# Cache validators

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

## Definition

ETag and Last-Modified values identify the version or modification time of a representation.

## How it works

Strong ETags describe byte identity, weak ETags describe semantic equivalence, and modification dates provide a coarser time-based validator.

## How to validate it

Derive validators from the actual representation or a versioned content build and test them across compressed and uncompressed variants.

## Common failure

Using deployment time as Last-Modified for every page makes untouched content look freshly changed and wastes recrawl attention.

## Related topics

- [Conditional requests](/atlas/conditional-requests)
- [Response compression](/atlas/compression)