'az vmss extension set' does not update an existing extension anymore

[HOMAG-Manuel-Schweikert]

Describe the bug

With the latest azure cli version (2.88.0), the command 'az vmss extension set' does not update an existing extension on a virtual machine scale set anymore.

This is a breaking change that was introduced with the latest azure cli version 2.88.0.
On earlier versions, it correctly updated the already existing vmss extension with the settings as set in the command.
This is also how it is documented here: https://learn.microsoft.com/en-us/cli/azure/vmss/extension?view=azure-cli-latest#az-vmss-extension-set
It says: 'Add an extension to a vmss or UPDATE AN EXISTING extension.'
This is no longer the case since it now returns this error.

Related command

az vmss extension set

Errors

(BadRequest) Multiple VMExtensions per handler not supported for OS type 'Linux'. VMExtension 'myScript' with handler 'Microsoft.Azure.Extensions.CustomScript' already added or specified in input.
Code: BadRequest
Message: Multiple VMExtensions per handler not supported for OS type 'Linux'. VMExtension 'myScript' with handler 'Microsoft.Azure.Extensions.CustomScript' already added or specified in input.

Issue script & Debug output

az vmss extension set --vmss-name myVMSS --resource-group myRG --name CustomScript --extension-instance-name myScript --version 2.1 --publisher Microsoft.Azure.Extensions --settings "..." --force-update

Expected behavior

If the extension custom script already exists, it should be updated. That's how it was working before version 2.88.0.
This is also how it is documented here: https://learn.microsoft.com/en-us/cli/azure/vmss/extension?view=azure-cli-latest#az-vmss-extension-set
It says: 'Add an extension to a vmss or UPDATE AN EXISTING extension.'

With this bug it is no longer possible to update the extension, since there is also no other command that can be used to update an existing custom script extension.

Environment Summary

az version
{
"azure-cli": "2.88.0",
"azure-cli-core": "2.88.0",
"azure-cli-telemetry": "1.1.0",
"extensions": {
"application-insights": "0.1.16",
"redisenterprise": "0.1.2"
}
}

Additional context

The returned error about the constraint of only having one customScript extension is known and ok.
However, since we have rolling updates enabled on this vmss, we can not do a work around by deleting the script and add it again, since this would lead to downtime / time where we can not reach the deployed api on the machines.
So we need the functionality as documented and as it was working before.

[yonzhan]

Thank you for opening this issue, we will look into it.

[azure-client-tools-agent]

Bug Analysis

az vmss extension set is regressing on Azure CLI 2.88.0: instead of updating an existing VMSS extension in place, it now attempts to add a duplicate extension, hitting the service-side error:

(BadRequest) Multiple VMExtensions per handler not supported for OS type 'Linux'. VMExtension 'myScript' with handler 'Microsoft.Azure.Extensions.CustomScript' already added or specified in input.

Repro:

az vmss extension set --vmss-name myVMSS --resource-group myRG --name CustomScript \
  --extension-instance-name myScript --version 2.1 --publisher Microsoft.Azure.Extensions \
  --settings "..." --force-update

Expected: When an extension with a matching name/instance-name already exists on the VMSS, the command should update it in place (as documented), not attempt to append a new one.

Likely area: src/azure-cli/azure/cli/command_modules/vm/custom.py — the set_vmss_extension (or equivalent) implementation that builds/patches the virtualMachineProfile.extensionProfile.extensions list. Check recent changes around extension matching-by-name logic (this looks like a regression introduced in the 2.88.0 cycle) — likely the lookup used to find-and-replace an existing extension entry by name/instance-name was changed or removed, causing a duplicate to be appended instead of an update.

Fix approach: Ensure the extension-set path locates any existing extension entry (matching by name and/or extension_instance_name) in the VMSS's extension_profile.extensions list and replaces/updates it in place instead of always appending a new entry.

Use this EXACT PR title: [VM] Fix #33751: az vmss extension set: Fix vmss extension set failing to update existing extension

PR title & description format (required)

This repo enforces a PR format (guide). Please author the PR exactly as follows or CI's Check the Format of Pull Request Title and Content will fail.

Use this EXACT PR title (copy verbatim, do not reword):

[VM] Fix #33751: `az vmss extension set`: Fix vmss extension set failing to update existing extension

Keep the backticks around the command and the Fix #33751: prefix. You may only adjust the wording after the command (the final summary) if the fix changes; the [VM] prefix, issue link, and backticked command must stay.

Description — follow the PR template and fill in:

• Link the issue — start the Description with a closing keyword so the PR auto-links and closes it: Fixes #33751.
• Related command — the az ... command this affects.
• Description (mandatory) — why the bug happens, what you changed, and the resulting behavior.
• Testing Guide — example command(s) showing the fix works.
• History Notes — leave the title to drive the history note, or add extra lines in the same format (component in brackets + the command in backticks), e.g. [VM] `az <command>`: <note>.
• Keep the template checklist and tick the items you've satisfied.

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