CAUTION provisional headers are shown in Chrome debugger

Question

I noticed a strange caution message when looking at downloaded resources using Google chrome inspector  F12       Caution provisional headers are shown     I found something possibly relevant  Network Panel  add caution about provisional request headers  but I could not fully understand it  Related questions can be found Chrome block requests as well as XMLHttpRequest cannot load  Unloaded resources show caution  Provisional headers are shown    Similar to the first question  my resource was blocked  but later automatically loaded the same resource  Unlike the second question  I don t want to fix anything  I want to know what this message means and why I received it

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This issue occurred to me when I was sending an invalid HTTP Authorization header  I forgot to base64 encode it

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Here is another solution   If you encounter this issue with  ajax   call  add http    before your serverhost will solve your problem   var requestURL    http       serverHost    ajax       dataType   json       url  requestURL      data  data      success  success

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I had this issue when sending parameters in the AJAX URL propertylike  http   10 165 10 160 82 services param1 xxxx amp param2 xxxx  If you want to perform a get request  e g  send parameterized url   do not add them in the url property  do it in data object instead            lt script gt            ajax           url   http   10 160 10 160 82 services STD ERROR php           data                StatusText  StatusText              Status  Status              UserCode  UserCode              FUNC CODE  FuncCode              ErrorDescription  ErrorDescription                    type   GET           crossDomain  true          cache  false        done data   gt            console log data         fail function xhr  status  error            console log xhr ErrorDescription           lt  script gt

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I ran this issue when I tried to load main js for require js for the second time after I made changes as a result from error    I just turned on in Developer Tools Settings   Disable Cache  When DevTools is Open     and that did the charm

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I came across this and it went away when I switched from https to http   The SSL certs we use in dev aren t verified by a 3rd party   They re just locally generated dev certs   The same calls work just fine in Chrome Canary and Firefox   These browsers don t appear to be as strict about the SSL cert as Chrome is   The calls would fail in Chrome with the  CAUTION  Provisional headers     message   I think hope that when we use a legit SSL cert in stage and prod  we won t see this behavior in Chrome anymore

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This caution message also occurs if the response is invalid and therefore dropped by the browser   In my case the request was correctly sent to the server  the server-side code then produced an error and my custom error handling returned the error message in the HTTP status message field  But this error was not received on the client side  due to invalid characters in the error message  described here http   aspnetwebstack codeplex com workitem 1386  which resulted in corrupt response headers

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This issue will also occur while using some packages like webpack-hot-middleware and open multiple pages at the same time  webpack-hot-middleware will create a connection for each page for listening to the changes of code then to refresh the page  Each browser has a max-connections-per-server limitation which is 6 for Chrome  so if you have already opened more than 6 pages in Chrome  the new request will be hung there until you close some pages

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Use this code fist of your code   header  Cache-Control  no-cache  no-store  must-revalidate    header  Pragma  no-cache    header  Expires  0      This works for me

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For chrome v72  what solved it for me was only this   go to chrome   flags  and disable this 3 flags   Disable site isolation Enable network service Runs network service in-process     or you can do it from command line    chrome --disable-site-isolation-trials --disable-features NetworkService NetworkServiceInProcess     why this happen      It seems that Google is refactoring their Chromium engine into modular structure  where different services will be separated into stand-alone modules and processes  They call this process servicification  Network service is the first step  Ui service  Identity service and Device service are coming up  Google provides the official information at the Chromium project site    is it dangerous to change that      An example is networking  once we have a network service we can choose to run it out of process for better stability security  or in-process if we re resource constrained  source

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I ran into this issue with an AJAX call that would never complete  I followed wvega s advice and tip about debugging with chrome   net-internals to eventually determine another click event handler in the page  listening on a parent node  was causing the browser to navigate to the same URL  so it wasn t easily noticeable    The solution was to add event stopPropagation   in a click handler on the form submit button to keep the click from bubbling up the DOM and canceling the AJAX request in progress  initiated via a submit handler on the form

