(Deprecated) I receive "Specify a cache validator" when testing with GTMetrix, why?

Note: this article is deprecated since November 16th 2020, due to the new changes GTmetrix implemented.

Sometimes, while running ShortPixel Adaptive Images on your website you can notice this issue when running a GTMetrix test:

Possible causes are:

  1. Transient issue. The images weren't yet fully processed and because of that they weren't yet cached by the CDN. This usually takes a few seconds and the solution to this problem is to run again the test a couple of times more in the following minutes and this error, in relation to ShortPixel, should disappear.
  2. Using a different CDN than ShortPixel CDN. Sometimes other CDNs (e.g. Cloudflare) can have settings that will remove the ETag sent by ShortPixel. The solution is to figure out a solution with your CDN provider. For Cloudflare you can read more about when ETag is removed here.
  3. Low Quality Placeholder. If you see that the URLs of the resources contain the string lqip in the URL, you can safely ignore them. It is a false positive. Those images you see are just low quality placeholders, not the "real images" that you see. This is an example of false positives:

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