FlyingCDN automatically converts and delivers images in WebP format to improve performance. However, in some cases, images may still be served in their original format. Here are the most common reasons:
Possible reasons
input_too_large
The image is too large or complex to convert.
Tip: Use images under 10MB and within 1000–2000 pixels in dimension.
not_compressed
The original image is already fully optimized, so no further compression is applied.
webp_bigger
The WebP version would be larger than the original, so it’s skipped to avoid a performance hit.
cannot_optimize
The image is incomplete or corrupted. Re-uploading a clean version may help.
format_not_supported
The image format is not supported for conversion (e.g., BMP or TIFF), or a conflicting plugin is altering it. Use standard formats like JPEG or PNG and avoid other optimization tools.
vary_header_present
Your server is sending a Vary header with unsupported values. Only accept-encoding should be present to allow proper optimization.
How to check WebP delivery
Open browser developer tools (usually F12).
Go to the Network tab and reload your site.
Click on an image request.
Under Response Headers, look for:
content-type: image/webp
This confirms that the image is being delivered in WebP format.
Additional insight
The Cf-Polished response header provides insights into the image processing and optimization attempts by FlyingCDN. It can indicate whether an image was converted, compressed, or skipped, along with the reason.