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HTTP 2 Pushed resources will produce Provisional headers are shown in the inspector for the same theory as  wvega posted in his answer above   e g  Since the server pushed the resource s  to the client  before the client requested them   the browser has the resources cached and therefore the client never makes needs a requests  So because           the real headers are updated when the server responds  but there is no response if the request was blocked

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Try reloading the page with ctrl   shift  F5 and then check the request headers again Full headers are not shown because there can be a chance that request is not going to the servers and instead is served directly from Browser s cache for example using service workers

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I encountered this issue  and I managed to identify a specific cause  which isn t mentioned above either in answers or the question   I am running a full js stack  angular front end and node back end on SSL  and the API is on a different domain running on port 8081  so I am doing CORS requests and withCredentials as I am dropping a session cookie from the API  So specifically my scenario was  POST request  withCredentials to port 8081 caused the  CAUTION  provisional headers are shown  message in the inspector and also of course blocked the request all together   My solution was to set up apache to proxy pass the request from the usual SSL port of 443 to the node SSL port of 8081  node has to be on a higher port as it cannot be ran as root in prod   So I guess Chrome doesn t like SSL requests to unconventional SSL ports  but perhaps their error message could be more specific

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In my case  the body parameters I was sending in the post request  and logic I have writting depending on the body parameters were wrong  so the response was not able to be sent  so I was getting this error    example  post request body  a  alsldfjfj  which I was sending   but I had written the code for validating  b  instead of  a

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I ve had this come up very recently  today in fact  where I ve had an AJAX call go out to the server and Chrome fires off the  Caution  Provisional headers are shown   In the server side PHP scripting  there are MySQL queries that can be pretty much instant or take a few seconds depending on the given scenario  My server response isn t sent back to the browser until the queries are completed  I ve found I get this error only when time consuming queries  up to a few seconds total  are being done and prevent the response from being sent back   My scenario involves the very rare possibility of having to alter a table by adding removing hundreds of columns for weather model output   hence the response lag from iterating through a loop of ALTER TABLE queries

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The reason why this header shows is that  your request don t send to remote  It usually caused by  Extension has blocks your request Chrome use own cache to fetch your resource  Chrome cannot get your request headers from a request that haven t made  A recent version of chrome has indicate these   Only Provisional headers are avaliable because this request was not sent over the network and instead was served from a local cache  which doesn t store the original request headers  Disable cache to see full request headers

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This was happening for me  when I had a download link and after clicking on it I was trying also to catch the click with jquery and send an ajax request  The problem was because when you are clicking on the download link  you are leaving the page  even it does not look so  If there would no file transfer  you would see the requested page   So I set a target   blank  for preventing this issue

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This message can occur when the website is protected using HSTS  Then  when someone links to the HTTP version of the URL  the browser  as instructed by HSTS  does not issue an HTTP request  but internally redirects to the HTTPS resource securely  This is to avoid HTTPS downgrade attacks such as sslstrip

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I had a similar issue with my MEAN app  In my case  the issue was happening in only one get request  I tried with removing adblock  tried clearing cache and tried with different browsers  Nothing helped   finally  I have figured out that the api was trying to return a huge JSON object  When I have tried to send a small object  it was working fine  Finally  I have changed my implementation to return a buffer instead  of a JSON   I wish expressJS to throw an error in this case

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In my case the cause was AdBlock extension   The request to server went through and I got the response but I could not see the request cookies due to  Provisional headers    being shown in Dev tools  After disabling AdBlock for the site  the warning went away and dev tools started to show the cookies again   For the change to take effect  it was also necessary to close the Dev tools and refresh the page

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Another possible scenario I ve seen - the exact same request is being sent again just after few milliseconds  most likely due to a bug in the client side   In that case you ll also see that the status of the first request is  canceled  and that the latency is only several milliseconds

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A common reason this happens is if you are tracking an event and you don t prevent the default action   For example  if you have a click event  then you will want to include   e preventDefault      or  return false    If you don t  you will see the provisional headers warning as well as a  canceled  status in the Network tab of your web console

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I saw this occur when the number of connections to my server exceeded Chrome s max-connections-per-server limit of 6

