Missing Last-Modified header: enable conditional GET

What it means

The HTTP response for this page includes no Last-Modified header. Last-Modified tells crawlers when the content last changed, enabling a conditional If-Modified-Since request on the next visit. Without it (and without an ETag), Googlebot must download the full page body every time it crawls the URL.

Why it matters

When a page returns Last-Modified, Googlebot stores the timestamp and revalidates with If-Modified-Since; an unchanged page then returns 304 Not Modified with no body. Without any validator this negotiation is impossible and the full body is re-fetched on every crawl. Google's crawl-budget guidance names proper conditional-GET support as a server-side crawl-efficiency win; the impact compounds on large sites with many stable pages. This is a lab-proxy read from the response headers, not a measure of crawl frequency.

How to fix it

1. On Apache/Nginx, static files serve Last-Modified automatically from filesystem mtime — confirm it is not stripped.
2. For dynamic pages, store the content's last-updated timestamp and emit it as Last-Modified in RFC 7231 HTTP-date format.
3. On Vercel/Netlify/CDN edge functions, set the header explicitly for dynamic responses.
4. If a timestamp is impractical, emit an ETag (content hash) instead — either header enables conditional GET.

When it's not a problem

Genuinely dynamic pages — session-specific, personalised, or generated per request — cannot reliably emit Last-Modified and the absence is intentional. Pages behind auth, or proxies that strip headers, may also omit it. Suppress the finding for URL patterns where dynamic generation makes the header inappropriate.

How common is it?

10852 audited sites in our corpus currently show this issue. The breakdowns below show which platforms, gatekeepers, verticals and countries are most exposed.