az bicep snapshot does not show changes when validation failes

[frits-becker]

Describe the bug

When the validation failes, there should be information what the snapshot changes are.

Related command

az bicep snapshot

Errors

Snapshot validation failed. Expected no changes, but found the following:
no information here

Issue script & Debug output

az bicep snapshot `
    --file $paramFile `
    --subscription-id $subscriptionId `
    --location 'westeurope' `
    --mode $mode

Expected behavior

Snapshot validation failed. Expected no changes, but found the following:
with the found changes here

Environment Summary

azure-cli 2.89.1

core 2.89.1
telemetry 1.1.0

Extensions:
acrcssc 1.0.0b7
azure-devops 1.0.6

Dependencies:
msal 1.36.0
azure-mgmt-resource 24.0.0

Python location 'C:\Program Files\Microsoft SDKs\Azure\CLI2\python.exe'
Config directory 'C:\Users\frits.azure'
Extensions directory 'C:\Users\frits.azure\cliextensions'

Python (Windows) 3.14.6 (tags/v3.14.6:c63aec6, Jun 10 2026, 10:26:10) [MSC v.1944 64 bit (AMD64)]

Additional context

No response

[yonzhan]

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

[azure-client-tools-agent]

Bug Analysis

Command: az bicep snapshot

Observed behavior: When az bicep snapshot validation fails because the current
deployment snapshot differs from the expected snapshot, the CLI reports:

Snapshot validation failed. Expected no changes, but found the following:

...but the actual list of detected changes/diffs is never appended, leaving the
message empty and unhelpful for diagnosing what changed.

Expected behavior: The error message should include the actual changes found
(e.g., a diff of the properties/resources that changed between the expected and
current snapshot), similar to:

Snapshot validation failed. Expected no changes, but found the following:
<actual diff/changes>

Likely cause: The code path that raises the "Expected no changes, but found the
following" error is not appending the computed diff/changes to the exception
message, or the diff computation result is not being passed to the error string
formatting. Locate the bicep snapshot validation logic (likely in
src/azure-cli/azure/cli/command_modules/resource, look for the snapshot/diff
comparison code path) and ensure the detected differences are serialized into the
error message.

Suggested fix: Update the snapshot validation error handling to format and
include the detected differences (e.g. added/removed/changed properties) in the
raised error message. Add/extend a unit test asserting the error message contains
the diff content when validation fails.

Use this EXACT PR title: [Resource] Fix #33922: az bicep snapshot: Include snapshot diff details in validation failure error message

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):

[Resource] Fix #33922: `az bicep snapshot`: Include snapshot diff details in validation failure error message

Keep the backticks around the command and the Fix #33922: prefix. You may only adjust the wording after the command (the final summary) if the fix changes; the [Resource] 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 #33922.
• 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. [Resource] `az <command>`: <note>.
• Keep the template checklist and tick the items you've satisfied.

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Posted by agent-assist (autonomous bug-fix pipeline).