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Edge CDN Status Codes and Behaviour

Understand how Prerender.io’s Edge CDN responds to requests and how to interpret headers and HTTP status codes for better troubleshooting and integration.

Overview

Prerender.io’s Edge CDN enhances performance and reliability by serving pre-rendered content closer to users and bots. To help developers and integrators better understand CDN behavior, a set of response headers and HTTP status codes is used. These indicators provide critical insights into whether content was served from cache, passed through for rendering, or failed due to authentication or configuration issues.

Understanding these signals allows for smarter debugging, better monitoring, and optimized integration, especially for SEO-driven workflows.

How It Works

Key Headers Explained

  1. X-Prerender-Request-ID

    • A unique identifier for every request.

    • Use it to trace logs or debug issues via support.

  2. X-Prerender-Mode

    • Values: CacheHit or CacheMiss

    • Indicates whether the response was served from cache or passed through for live rendering.

 

Cache Modes and Response Headers

We have implemented some new headers with the Edge CDN offering to improve your integration capabilities.

PassThrough headers (CacheMiss)

  • Cache-Control, Pragma, Expires - The following headers are applied to indicate to the crawler to not to cache the current response as this comes from a pass-through mode, and the expiration is set 60 second in the future. This will allow us to render the content and wait for the crawler to check back for the rendered content.

CacheHit headers

  • Expires, Last-Modified headers are attached to the content and configured respectfully to the cache's state. With this we can indicate that the content was last rendered at X, and there will be new content at Y date. The expiration is calculated based on the recache interval.

 

Authentication and Edge Access (HTTP 401)

Indicates that the request is not properly authenticated or the account does not have access for the Edge CDN network.

To authenticate the request, your integration has to send the following HTTP header:

X-Prerender-Token: %YOUR_RENDER_TOKEN%

Each response comes with additional hints:

  • Missing [X-Prerender-Token] header when the header is not present in the request
  • Invalid environment token when the token's format is not matching the /^[a-zA-Z0-9]{20}$/ pattern
  • Environment token is not registered. Please visit your dashboard and turn on the Edge option for your account. when your account does not

 

HTTP 500 – Internal Server Error

Uncommon response in case your environment configuration contains some irregurality. Please open a ticket with our support team, as this case requires us to verify your configuration.

Addition hints:

  • Invalid configuration is detected, please notify our support team

  • Could not fetch from the origin, please notify our support team can happen when your servers are blocking our request or the requested domain does not exists.

 

HTTP 502 – Bad Gateway

Caused by improper requests, this usually indicates an integration that forwards requests to our service when it shouldn’t.

Additional hints:

  • Prerender loop detected, please verify your integration when the user agent string contains the Prerender word, it happens when the integration is forwarding our own rendering process to our proxy. Please verify your forward rules.

  • %path% is not renderable hints that our system received a static resource call which should not be served by Prerender but the origin server. Can be corrected by filtering forward request for non static resources only.

 

HTTP 400 – Bad Request (Handled Internally)

Outstanding case when our integration detected an issue in the process and abandoned it. We log those errors and when a HTTP 400 is responded it means that we need to improve on our error messages.

Please note, this error can contain hints about the problem to help debugging but not an error which need to be handled by your team / integration.

Tips

  • Don’t forward static resources like images or scripts to Prerender — only HTML pages.

  • Avoid infinite loops by filtering requests with Prerender in the User-Agent.

  • Always use a valid X-Prerender-Token — and ensure your environment is registered for Edge CDN.

  • Use the X-Prerender-Mode header to verify whether a page is being cached properly.

  • Set realistic recache intervals to balance freshness with rendering load.

 

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