diff --git a/.claude/rules/testing/playwright-tests.md b/.claude/rules/testing/playwright-tests.md
index 2520787ad3..6b7370f866 100644
--- a/.claude/rules/testing/playwright-tests.md
+++ b/.claude/rules/testing/playwright-tests.md
@@ -119,8 +119,11 @@ uv run python -m http.server 8765
 # Serves from tests/docs/playwright/
 ```
 
-Locally (`reuseExistingServer: !isCI`), Playwright reuses whatever already listens on that port, even another app.
-Every spec then fails with element-not-found errors; set `QUARTO_PLAYWRIGHT_PORT` to a free port.
+The fixture server is never reused (`reuseExistingServer: false`): if another process answers on the port, the run stops with `http://127.0.0.1:<port> is already used`.
+Each concurrent run (another worktree, a second terminal) needs its own free `QUARTO_PLAYWRIGHT_PORT`; the default 8765 suits only one run at a time.
+A process answering 404 on the port is not detected as busy; the run then times out after 120 s (`Timed out waiting 120000ms from config.webServer`), so pick another port.
+
+The revealjs multiplex server has a fixed port, 1948 (set in `tests/docs/playwright/revealjs/multiplex.qmd`), and is reused locally: concurrent runs share the first run's instance, which serves no test fixtures.
 
 ## Best Practices
 
diff --git a/tests/integration/playwright/playwright.config.ts b/tests/integration/playwright/playwright.config.ts
index c715b56090..689814dc28 100644
--- a/tests/integration/playwright/playwright.config.ts
+++ b/tests/integration/playwright/playwright.config.ts
@@ -100,7 +100,10 @@ export default defineConfig({
       // HTTP server for rendered HTML files
       command: `uv run python -m http.server ${fixtureServerPort} --bind 127.0.0.1`,
       url: fixtureServerUrl,
-      reuseExistingServer: !isCI,
+      // Never reuse: a server already on the port may be another app, or another
+      // checkout serving its own tests/docs/playwright, so tests would run
+      // against the wrong HTML. A busy port fails the run instead.
+      reuseExistingServer: false,
       cwd: '../../docs/playwright',
       // http.server does a reverse DNS lookup before listening, slow on some hosts
       timeout: 120000,
@@ -110,6 +113,9 @@ export default defineConfig({
       // Socket.IO multiplex server for RevealJS
       command: 'npm start',
       url: 'http://127.0.0.1:1948',
+      // Reused locally: it relays socket messages and serves no test fixtures,
+      // and its port is fixed in revealjs/multiplex.qmd, so concurrent runs
+      // from several checkouts share one instance.
       reuseExistingServer: !isCI,
       cwd: './multiplex-server',
       timeout: 10000,