Azure container app - Kestrel - Http 431


This post is about an error over which I spent hours trying to get anywhere until my colleague Sebastian Wiejas spotted the issue and suggested the solution.

Overall scenario

We have a dotnet api deployed as a docker image which is hosted on Azure Container Apps instance. But whenever we would try to hit an endpoint, from the UI app (which is hosted as a separate) app, we will get the following error

Request URL: https://testapi.containerapp.io/api/v1/endpoint1
Request Method: OPTIONS
Status Code: 431 
Remote Address:
Referrer Policy: strict-origin-when-cross-origin

Further investigating into Http 431 status code, it says 431 Request Header Fields Too Large, which implies we are sending too many headers or something is limiting our allowed header count.

The solution or maybe the workaround

Given it is a dotnet app and we know from the deployment image that it is using kestrel web server, so we investigated further into Kestrel settings.

One of the settings that stood out was KestrelServerLimits.MaxRequestHeaderCount Property. Although it defaults to 100, in our case, as it turns out it was set to a low number (I think 10) in the app’s appsettings.json. So the way we got the app to work is to allow more headers by setting the MaxRequestHeaderCount property in appsettings.json

  "Kestrel": {
    "Endpoints": {
      "Https": {
        "Url": "https://*:443",
        "Certificate": {
      "Http": {
        "Url": "http://*:80"
    "Limits": {
      "MaxRequestHeaderCount": 50 // Default is 100


Hope this was useful and saved you some time. And again many thanks to Sebastian for sharing these details. Overall, the app being deployed as a container on Azure container app doesn’t have anything to do with the issue. It was more the server settings which broke cors requirements. Please do share your learnings. If you have any thoughts or comments please do get in touch with me on Twitter @rubberduckdev. Or use the Disqus plugin below.



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Azure container app - Kestrel - Http 431