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Guard monthly cost change against a zero prior month #2327

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8f05194
fix(queries): guard monthly cost change against a zero prior month
RolandKrummenacher Sep 16, 2026
d280609
docs: add changelog entry for the monthly cost change fix
RolandKrummenacher Sep 16, 2026
bae6d5a
chore: Update ms.date in docs-mslearn files
github-actions[bot] Sep 16, 2026
a917d05
Merge branch 'dev' into RolandKrummenacher/monthly-cost-change-divide…
RolandKrummenacher Sep 16, 2026
927c9fe
Merge origin/dev into RolandKrummenacher/monthly-cost-change-divide-b…
RolandKrummenacher Oct 15, 2026
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3 changes: 2 additions & 1 deletion docs-mslearn/toolkit/changelog.md
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Expand Up @@ -3,7 +3,7 @@ title: FinOps toolkit changelog
description: Review the latest features and enhancements in the FinOps toolkit, including updates to FinOps hubs, Power BI reports, and more.
author: MSBrett
ms.author: brettwil
ms.date: 10/14/2026
ms.date: 10/15/2026
ms.topic: reference
ms.service: finops
ms.subservice: finops-toolkit
Expand Down Expand Up @@ -42,6 +42,7 @@ The following section lists features and enhancements that are currently in deve
- Switched dimension enrichment in the v1_0/v1_2 ingestion transforms (`PricingUnits`, `Regions`, `ResourceTypes`, `Services`) from `join` to the broadcast-optimized `lookup` operator and deduplicated the `Services` mapping per resource type to prevent cost row fan-out ([#2225](https://github.com/microsoft/finops-toolkit/pull/2225)).
- Fixed FinOps hub dashboard "Governance, policy, and risk" and "FinOps tools and services" links that landed on a 404 page by linking directly to the Microsoft Learn pages ([#2319](https://github.com/microsoft/finops-toolkit/issues/2319)).
- Fixed the v1_0 and v1_2 price transforms setting `x_CommitmentDiscountSpendEligibility` from reservation meters and `x_CommitmentDiscountUsageEligibility` from savings plan meters, the opposite of the FOCUS `CommitmentDiscountCategory` value the same transform assigns. The spend column now reports savings plan pricing and the usage column reports reservation pricing. Prices ingested after upgrading carry the corrected values; rows ingested earlier keep the old values until they are reingested ([#2279](https://github.com/microsoft/finops-toolkit/issues/2279)).
- Fixed `monthly-cost-change-percentage` returning `inf` when the prior month's cost was exactly zero. The divide-by-zero guard used `isempty()`, which is true for the null that `prev()` returns on the first row but false for `0.0`, and KQL evaluates `x / 0.0` as an infinity rather than raising. A zero prior month is reachable because `BilledCost` is zero on usage rows covered by a commitment discount ([#2323](https://github.com/microsoft/finops-toolkit/issues/2323)).

### [FinOps workbooks](workbooks/finops-workbooks-overview.md)

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4 changes: 2 additions & 2 deletions src/queries/catalog/monthly-cost-change-percentage.kql
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Expand Up @@ -22,5 +22,5 @@ Costs()
| extend PreviousBilledCost = prev(BilledCost)
| extend PreviousEffectiveCost = prev(EffectiveCost)
| project ChargePeriodStart,
BilledCostChangePct = iff(isempty(PreviousBilledCost), 0.0, toreal((BilledCost - PreviousBilledCost) * 100.0 / PreviousBilledCost)),
EffectiveCostChangePct = iff(isempty(PreviousEffectiveCost), 0.0, toreal((EffectiveCost - PreviousEffectiveCost) * 100.0 / PreviousEffectiveCost))
BilledCostChangePct = iff(isempty(PreviousBilledCost) or PreviousBilledCost == 0.0, 0.0, toreal((BilledCost - PreviousBilledCost) * 100.0 / PreviousBilledCost)),
EffectiveCostChangePct = iff(isempty(PreviousEffectiveCost) or PreviousEffectiveCost == 0.0, 0.0, toreal((EffectiveCost - PreviousEffectiveCost) * 100.0 / PreviousEffectiveCost))
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