Azure Container Instances won't deploy or start due to host name invalid.

Question

Azure Container Instances won't deploy or start due to host name invalid.

William Kruse 5

Hello!
I have been running a bunch of container instances for months now on a pretty basic setup.
Now all of a sudden the containers wont start and I can't deploy new ones. The error i am getting is like this:

"Failed to start the container group '<containerName>'. Error: The server '<server>.azurecr.io' in the 'imageRegistryCredentials' of container group '<container name>' is invalid. It should be a valid host name without protocol."

My server hostname has not changed. Same container, same registry. The hostname is: companynamecontainerregistry-fthyjju6ymqerta1.azurecr.io

*
*I have alwayd used the Azure Portal GUI to deploy, and have never configures the servername or host myself. It is done through a dropdown choosing between both my Azure Container Registries and the images they have.

I have a VNET setup, but have been running the Container Registry on BASIC tier ( no private endpoint ). But It has worked flawless for months and now suddenly not.

Any idea why this might have happened?

I have tries creating new Registries and new Container Instanceses pointing to thoose images but with the same error.

Thank you.

Jack B 0 Reputation points

2026-08-21T10:05:29.2633333+00:00
Hi,

I'm also experiencing the same issue today. Previously working deployments are now returning the error:

Message: The server '<registry name>-********.azurecr.io' in the 'imageRegistryCredentials' of container group '<container group name>' is invalid. It should be a valid host name without protocol.

I'm typically deploying via a service principle, I've confirmed that I can login successfully to the container registry using this service principle.

If I get the login server via:

az acr show \ --name <registry-name> \ --query loginServer \ --output tsv

...it matches what I'm passing to az container create. Adding --debug to az container create, I can see that it's passing the correct server name "imageRegistryCredentials": [{"server": "<registry name>-********.azurecr.io"....

I've also confirmed that the service principal credential is valid and does not expire until 2027.

As an additional test, I enabled the admin user on the container registry and deployed via the portal, I'm also getting the same error there.

Deploying an ACI using a public image from mcr.microsoft.com works successfully, so ACI itself appears to be functioning.

This was a previously working configuration that started failing last night, without any changes to the registry, credentials, networking, or deployment configuration.

Did you find what was causing the issue?

Many thanks
Mohammed Muddasar 15 Reputation points

2026-08-25T21:23:12.59+00:00

As of 25 August 2026, 21:50 BST, the issue appears to have been resolved. Container image is now being fetched successfully. I made no changes, it just started to work again.

2 answers

Your answer

Jack B 0 Reputation points

2026-08-21T10:05:29.2633333+00:00

Hi,

I'm also experiencing the same issue today. Previously working deployments are now returning the error:

Message: The server '<registry name>-********.azurecr.io' in the 'imageRegistryCredentials' of container group '<container group name>' is invalid. It should be a valid host name without protocol.

I'm typically deploying via a service principle, I've confirmed that I can login successfully to the container registry using this service principle.

If I get the login server via:

az acr show \ --name <registry-name> \ --query loginServer \ --output tsv

...it matches what I'm passing to az container create. Adding --debug to az container create, I can see that it's passing the correct server name "imageRegistryCredentials": [{"server": "<registry name>-********.azurecr.io"....

I've also confirmed that the service principal credential is valid and does not expire until 2027.

As an additional test, I enabled the admin user on the container registry and deployed via the portal, I'm also getting the same error there.

Deploying an ACI using a public image from mcr.microsoft.com works successfully, so ACI itself appears to be functioning.

This was a previously working configuration that started failing last night, without any changes to the registry, credentials, networking, or deployment configuration.

Did you find what was causing the issue?

Many thanks
Mohammed Muddasar 15 Reputation points

2026-08-25T21:23:12.59+00:00

As of 25 August 2026, 21:50 BST, the issue appears to have been resolved. Container image is now being fetched successfully. I made no changes, it just started to work again.

Answer 1

SUNOJ KUMAR YELURU 18,736 MVP Volunteer Moderator

Hello @William Kruse

Your registry name is too long: companynamecontainerregistry-fthyjju6ymqerta1 = 45 chars. ACI now validates the first label (before .azurecr.io) against DNS spec - and combined with internal suffixes it exceeds 63 chars / fails regex.

It's a known bug - Portal dropdown still shows it, but API rejects it.

Fix: Create a new ACR with short name < 25 chars, e.g. companynameacr1.azurecr.io, push your images there, deploy ACI to that.

Workaround: Deploy via ARM/Bicep and use imageRegistryCredentials without protocol - Portal GUI is adding https://

If this answers your query, do click Accept Answer and Up-Vote for the same. And, if you have any further query do let us know.

Mohammed Muddasar 15 Reputation points

2026-08-23T12:52:05.17+00:00

My ACR name is shorter than 25 chars and getting same issue. If portal GUI is adding protocol "behind the scenes" as you suggest then deploying from, e.g. A logic app's create container group action, is fundamentally broken and not to do with ACR name length, but where's the official Microsoft comms for any of this (or am I expecting too much).

Answer 2

Hello @William Kruse

The error is interesting because the hostname you've shown appears syntactically valid. For ACI, imageRegistryCredentials.server must contain only the registry login server hostname, without https://, a port, repository path, or image tag. Microsoft documents this explicitly.

For example:

server: companynamecontainerregistry-fthyjju6ymqerta1.azurecr.io

image: companynamecontainerregistry-fthyjju6ymqerta1.azurecr.io/repository/image:tag

The -fthyjju6ymqerta1 portion is also not necessarily an error. Azure Container Registry can now use Domain Name Label (DNL) protection, where the login server includes a DNS hash such as myregistry-abc123.azurecr.io.

I would first verify what Azure considers the actual login server:

az acr show \
  --name <registry-name> \
  --query loginServer \
  --output tsv

Then inspect what ACI actually stored:

az container show \
  --resource-group <resource-group> \
  --name <container-group> \
  --query properties.imageRegistryCredentials

The server value should exactly match the ACR loginServer value.

Since you're selecting the registry through the Azure Portal and you've reproduced this even with newly created registries/container instances, I'd also try one deployment using CLI rather than the portal:

az container create \
  --resource-group <resource-group> \
  --name aci-test \
  --image <login-server>/<repository>:<tag> \
  --registry-login-server <login-server> \
  --registry-username <username> \
  --registry-password <password>

Microsoft documents this syntax for ACI pulling from ACR.

If the CLI deployment succeeds while the Portal-generated deployment fails with InvalidImageRegistryServer, that strongly points to a Portal-generated configuration issue rather than your VNet or ACR Basic tier.

Also, this error occurs while ACI validates the registry server value. A registry network-access problem would more typically surface later as an image accessibility/pull/connectivity failure, so I wouldn't start changing your VNet configuration yet.

If CLI fails as well, please share the output of:

az acr show -n <registry-name> --query loginServer -o tsv

and the imageRegistryCredentials section from:

az container show -g <resource-group> -n <container-group>

with credentials removed. That should show whether ACI is receiving a malformed hostname or whether this needs investigation as a service-side regression.

Sharing this reference with you:

Microsoft: Deploy an ACR image to Azure Container Instances

https://learn.microsoft.com/en-us/azure/container-instances/container-instances-using-azure-container-registry

Please "Accept the Answer" if this information helped you. This will help us and others in the community.

Allan Solomon Mejia 7,590 Reputation points

2026-08-21T13:36:02.66+00:00

Hello @William Kruse

Please let me know if you need any further assistance. Otherwise, if the information provided resolved your issue, please click “Accept Answer” to help others who may encounter the same problem.

Thank you!