# Content negotiation

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

## Definition

Content negotiation selects a representation using request headers such as Accept, Accept-Language, and Accept-Encoding.

## How it works

One URL can return different media types or languages, but caches and crawlers need explicit Vary metadata and stable canonical relationships.

## How to validate it

Replay controlled header matrices and hash every returned representation so variation is observable rather than guessed from the URL.

## Common failure

Serving materially different facts to crawler user-agents creates cloaking risk and destroys reproducibility.

## Related topics

- [Response compression](/atlas/compression)
- [hreflang alternates](/atlas/hreflang)