About this tool
Audit image response exports before deployment so format-negotiated AVIF/WebP delivery includes Vary: Accept and avoids cache-fragmenting header patterns.
The Image Vary Accept Header Checker inspects HTTP response headers on image URLs to verify that endpoints performing content negotiation include the correct Vary: Accept directive. When a server selects between AVIF, WebP, and legacy formats based on the request's Accept header but omits Vary, shared caches and CDNs may serve the wrong format to subsequent visitors. This tool surfaces every image where that mismatch risk exists.
- Parses rows in URL|image-url|accept|vary|content-type|status|context|bytes format and normalizes relative URLs with an optional base URL.
- Flags missing Vary headers, missing Vary: Accept on format-negotiated image responses, wildcard Vary usage, and high-cardinality Vary tokens.
- Highlights critical-image risk bytes per page so teams can prioritize Vary header fixes for LCP-sensitive assets first.
How to use Vary Accept Checker
Provide a page URL and the tool fetches each image resource, examines the response's Content-Type and Vary headers, and flags images where format negotiation is happening without a matching Vary: Accept token. Results are grouped by severity so you can prioritize high-traffic or LCP-critical images first. After updating your CDN or origin server rules, re-run the check to confirm every negotiated image carries the proper cache-key signal.
When this tool is useful
- Audit image response headers before launch when AVIF/WebP format negotiation is handled through CDNs or edge transforms.
- Catch missing Vary: Accept directives that can return the wrong image variant from shared caches.
- Prioritize high-impact hero/LCP images where missing Vary coverage can create both SEO and UX regressions.
Practical tips
- If an endpoint negotiates format by Accept header, keep Vary: Accept explicit even when the CDN appears to auto-handle variants.
- Avoid Vary: * and high-cardinality Vary tokens unless you have a measured reason; they fragment cache keys and reduce hit rates.
- Track critical-image byte risk by page so missing Vary fixes land first on LCP-sensitive templates.
Why people use this tool
A missing Vary: Accept header on a format-negotiated image means a CDN edge node could cache the WebP variant and serve it to a browser that only supports JPEG, or vice versa. This leads to broken images, inflated byte transfers, and inconsistent rendering across user agents. Correct Vary directives ensure each cached variant is keyed to the requesting client's capabilities, protecting both performance and visual reliability at scale.
Related search intents
image vary accept checker, vary accept header validator, image format negotiation checker, image cache vary header audit.