304 Not Modified
304 Not Modified is a server's reply saying a page hasn't changed since the visitor last fetched it, so the stored copy can be reused.
304 Not Modified is a reply your server sends when a browser or bot asks for a page it has already fetched, and the page hasn't changed since. Instead of sending the whole page again, the server says "nothing new here," and the visitor uses the copy it already has.
You won't see it as a person browsing. It matters to marketers because crawlers, search bots and AI bots among them, come back to your pages again and again, and a 304 lets those visits cost your server very little.
How it works
When a visitor first fetches a page, your server can include a date it last changed, or a short fingerprint of the page. The next time, the visitor sends that date or fingerprint back and asks, in effect, "has this changed?" If it hasn't, the server answers 304 with no page attached. If it has, it sends the new page as normal. Mozilla's page on 304 Not Modified has the technical detail.
Google's crawlers treat a 304 as a sign that the content is the same as last time, according to Google's guide to HTTP status codes.
When it goes wrong
A 304 is only useful if it's honest. If your server or CDN answers 304 when the page has in fact changed, crawlers keep the old version. You update your prices, and the bots carry on reading last month's.
For example, Kestrel Roasters raises the price of its espresso blend, but a caching rule on its CDN keeps telling crawlers the page hasn't changed. For weeks, the old price is what any crawler that relies on that reply has on file, and what an answer built from that copy might repeat.
Why it matters for AI search
AI platforms work from the copies of your pages they fetched. When you change something that matters, like a price, a product name or what you offer, you want that change picked up. Check that your important pages return a fresh page after you edit them, and a 304 only when nothing has changed.