# Conditional requests

Canonical HTML: /atlas/conditional-requests
Atlas version: atlas-2026-07-28-v1
Category: HTTP behavior and efficiency

## Definition

Conditional requests let a crawler ask whether a previously fetched representation has changed.

## How it works

The client sends validators such as If-None-Match or If-Modified-Since; an unchanged resource can return 304 without retransmitting the body.

## How to validate it

Log validator presence and response status, then verify that ETags and Last-Modified values change whenever meaningful content changes.

## Common failure

A static validator attached to changing content can prevent crawlers from seeing updates even while the server returns successful responses.

## Related topics

- [Cache validators](/atlas/cache-validators)
- [Content freshness](/atlas/content-freshness)