diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/README.md b/crates/vp_cli_snapshots/tests/cli_snapshots/README.md
index fc3547a9f2..6ce4e4ea36 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/README.md
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/README.md
@@ -306,3 +306,14 @@ Fixture trees are excluded from repo-wide fmt, lint, typecheck, and vitest
 (`vite.config.ts`, `tsconfig.json`); recorded snapshots and
 `snapshots.toml` are runner metadata and never appear inside the staged
 workspace a test runs in.
+
+Test servers must request port `0` so the OS selects an available port at bind
+time. Fixed ports can be reserved on Windows, where `EACCES` does not trigger
+Vite's port-in-use fallback. Vitest 5.0.4 replaces a plain `test.api.port: 0`
+with its default, including when it comes from `--api.port=0`. Set the port in
+a post-configuration hook instead; see
+[`test_v5_browser_defines/config.mjs`](fixtures/test_v5_browser_defines/config.mjs)
+for independent, inherited, referenced, and injected projects, and
+[`test_v5_optional_packages/verify-ui.mjs`](fixtures/test_v5_optional_packages/verify-ui.mjs)
+for the UI server. Assert that each resolved API port is `0` so regressions fail
+on every platform, even when the default port is available.
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/config.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/config.mjs
index 1eada66183..33ed4c03de 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/config.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/config.mjs
@@ -10,6 +10,18 @@ export const executablePath = existsSync('chromium-path.json')
 
 export function project(name, browser = true) {
   return {
+    plugins: [{
+      name: 'browser-server-port',
+      // Each project needs its own hook, even with extends: true.
+      // Vitest replaces a plain api.port: 0 with its default. Override it last.
+      enforce: 'post',
+      config: {
+        order: 'post',
+        handler(config) {
+          config.test.api.port = 0;
+        },
+      },
+    }],
     define: {
       __VP_STRING_DEFINE__: JSON.stringify('/messages?locale="en"'),
       __VP_FALSE_DEFINE__: 'false',
@@ -43,7 +55,7 @@ export function configFor(name) {
   switch (name) {
     case 'injected-hook': {
       const config = project('root');
-      config.plugins = [{
+      config.plugins.push({
         name: 'inject-browser-project',
         configureVitest: {
           order: 'pre',
@@ -56,7 +68,7 @@ export function configFor(name) {
             await injectTestProjects({ extends: false, ...injected });
           },
         },
-      }];
+      });
       return config;
     }
     case 'injected': return project('root');
@@ -78,15 +90,18 @@ export function configFor(name) {
     case 'inherited': {
       const config = project('root');
       config.test.projects = [
-        { test: project('shared').test },
-        { extends: true, test: project('inherited').test },
+        { plugins: config.plugins, test: project('shared').test },
+        { extends: true, plugins: config.plugins, test: project('inherited').test },
       ];
       return config;
     }
     case 'separate': {
       const config = project('root');
       config.test.sharedViteServer = false;
-      config.test.projects = [{ test: project('first').test }, { test: project('second').test }];
+      config.test.projects = [
+        { plugins: config.plugins, test: project('first').test },
+        { plugins: config.plugins, test: project('second').test },
+      ];
       return config;
     }
     case 'independent': return {
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/verify.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/verify.mjs
index 7035c9983e..a588657cfd 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/verify.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_browser_defines/verify.mjs
@@ -39,6 +39,11 @@ try {
         ]);
       }
       assert.equal(runner.projects.length, count, name);
+      for (const project of runner.projects) {
+        if (project.config.browser.enabled) {
+          assert.equal(project.config.api.port, 0, `${name}: ${project.name} must use an OS-assigned port`);
+        }
+      }
       const servers = new Set(runner.projects.map((project) => project.vite));
       if (name === 'shared' || name === 'shared-node') assert.equal(servers.size, 1);
       if (name === 'separate') assert.equal(servers.size, 2);
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_optional_packages/verify-ui.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_optional_packages/verify-ui.mjs
index 27ac337d1e..0040653f63 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_optional_packages/verify-ui.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_optional_packages/verify-ui.mjs
@@ -4,9 +4,22 @@ import { createVitest } from 'vite-plus/test/node';
 
