TL;DR — A custom session-cookie login flow appeared to succeed on localhost (the OTP verified, the response looked fine) but every subsequent request to a login-gated page treated the visitor as logged out. Identical code worked fine on the live HTTPS site. The cookie's Secure attribute was hardcoded to true — and per the cookie spec, browsers never store or send a Secure cookie over a plain, non-HTTPS connection.

Diagnostic

Check the actual Set-Cookie response header and the browser's own cookie storage panel — on localhost over http://, the cookie is sent by the server but never actually stored by the browser.

Root cause

// before -- assumes the app is always served over HTTPS
setCookie("session", token, { secure: true, httpOnly: true });

A cookie config that quietly assumes "we're always on HTTPS" breaks the instant you test over plain HTTP, which local dev servers commonly are.

Fix

// after -- derive secure from the actual request protocol
const isHttps = request.url.startsWith("https://");
setCookie("session", token, { secure: isHttps, httpOnly: true });

Lessons learned

• Any Secure-flagged cookie needs to key off the real request scheme, not an assumption baked in once at cookie-creation time.
• "Works in production, silently fails in local dev" is a strong signal to check cookie flags before anything else in an auth flow.
• Check other cookies in the same codebase for the same hardcoded assumption — if one cookie has this bug, sibling cookies set the same way are worth auditing too.

TL;DR — GA4's gtag.js snippet was installed correctly, the page loaded with no visible JavaScript error, and GA4's real-time report still showed zero activity. The CMS's default Content-Security-Policy had no allowance for analytics domains and no config option to add one — so every request to Google's tracking endpoints was blocked at the browser level before it could fail loudly.

Diagnostic

Check the browser's dedicated CSP violation reporting, not the regular console error list — CSP blocks are reported through their own channel, not thrown as normal script errors, so "no console errors" doesn't mean nothing was blocked.

Root cause

The CSP's script-src and connect-src directives had no entry for googletagmanager.com or google-analytics.com, and the CMS exposed no configuration surface to add one — the only way in was patching the CSP directives directly.

// patch-package: add analytics domains to the existing CSP directives
scriptSrc.push("https://www.googletagmanager.com");
connectSrc.push("https://www.google-analytics.com", "https://www.googletagmanager.com");

Lessons learned

• "No console errors" is not proof nothing was blocked — CSP violations live in their own reporting surface and are easy to miss if you're only scanning for red error text.
• Before adding any third-party script tag to a site with a CSP already in place, check the CSP's directives first rather than assuming a silently-empty analytics dashboard means a snippet-installation mistake.