fix: do not throw when browser mode is used without explicit projects (#9801)

packages/vitest/src/node/config/resolveConfig.ts

@@ -314,17 +314,23 @@ export function resolveConfig(
     }
   }
 
-  // apply browser CLI options only if the config already has the browser config and not disabled manually
+  // apply browser CLI options only if the config already has the browser config,
+  // OR if this is the root config and we explicitly passed CLI options.
   if (
     vitest._cliOptions.browser
-    && resolved.browser
+    && (resolved.browser || vitest.vite?.config === viteConfig)
     // if enabled is set to `false`, but CLI overrides it, then always override it
-    && (resolved.browser.enabled !== false || vitest._cliOptions.browser.enabled)
+    && (resolved.browser?.enabled !== false || vitest._cliOptions.browser.enabled)
   ) {
     resolved.browser = mergeConfig(
       resolved.browser,
       vitest._cliOptions.browser,
     ) as ResolvedBrowserOptions
+    if (resolved.browser.instances) {
+      resolved.browser.instances.forEach((instance) => {
+        instance.name ??= resolved.name ? `${resolved.name} (${instance.browser})` : instance.browser
+      })
+    }
   }
 
   resolved.browser ??= {} as any

test/cli/test/config/browser-configs.test.ts

@@ -69,6 +69,55 @@ test('filters projects with a wildcard', async () => {
   ])
 })
 
+test('allows browser mode without projects', async () => {
+  await vitest({}, {
+    browser: {
+      enabled: true,
+      headless: true,
+      provider: playwright(),
+      instances: [{ browser: 'chromium' }],
+    },
+  })
+})
+
+test('applies browser CLI options to root config without existing browser config', async () => {
+  const { projects } = await vitest({
+    browser: {
+      enabled: true,
+      provider: preview(),
+      headless: true,
+      instances: [
+        { browser: 'chromium' },
+        { browser: 'firefox' },
+      ],
+    },
+  }, {})
+  expect(projects.map(p => p.name)).toEqual([
+    'chromium',
+    'firefox',
+  ])
+})
+
+test('assigns instance names from CLI options without existing browser config', async () => {
+  const { projects } = await vitest({
+    browser: {
+      enabled: true,
+      provider: preview(),
+      headless: true,
+      instances: [
+        { browser: 'chromium' },
+        { browser: 'firefox', name: 'my-firefox' },
+      ],
+    },
+  }, {
+    name: 'myproject',
+  })
+  expect(projects.map(p => p.name)).toEqual([
+    'myproject (chromium)',
+    'my-firefox',
+  ])
+})
+
 test('assigns names as browsers in a custom project', async () => {
   const { projects } = await vitest({}, {
     projects: [