 const runner = await createVitest({
   config: false, watch: false, reporters: [], ui: true, open: false,
-  api: { host: '127.0.0.1', port: 0 },
-}, defineConfig({}));
+  api: { host: '127.0.0.1' },
+}, defineConfig({
+  plugins: [{
+    name: 'ui-server-port',
+    // Vitest replaces a plain api.port: 0 with its default. Override it last.
+    enforce: 'post',
+    config: {
+      order: 'post',
+      handler(config) {
+        config.test.api.port = 0;
+      },
+    },
+  }],
+}));
 try {
+  assert.equal(runner.config.api.port, 0);
   if (!runner.vite.httpServer.listening) await runner.vite.listen(0);
   const base = `http://127.0.0.1:${runner.vite.httpServer.address().port}`;
   const ui = new URL(runner.config.uiBase, base);
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_preview/verify.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_preview/verify.mjs
index 54108af36c..5abb89f12f 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_preview/verify.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_preview/verify.mjs
@@ -44,6 +44,7 @@ try {
       browser: { enabled: true, provider, headless: false, instances: [{ browser: 'chromium' }] },
     },
   }));
+  assert.equal(runner.projects[0].config.api.port, 0);
   const result = await runner.start();
   assert.equal(result.unhandledErrors.length, 0, JSON.stringify(result.unhandledErrors));
   assert.equal(runner.state.getFiles().length, 1);
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_screenshots/verify.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_screenshots/verify.mjs
index 37307f3255..6bdf19f276 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_screenshots/verify.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_screenshots/verify.mjs
@@ -38,6 +38,7 @@ for (const phase of ['record', 'compare', 'failure']) {
     },
   }));
   try {
+    assert.equal(runner.projects[0].config.api.port, 0);
     const result = await runner.start();
     assert.equal(result.unhandledErrors.length, 0, JSON.stringify(result.unhandledErrors));
     assert.equal(runner.state.getFiles().length, 1);
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_webdriverio/verify.mjs b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_webdriverio/verify.mjs
index 29c77f8283..918aab4085 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_webdriverio/verify.mjs
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/test_v5_webdriverio/verify.mjs
@@ -47,18 +47,32 @@ provider.providerFactory = (project) => {
   };
   return instance;
 };
-const runner = await createVitest({ config: false, watch: false, reporters: [] }, defineConfig({ test: {
-  include: ['browser.test.js'],
-  browser: {
-    enabled: true, headless: true, provider, instances: [{ browser: 'chrome' }],
-    viewport: { width: 800, height: 600 },
-    commands: { inspectLocator(context, locator) {
-      assert.equal(typeof locator, 'object');
-      return { ...locator, session: typeof context.sessionId === 'string' && context.sessionId.length > 0 };
-    } },
+const runner = await createVitest({ config: false, watch: false, reporters: [] }, defineConfig({
+  plugins: [{
+    name: 'browser-server-port',
+    // Vitest replaces a plain api.port: 0 with its default. Override it last.
+    enforce: 'post',
+    config: {
+      order: 'post',
+      handler(config) {
+        config.test.api.port = 0;
+      },
+    },
+  }],
+  test: {
+    include: ['browser.test.js'],
+    browser: {
+      enabled: true, headless: true, provider, instances: [{ browser: 'chrome' }],
+      viewport: { width: 800, height: 600 },
+      commands: { inspectLocator(context, locator) {
+        assert.equal(typeof locator, 'object');
+        return { ...locator, session: typeof context.sessionId === 'string' && context.sessionId.length > 0 };
+      } },
+    },
   },
-} }));
+}));
 try {
+  assert.equal(runner.projects[0].config.api.port, 0);
   const result = await runner.start();
   assert.equal(result.unhandledErrors.length, 0, inspect(result.unhandledErrors));
   assert.equal(runner.state.getCountOfFailedTests(), 0, JSON.stringify(runner.state.getFiles().flatMap((file) => [file.result, ...file.tasks.map((task) => task.result)])));
diff --git a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/vitest_browser_mode/vitest.config.ts b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/vitest_browser_mode/vitest.config.ts
index 00dd92e6b7..5a73fc804c 100644
--- a/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/vitest_browser_mode/vitest.config.ts
+++ b/crates/vp_cli_snapshots/tests/cli_snapshots/fixtures/vitest_browser_mode/vitest.config.ts
@@ -1,21 +1,29 @@
+import assert from 'node:assert/strict';
 import { defineConfig } from 'vite-plus';
 import { playwright } from 'vite-plus/test/browser-playwright';
 
 export default defineConfig({
   plugins: [
+    {
+      name: 'browser-server-port',
+      // Vitest replaces a plain api.port: 0 with its default. Override it last.
+      enforce: 'post',
+      config: {
+        order: 'post',
+        handler(config) {
+          config.test.api.port = 0;
+        },
+      },
+    },
     {
       name: 'vitest-browser-mode-suppress-known-vite-logs',
       configResolved(config) {
-        // "is in use, trying another one": port fallback depends on other
-        // concurrently running servers. "[optimizer]" progress lines fire on
-        // a 1s timer, so they only appear on slow cold-cache runs.
-        const nonDeterministicLogs = [
-          'is in use, trying another one',
-          '[optimizer]',
-        ];
+        assert.equal(config.test.api.port, 0);
+        // "[optimizer]" progress lines fire on a 1s timer, so they only
+        // appear on slow cold-cache runs.
         const info = config.logger.info;
         config.logger.info = (message, options) => {
-          if (!nonDeterministicLogs.some((log) => message.includes(log))) {
+          if (!message.includes('[optimizer]')) {
             info(message, options);
           }
         };