AKS Ubuntu 26.04 Preview: Enable Ubuntu2604 Node Pools on Kubernetes 1.36+

The update page shows only a title. The useful details are in Microsoft's AKS documentation: the Kubernetes versions that qualify, the feature flag you have to register, the hardware requirements, the configurations that don't work yet, and the CLI commands. This article pulls those details together.

What's in the preview​

Microsoft Learn's AKS guide to upgrading operating system versions lists a new versioned OS SKU. This OS SKU is supported in preview in Kubernetes version 1.36 and later. The same table says Microsoft recommends it if you want to migrate to the new OS version without upgrading your Kubernetes version. Requires the Ubuntu2604Preview feature flag.

You don't get the new OS by accident. It only runs on node pools that you explicitly set to --os-sku Ubuntu2604. The plain Ubuntu SKU stays on its current default. Ubuntu 22.04 is default for Kubernetes versions 1.25 to 1.34. Ubuntu 24.04 is default for Kubernetes versions 1.35+. The rollback section of the same page says Ubuntu 24.04 is the default for the Ubuntu SKU on Kubernetes 1.35 through 1.37.

Quick summary:

• OS: Ubuntu 26.04 LTS on AKS Linux node pools
• Status: Public preview
• Kubernetes floor: 1.36 or later
• How you choose it: --os-sku Ubuntu2604
• Gate: the Ubuntu2604Preview subscription feature flag

What Ubuntu 26.04 is​

Canonical shipped the release on April 23, 2026. Canonical calls "Resolute Raccoon" the 11th long-term supported release of Ubuntu. Much of the launch coverage was about the desktop, which doesn't matter on a Kubernetes node, but some server changes do. Canonical says Canonical Livepatch now extends its rebootless kernel patching capability to Arm64 for the first time. Canonical also says the release is the first LTS to expand the number of memory safe system components, including sudo-rs and the uutils coreutils. Computing for Geeks reports five years of free security updates through April 2031, extendable to ten years with Ubuntu Pro.

Don't assume every Canonical feature is in the AKS image. Microsoft builds its own node images, and the documentation for this preview says they are minimal images. It's safest to treat the AKS image as its own product that happens to be based on Ubuntu 26.04, not as a copy of the retail server install.

Prerequisites and limits​

Microsoft's migration section lists several conditions. Register the Ubuntu2604Preview feature flag before you create or update a node pool to Ubuntu 26.04. Ubuntu 26.04 requires a VM size that supports Generation 2 VMs. The same documentation also says:

• Ubuntu 26.04 uses minimal node images for AMD64 and Arm64.
• FIPS, Confidential VM and Trusted Launch aren't supported.
• You need the preview Azure CLI at version 21.0.0b14 or later. Run az --version to check what you have.

A separate Microsoft Learn page on AKS node images backs up the hardware rule. Its table for the standard "Ubuntu with containerd and Gen 1" image says Ubuntu 26.04 doesn't support that image. That means a node pool on a Gen 1-only VM size can't move to 26.04 at all.

The unsupported features rule out a lot of real deployments:

Regulated and security-hardened environments are likely to rely on FIPS or Trusted Launch. For them, this preview is something to watch for now, not something to use.

Step by step: moving a test node pool to Ubuntu 26.04​

These steps follow Microsoft's documented process. Run them against a nonproduction cluster on Kubernetes 1.36 or later.

1. Install and update the preview extension.
   
   Code:

   az extension add --name aks-preview
   az extension update --name aks-preview

   Microsoft notes that in order to access preview functions, make sure you have the preview extension installed.
2. Register the feature flag.
   
   Code:

   az feature register \
     --namespace Microsoft.ContainerService \
     --name Ubuntu2604Preview

3. Wait until the flag shows as registered. Microsoft says this takes a few minutes.
   
   Code:

   az feature show \
     --namespace Microsoft.ContainerService \
     --name Ubuntu2604Preview \
     --query properties.state

   Continue when the output says Registered.
4. Refresh the resource provider registration.
   az provider register --namespace Microsoft.ContainerService
5. Update the node pool's OS SKU.
   
   Code:

   az aks nodepool update \
     --resource-group $RESOURCE_GROUP \
     --cluster-name $CLUSTER_NAME \
     --os-sku Ubuntu2604 \
     --name $NODE_POOL_NAME

If it works, the node pool moves to Ubuntu 26.04 node images and your Kubernetes version stays the same. That's the whole reason to use a versioned SKU.

