diff --git a/tools/lib/harness.mjs b/tools/lib/harness.mjs
index 229d15b5..06e837a0 100644
--- a/tools/lib/harness.mjs
+++ b/tools/lib/harness.mjs
@@ -424,6 +424,9 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
    */
   const signIn = async () => {
     await cdp.send('Network.enable');
+    // The browser keeps the default profile, disk cache included, and nginx sends no
+    // Cache-Control: a run could test the index.html of a build before the last one.
+    await cdp.send('Network.setCacheDisabled', { cacheDisabled: true });
     await cdp.send('Network.setCookie', {
       name: SESSION_COOKIE,
       value: await sessionToken(baseUrl),