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Just throwing in my two cents  I m writing a Web Application using CORS requests and a full RESTful web service  I have found chrome will throw this error when I have an unhanded exception or a PHP Error thrown  Just incase anyone else runs into the problem  I found that when this happens I can fire up the Chrome App  Postman - Rest Client  and run the exact same request but in the Chrome App I ll actually get the PHP Error thats being thrown instead of this non descriptive error

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In my case it was just a false set path in  to a resource  svg   img

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I believe it happens when the actual request is not sent  Usually happens when you are loading a cached resource

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Caution  provisional headers are shown  message can be shown when website hosted on HTTPS invokes a calls to WebApi hosted on HTTP  You can check all if all your Api s are HTTPS  Browser prevents to do a call to insecure resource  You can see similar message in your code when use FETCH API to domain with HTTP   Mixed Content  The page at  https   website com  was loaded over HTTPS  but requested an insecure resource  http   webapi com   This request has been blocked  the content must be served over HTTPS

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My situation is cross-origin related  Situation  Browser sends OPTIONS request before sending the real request like GET or POST  Backend developer forgets to deal with the OPTIONS request  letting it go through the service code  making the processing time too long  Longer than the timeout setting I wrote in the axios initialization  which is 5000 milliseconds  Therefore  the real request couldn t be sent  and then I encountered the provisional headers are shown problem  Solution  When it comes to OPTIONS request  backend api just return result  it makes the request faster and the real request can be sent before timeout

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I doubt my answer is in time to help you but others might find it helpful  I experienced a similar issue with a jQuery Ajax Post script that i created   It turned out that i had a typo in the href attribute of the A tag that i was using to fire the post  I had typed href  javacsript     reversing the  s  and the  c      this caused the script to try to refresh the page while the post was attempting to fire  corrected the typo and it worked perfectly fine for me

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This can also happen  for cross-origin requests only  because of a new feature called site isolation  This page details the issue and a work-around  Which is to go to chrome   flags  site-isolation-trial-opt-out in chrome and change that setting to  Opt-out  and reload chrome   It s a known issue  However that page says it s fixed in chrome 68  but I m running chrome 68 and I still have the issue

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The resource could be being blocked by an extension  AdBlock in my case    The message is there because the request to retrieve that resource was never made  so the headers being shown are not the real thing  As explained in the issue you referenced  the real headers are updated when the server responds  but there is no response if the request was blocked     The way I found about the extension that was blocking my resource was through the net-internals tool in Chrome   For Latest Versions of chrome   Type chrome   net-export  in the address bar and hit enter  Start Recording  And save Recording file to local  Open the page that is showing problems  Go back to net-internals You can view Recorded Log file Here https   netlog-viewer appspot com  import click on events       and use the textfield to find the event related to your resource  use parts of the URL   Finally  click on the event and see if the info shown tells you something    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 use the textfield to find the event related to your resource  use parts of the URL   Finally  click on the event and see if the info shown tells you something

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If you are developing an Asp Net Mvc application and you are trying to return a JsonResult in your controller  make sure you add JsonRequestBehavior AllowGet to the Json method  That fixed it for me   public JsonResult GetTaskSubCategories int id        var subcategs   FindSubCategories id        return Json subcategs  JsonRequestBehavior AllowGet       lt -- Notice it has two parameters

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This could be a CORS issue  try enabling CORS for you api  For WebApi   var cors   new EnableCorsAttribute  quot   quot    quot   quot    quot   quot                config EnableCors cors

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That might because you sent an Ajax request  at the same time you jump your page to another one using location href or something like that  So the previous request failed

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I got this error when I tried to print a page in a popup  The print dialog was show and it still waiting my acceptance or cancellation of the printing in the popup while in the master page also was waiting in the background showing the message CAUTION provisional headers are shown when I tried to click another link   In my case the solution was to remove the window print     script that it was executing on the  lt body gt  of the popup window to prevent the print dialog

[google-chrome] "CAUTION: provisional headers are shown" in Chrome debugger

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