If it fails, check these first. Microsoft warns that az aks nodepool update might fail if the target OS doesn't have a supported node image for your Kubernetes version, VM size, or FIPS enablement. (That warning appears on the same Learn page.) In practice, look at:

• Kubernetes version: Is the pool below 1.36?
• VM size: Is it Gen 1-only?
• FIPS, Trusted Launch or Confidential VM: Is any of them enabled on the pool?
• Feature flag: Did you run the provider refresh after it showed Registered?
• CLI: Is your CLI older than 21.0.0b14, or is aks-preview out of date?

The documentation says to register the flag before you create or update a node pool, so new pools are clearly meant to be supported. However, the step-by-step instructions only show the update command.

Rolling back​

Microsoft documents a way back. In Kubernetes versions where multiple OS versions are supported, you can use the az aks nodepool update command to roll back to a previous OS version. One of the listed reasons is testing a new OS version and running into problems. On Kubernetes 1.36, setting the pool back to --os-sku Ubuntu should return it to the default, which the documentation lists as Ubuntu 24.04. Microsoft also warns that changing the SKU might trigger an automatic reimage if the OS version changes. Plan for nodes being replaced and pods being evicted, and don't run this in the middle of the workday.

How this preview fits Microsoft's usual pattern​

Microsoft describes a set routine for new OS versions on AKS. When a new OS version releases on AKS, it's initially supported in preview. After testing in preview for a few months, AKS makes the new OS version generally available (GA) and then updates the default OS SKU (Ubuntu or AzureLinux) to the latest GA OS version. This default update occurs with a new Kubernetes version release.

The following reading is my own, based on that documented routine, not a Microsoft announcement. If you use the plain Ubuntu SKU, 26.04 will eventually reach your clusters as part of a Kubernetes upgrade, once GA arrives and a later Kubernetes release makes it the default. Microsoft hasn't given a date. Testing during the preview is your chance to find broken DaemonSets, kernel-module assumptions and old agents before the change arrives with a routine version upgrade.

The same documentation also lists support windows for the other Ubuntu SKUs:

• Ubuntu2204 is supported on Kubernetes 1.25 to 1.36.
• Ubuntu2404 is supported only on 1.32 to 1.38.
• AKS support for Ubuntu 22.04 ends June 30, 2027, and node images are removed on April 30, 2028.
• Ubuntu 20.04 support ends March 17, 2027.

If you're still on 22.04, your deadline is the move to 24.04. Ubuntu 26.04 is the step after that.

Other Microsoft services are ahead of AKS here. On GitHub's runner-images repository, Microsoft announced that Ubuntu 26.04 and Ubuntu 26.04 Arm64 are now generally available for GitHub Actions and Azure DevOps. The runner-images readme also says the ubuntu-latest label will use Ubuntu 26.04 in November 2026. Many teams will soon be building containers on 26.04 CI runners while running them on 22.04 or 24.04 nodes. Container images carry their own userland, but anything that depends on the host kernel (eBPF tooling, security agents, GPU drivers, storage plugins) runs on the node OS. The AKS preview lets you test the node side too.

Should you try it?​

Yes, on a test cluster, if:

• you run Kubernetes 1.36 or later on Gen 2 VM sizes
• you rely on node-level agents or kernel-sensitive tooling that needs checking
• you run Arm64 nodes and want an early look at the minimal Arm64 image

Not yet, if:

• your node pools need FIPS, Trusted Launch or Confidential VMs
• you want an SLA. Microsoft states that previews are provided "as is" and "as available," and they're excluded from the service-level agreements and limited warranty. AKS previews are partially covered by customer support on a best-effort basis.

A practical tip: create a separate test node pool with a taint instead of converting a pool your workloads already use. Then place a few canary deployments on it with matching tolerations. If something breaks, you remove that one pool and nothing else is affected.

The bottom line​

The Ubuntu 26.04 preview on AKS is small but useful. It requires Kubernetes 1.36 or later, a registered Ubuntu2604Preview flag, Gen 2 VM sizes, and a pool with no FIPS, Confidential VM or Trusted Launch. You get minimal AMD64 and Arm64 images and a documented way to switch a pool in place without upgrading Kubernetes. It isn't ready for production, and Microsoft says so plainly. For teams that want to test the next Ubuntu LTS before it becomes the default on AKS, you can start now.