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AB-028

Vary Header

🔄 Content Negotiation Pillar: Discovery Quick fix Severity: low Est. cost: $0 Gap: documentation Fix: deterministic

What it means

Your site tells caches that content varies based on the requested format.

Why it matters

Without a Vary header, caches may serve the wrong format to agents — JSON to someone who asked for HTML.

Evidence expected

This rule is verified by:

http Source: Runtime probe
✗

What's wrong

No Vary header. Caches serve stale or wrong formats.

✓

What's right

Vary header ensures each agent gets the correct format from caches.

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Related rules in Content Negotiation

🤖 Agent User-Agent Format
📊 JSON Format Support
📝 Plain Text Format Support
⬇️ Markdown Format Support
🎯 Preferred Format Selection
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Machine-Readable

This rule is also available as JSON: /rules/AB-028.json

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