diff --git a/tests/playwright-config.test.mjs b/tests/playwright-config.test.mjs
new file mode 100644
index 00000000..cff4a229
--- /dev/null
+++ b/tests/playwright-config.test.mjs
@@ -0,0 +1,101 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { existsSync, readdirSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { join } from 'node:path';
+
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+
+// playwright.config.js reads process.env at module scope, so every branch has
+// to be observed through a fresh import under the env it depends on.
+async function loadConfig(env = {}) {
+  const previous = {};
+  for (const [key, value] of Object.entries(env)) {
+    previous[key] = process.env[key];
+    if (value === undefined) delete process.env[key];
+    else process.env[key] = value;
+  }
+  try {
+    const suffix = `?load=${Math.random()}`;
+    return (await import(`../playwright.config.js${suffix}`)).default;
+  } finally {
+    for (const [key, value] of Object.entries(previous)) {
+      if (value === undefined) delete process.env[key];
+      else process.env[key] = value;
+    }
+  }
+}
+
+const cleanEnv = { CI: undefined, E2E_PORT: undefined };
+
+test('defaults to port 3000 when E2E_PORT is unset', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.equal(config.use.baseURL, 'http://localhost:3000');
+  assert.equal(config.webServer.url, 'http://localhost:3000');
+  assert.match(config.webServer.command, /--port 3000$/);
+});
+
+test('threads E2E_PORT through baseURL and the web server command', async () => {
+  const config = await loadConfig({ ...cleanEnv, E2E_PORT: '4321' });
+  assert.equal(config.use.baseURL, 'http://localhost:4321');
+  assert.equal(config.webServer.url, 'http://localhost:4321');
+  assert.match(config.webServer.command, /--port 4321$/);
+});
+
+test('relaxes CI-only guards when CI is unset', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.equal(config.forbidOnly, false);
+  assert.equal(config.retries, 0);
+  assert.equal(config.reporter, 'list');
+  assert.equal(config.webServer.reuseExistingServer, true);
+});
+
+test('tightens CI-only guards when CI is set', async () => {
+  const config = await loadConfig({ ...cleanEnv, CI: '1' });
+  assert.equal(config.forbidOnly, true);
+  assert.equal(config.retries, 1);
+  assert.equal(config.reporter, 'github');
+  assert.equal(config.webServer.reuseExistingServer, false);
+});
+
+test('treats an empty CI value as not running in CI', async () => {
+  const config = await loadConfig({ ...cleanEnv, CI: '' });
+  assert.equal(config.forbidOnly, false);
+  assert.equal(config.retries, 0);
+  assert.equal(config.reporter, 'list');
+  assert.equal(config.webServer.reuseExistingServer, true);
+});
+
+test('runs the suite fully parallel and retains traces on failure', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.equal(config.fullyParallel, true);
+  assert.equal(config.use.trace, 'retain-on-failure');
+});
+
+test('declares a single Desktop Chrome project', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.deepEqual(
+    config.projects.map((project) => project.name),
+    ['chromium'],
+  );
+  assert.equal(config.projects[0].use.defaultBrowserType, 'chromium');
+  assert.equal(config.projects[0].use.isMobile, false);
+  assert.match(config.projects[0].use.userAgent, /Chrome\//);
+});
+
+test('points testDir at a directory that holds spec files', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.equal(config.testDir, 'tests/e2e');
+  const testDir = join(repoRoot, config.testDir);
+  assert.ok(existsSync(testDir), `${config.testDir} does not exist`);
+  const specs = readdirSync(testDir).filter((entry) =>
+    entry.endsWith('.spec.js'),
+  );
+  assert.ok(specs.length > 0, `${config.testDir} contains no .spec.js files`);
+});
+
+test('gives the web server a bounded startup timeout', async () => {
+  const config = await loadConfig(cleanEnv);
+  assert.equal(typeof config.webServer.timeout, 'number');
+  assert.ok(config.webServer.timeout > 0);
+});