“Provisional headers are shown” in Chrome – everything you need to know

If you're familiar with Chrome DevTools, you certainly have seen the "Provisional headers are shown" warning message popped up in certain network requests.

In this article, we will explain what exactly "Provisional headers" are, and what does "Provisional headers are shown" means in Google Chrome/Chromium-based browsers.

Provisional headers in Chromium/Chrome?

HTTP headers are the name or value pairs that are displayed in the request and response messages of message headers for Hypertext Transfer Protocol (HTTP). Request headers contain more information about the resource to be fetched, or about the client requesting the resource. Response headers hold additional information about the response, like its location or about the server providing it. These headers are usually invisible to the end-user and are only processed or logged by the server and client applications.

HTTP Headers displayed for pending requests are provisional. They represent what request was sent from Blink to Chromium/Chrome. To those of you who didn't know, Blink is the open-source browser layout engine that powers Chromium (and therefore Chrome as well).

Chromium/Chrome do not update headers until server responds to avoid additional notification used only by DevTools. In short, provisional headers mean the requests they are not yet responded to. Please do note that this is just a notification, not an actual error message.

If the request is meant to be responded properly, but in reality, "Provisional headers are shown" are displayed, it's usually because one of these reasons:

  • An extension has blocked your request
  • Chrome use its own cache to fetch your resource. The request was not sent over the network and instead was served from a local cache, which doesn't store the original request headers.
  • Internal error which made Chrome cannot get your request headers from a request that haven't made.

Check your adblockers

One of the most common reasons that causes "Provisional headers are shown" are Chrome extensions. The resource you're trying to load could be being blocked by an extension, usually ad-blockers such as AdBlock, uBlock Origin or Adguard.

Most adblockers work the same way, which is blocking the requests based on pattern matching. If an URL contains the word "advert", "ads" or something similar, it would be blocked from loading.

In order to make "Provisional headers are shown" disappear, you'll have to whitelist the URL out of adblockers block list. Below are instructions on how to do that with popular extensions like Adblock Plus and uBlock Origin.

Whitelist your site from Adblock Plus

  1. Navigate to the URL which have been blocked and shown "Provisional headers are shown".
  2. Click on the AdBlock Plus icon in the Chrome toolbar. If you don't see the icon, it may be hidden by Chrome/Chromium itself. Clicking the "jigsaw" icon will bring you the full list of all installed extensions.
  3. Click the toggle in Block ads on this website section to whitelist the whole domain out of Adblock Plus blocklist. Alternatively, you can whitelist only the URL you're visiting by clicking the smaller toggle on This page section.

Whitelist your site from uBlock Origin

  1. Navigate to the URL which have been blocked and shown "Provisional headers are shown".
  2. uBlock circumvents immediately and show you a message which notify you that the URL has been blocked. You can either click Temporarily to unblock the URL until you close the browser, or click Permanently to unblock the URL forever.

Find the troublesome extension (Advanced)

In case your adblocker isn't the cause of the error, there might be another extension interfering with the request-response cycle.

In order to pinpoint the exact troublesome extension, you have to use Chrome net-internals tools. How to access the tool depends on which version of Chromium/Chrome you're using.

Latest Versions of Chrome

  • Navigate to chrome://net-export/.
  • Click Start Logging to Disk to save the logs to the hard drive. The button should change to Stop Logging
  • Open the page that is was thrown "Provisional Headers are shown".
  • Go back to net-export page and click Stop Logging.
  • Go to https://netlog-viewer.appspot.com/#import and drag-drop the log file you've just saved into the webpage.
  • Click on Events (###) section in the sidebar and use the input box to find the event related to your resource (tips: use parts of the URL as search query).
  • Finally, click on the event and see if the info shown tells you something interesting.

For Older Versions of chrome

  • Type chrome://net-internals in the address bar and hit enter.
  • Open the page that is showing problems.
  • Go back to net-internals, click on Events (###) and search using input box to find the event related to your resource.
  • Finally, click on the event and see if the info shown tells you something. From these information, you can pinpoint exactly what happened and if there's any extension breaks the request-response cycle.

Disable site isolation/network service

"Provisional Headers are shown" can also happen (for cross-origin requests only) because of a Chrome feature called site isolation. If none of the above methods work for you, you may have to try messing with site isolation/network service.

First, open up the Disable site isolation setting by navigating to chrome://flags/#chrome://flags#Disable%20site%20isolation and disable it.

Then, open up Enable Network Service setting by navigating to chrome://flags/#chrome://flags#Enable%20network%20service, and disable that:

Once you're done, relaunch the browser and check if the issue still persists. In a few cases, users have reported that disabling cache in Chrome DevTools also help.

Alternatively, you can start Chrome with the aforementioned settings disabled by running the following command (either in Command Prompt or Linux terminal):

chrome --disable-site-isolation-trials --disable-features=NetworkService,NetworkServiceInProcess

We hope that the article offer useful information about "Provisional headers are shown" notification in Chromium/Chrome.

We've also written a few other guides for fixing common Chrome errors, such as Fix ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_DISPOSITION error in Google Chrome/Microsoft Edge, Fix “Requests to the server have been blocked by an extension” in Chrome/Microsoft Edge or Fix “Access to XMLHttpRequest has been blocked by CORS policy” in Chrome . If you have any suggestion, please feel free to leave a comment below.

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