++ b/packages/core/src/utils/constants.ts
@@ -21,26 +21,6 @@ export const DEFAULT_CONFIG_EXTENSIONS = [
   '.cts',
 ] as const;
 
-export const BROWSER_VIEWPORT_PRESET_IDS = [
-  'iPhoneSE',
-  'iPhoneXR',
-  'iPhone12Pro',
-  'iPhone14ProMax',
-  'Pixel7',
-  'SamsungGalaxyS8Plus',
-  'SamsungGalaxyS20Ultra',
-  'iPadMini',
-  'iPadAir',
-  'iPadPro',
-  'SurfacePro7',
-  'SurfaceDuo',
-  'GalaxyZFold5',
-  'AsusZenbookFold',
-  'SamsungGalaxyA51A71',
-  'NestHub',
-  'NestHubMax',
-] as const;
-
 export const globalApis: (keyof Rstest)[] = [
   'test',
   'describe',
diff --git a/packages/core/tests/config.test.ts b/packages/core/tests/config.test.ts
index ba470adfc..b84b959d6 100644
--- a/packages/core/tests/config.test.ts
+++ b/packages/core/tests/config.test.ts
@@ -92,26 +92,22 @@ describe('mergeRstestConfig', () => {
   });
 });
 
-describe('withDefaultConfig browser validation', () => {
-  it('should throw error when browser.enabled is true but provider is missing', () => {
-    // Use 'as any' to bypass TypeScript check and test runtime validation
+describe('withDefaultConfig browser normalization', () => {
+  it('should not throw when browser.enabled is true and provider is missing', () => {
     const config = {
       browser: { enabled: true },
     } as RstestConfig;
 
-    expect(() => withDefaultConfig(config)).toThrow(
-      'browser.provider is required when browser.enabled is true.',
-    );
+    expect(() => withDefaultConfig(config)).not.toThrow();
   });
 
-  it('should throw error when browser.enabled is true but provider is invalid', () => {
+  it('should preserve custom provider value for browser loader validation', () => {
     const config = {
       browser: { enabled: true, provider: 'invalid' },
     } as unknown as RstestConfig;
 
-    expect(() => withDefaultConfig(config)).toThrow(
-      'browser.provider must be one of: playwright.',
-    );
+    const normalized = withDefaultConfig(config);
+    expect(normalized.browser.provider).toBe('invalid');
   });
 
   it('should not throw when browser.enabled is true and provider is playwright', () => {
@@ -122,7 +118,7 @@ describe('withDefaultConfig browser validation', () => {
     expect(() => withDefaultConfig(config)).not.toThrow();
   });
 
-  it('should throw when browser.viewport preset id is invalid', () => {
+  it('should preserve viewport value for browser loader validation', () => {
     const config = {
       browser: {
         enabled: true,
@@ -131,9 +127,8 @@ describe('withDefaultConfig browser validation', () => {
       },
     } as unknown as RstestConfig;
 
-    expect(() => withDefaultConfig(config)).toThrow(
-      'browser.viewport must be a valid preset id.',
-    );
+    const normalized = withDefaultConfig(config);
+    expect(normalized.browser.viewport).toBe('iPhone99');
   });
 
   it('should not throw when browser.viewport preset id is valid', () => {