Error Codes for Failed Optimization Attempts

In this article, you can find details about the error codes for failed optimizations and what to do about them.

You can find the specific error code by hovering over the "Skipped" text.


Skipped 2000: Optimization was skipped because X-Nitro-Header: Disabled was detected

This means that at the time of the optimization attempt, one of the following was true:

  • The service was in Safe Mode
  • The page was excluded from optimization via the Excluded URLs setting or from the “Select what post/page types get optimized” setting in the NitroPack plugin.
  • The Included URLs setting is active, allowing NitroPack to optimize only the list of included URLs and the skipped URL is not part of the list.
  • NitroPack does not optimize the page type by default (e.g., checkout, cart and user profile pages).

Make sure to perform an individual purge of the URL after fixing the issue. The page should be optimized soon after that.

Skipped 2006: Optimization has been skipped due to a third party caching system detected 

  • You may encounter this error if you’re using another caching plugin. In that case, please turn off all other caching plugins listed in this article
  • This error could also be caused by a lack of integration with Cloudflare, Sucuri, or a Reverse Proxy. Follow the instructions in the linked articles if you’re using one of these services.

Redirected 301/302: URL is permanently/temporarily redirected to another URL

  • Check if the URL in question actually redirects to another one. If it does, remove it from the sitemap, if you’ve submitted one in the Cache Warmup panel; 
  • Also, please make sure that the sitemap is up to date with any changes you’ve made.

Forbidden 403: Your server did not allow us to optimize the page

404: Not Found 

  • This error means that the page was not found. This can indicate the presence of dead links somewhere on the site or the sitemap. Please make sure that all links on your site/sitemap are up to date.
  • When a URL returns a 404 Not Found error, NitroPack cannot optimize it. 

5000: Unknown 

  • This is an unexpected internal issue and may be fixed by purging the cache for the page
  • If the issue persists after purging the cache, please contact our support team at

5002: Reading response timed out

5005: Skipped 

  • The error means that Critical CSS extraction failed for the page.
  • A quick workaround is to perform an individual purge for the page until it’s optimized. 
  • In case a large number of pages are affected by this issue, please contact 

5010: Skipped

  • The error means that removing unused CSS failed for the page.
  • A quick workaround is to perform an individual purge for the page until it’s optimized. 
  • In case a large number of pages are affected by this issue, please contact 

5009: Not an HTML page 

  • This error appears when the system receives an optimization request for an URL that doesn’t point to a web page. For example, a URL that points to a CSS file directly.

5011: The page source file is too big

  • The page's HTML file has to be under 5MB for NitroPack to optimize it. This does not include external resources like CSS, JavaScript and images.
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  • I was told that reducing the level from Ludicrous to Strong might also eliminate some of these error codes. Is that true? And after I've adjusted the setting, how do I re-run the optimization. And a related question: where can I see the Before/After optimization breakdown? 

  • Hi Brian,

    Each error code is very specific, so you should follow the instructions in the article and contact our support team if you need assistance with anything. 

    We usually recommend switching from Ludicrous to Strong for websites that rely heavily on JavaScript to render content. In other words, it usually solves problems related to content rendering, not failed optimization attempts.

    After switching between modes and re-adjusting your settings, it's a good idea to purge your website's cache from the Dashboard. If you have Cache Warmup enabled, NitroPack will re-optimize your site automatically. If not, our service will start optimizing each page once it detects traffic to it.

    You can track the optimization process for each page from the Optimizations panel. Also, it's easy to see the difference between the optimized and unoptimized version of any page by adding ?nonitro to its URL, as explained in this video.

    Hope this helps.


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