AI AI Tools Guidebook
Troubleshooting

Last-Modified Response Header Missing

Page updated but Google never re-crawls — server isn't returning `Last-Modified` / `ETag`.

Published: Author: AI Productivity Guide Team 🌐 查看中文版本

You updated an article. Days pass. Google’s cached version still shows the old content. dateModified in your JSON-LD is correct, the page itself renders the new content — but Google hasn’t re-crawled. One likely reason: your server isn’t returning a Last-Modified HTTP header (or returns the same one for every request regardless of content change). Without Last-Modified, Google can’t tell if a page changed since its last crawl, and de-prioritizes recrawl in favor of other pages.

This is most common on static sites: Vercel and Netlify usually set it correctly, but Firebase Hosting and some self-hosted setups don’t.

Common causes

Ordered by hit rate, highest first.

1. Hosting platform doesn’t set Last-Modified

Firebase Hosting, GitHub Pages, and some custom nginx configs don’t automatically set Last-Modified for static files.

How to spot it:

curl -sI "https://yoursite.com/article" | grep -i last-modified

Empty result = not set.

2. CDN strips the header

Origin returns Last-Modified. Cloudflare or another CDN strips it for caching reasons.

How to spot it: Two curl -I commands — one against your origin (bypassing CDN), one normal. If origin has Last-Modified but the CDN-served version doesn’t, CDN is the issue.

3. Last-Modified is always the same value (deploy time)

Some static-site generators set Last-Modified to the build time, so every page on the site has the same value. Google sees no diff between pages and treats it as a non-signal.

How to spot it: curl -I two different pages. If they share the same Last-Modified, this is the bug.

4. Cache-Control: no-store overrides

If you set Cache-Control: no-store, Last-Modified becomes meaningless because clients won’t cache anyway. Google may also discount the signal.

How to spot it:

curl -sI "https://yoursite.com/article" | grep -E 'cache-control|last-modified'

If cache-control: no-store and last-modified both present, conflict.

5. Using ETag instead, but missing Last-Modified

ETag is fine but Google prefers Last-Modified for HTML pages. If you only emit ETag, you’re partially right but missing the more useful signal.

How to spot it: Headers have ETag but no Last-Modified. Add both.

6. Page is served by a worker / SSR function that doesn’t set headers

Cloudflare Worker or Vercel Edge function generating the page doesn’t add Last-Modified by default.

How to spot it: Page is dynamically generated (check cf-ray or server header) AND Last-Modified is missing.

Shortest path to fix

Step 1: Verify the missing header

curl -sI "https://yoursite.com/article" | head -10

Look for Last-Modified line. Absent = need to add.

Step 2: Enable per-platform

Vercel — works by default for static files. For SSR routes, add to response:

return new Response(html, {
  headers: { 'Last-Modified': new Date(article.modifiedAt).toUTCString() },
});

Netlify — works by default. Custom headers via _headers file:

/article/*
  Last-Modified: ...

(Note: dynamic value isn’t supported in _headers; use Netlify Functions for that.)

Firebase Hosting — explicit headers in firebase.json:

{
  "hosting": {
    "headers": [{
      "source": "**/*.html",
      "headers": [
        { "key": "Last-Modified", "value": "Wed, 21 Oct 2025 07:28:00 GMT" }
      ]
    }]
  }
}

Firebase static doesn’t compute it dynamically; you can set a per-deploy value via build script.

GitHub Pages — doesn’t support custom headers. Move off it if Last-Modified matters.

Cloudflare Workers — manually add to Response:

return new Response(body, {
  headers: {
    'Last-Modified': lastModifiedDate.toUTCString(),
    'Cache-Control': 'public, max-age=3600',
  },
});

Step 3: Make the value reflect actual content change

Per article:

const lastModified = new Date(article.modifiedAt || article.publishedAt).toUTCString();

NOT build time for all pages.

Step 4: Don’t let CDN strip it

Cloudflare → Caching → Configuration → ensure Last-Modified isn’t in any transform rules. By default it’s preserved.

Step 5: Verify both origin and CDN

# Origin (replace with your origin IP or skip-CDN URL)
curl -sI "https://origin.yoursite.com/article" | grep -i last-modified

# CDN-served
curl -sI "https://yoursite.com/article" | grep -i last-modified

Both should show Last-Modified. If origin has it but CDN doesn’t, CDN is stripping.

Step 6: Use If-Modified-Since to test caching behavior

LM=$(curl -sI "https://yoursite.com/article" | grep -i last-modified | cut -d' ' -f2-)
curl -sI -H "If-Modified-Since: $LM" "https://yoursite.com/article"

Should return 304 Not Modified if the page hasn’t changed. If returns 200, your caching isn’t honoring the header.

Prevention

  • Right after every deploy, curl -I three representative pages and confirm Last-Modified is present and per-page distinct.
  • Set Last-Modified from the article’s actual modifiedAt field, not from build time.
  • Don’t strip the header in CDN config.
  • Pair Last-Modified with ETag (both is best practice).
  • For CDN-served HTML, add cache rules that respect Last-Modified and If-Modified-Since.

Tags: #SEO #Troubleshooting

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