azure-cost: Query Your Bill Before You Optimize

Most Azure cost advice tells you to “eliminate waste” and stops there. Microsoft’s azure-cost skill (v1.3.1, MIT) works the other way: it reads your actual bill through the Cost Management API before it recommends anything. I read the full SKILL.md plus five sub-files — the cost-query workflow, its guardrails, the optimization workflow, the Azure Quick Review reference, and the tools-and-best-practices file — then scraped skills.sh on 2026-09-14: azure-cost ranks 132 with 382,419 installs.

Three Workflows, One Router

The SKILL.md file is 1,886 bytes — a routing table, not a tutorial. Three user intents land on three different workflows, each with its own reference tree: “how much am I spending” routes to cost-query, “reduce costs / find waste” to cost-optimization, “project future costs” to cost-forecast. Scopes run the full ladder — subscription, resource group, management group, or billing account — and the routing table explicitly bans the skill from deployment, provisioning, diagnostics, and security audits.

Two REST endpoints carry the work. Both use api-version=2023-11-01: POST {scope}/providers/Microsoft.CostManagement/query and the sibling forecast call. The required role stack is Cost Management Reader plus Monitoring Reader plus Reader on the target scope — the README warns that cost queries fail with 403, not a friendly message, when any of the three is missing.

The Hard Numbers the Workflow Enforces

The guardrails file reads like a contract, and agents skip these at their own cost. Daily granularity covers at most 31 days; monthly or none covers 12; the absolute API ceiling is 37 months. GroupBy takes up to two dimensions, and ResourceId grouping works only at subscription and resource group scope — ask for it at management group level and the API rejects the body. Pages return up to 5,000 rows (1,000 by default), so nextLink pagination is not optional on big tenants.

The rate limit section is the part most people don’t know. Sequential queries to the same scope share a 4-request-per-minute budget. A 429 response carries three x-ms-ratelimit-microsoft.costmanagement-*-retry-after headers — qpu, entity, tenant — and the documented rule is to wait the longest value and never retry more than three times. The workflow also bypasses az costmanagement query entirely, calling az rest with a JSON body file instead, and every request carries a ClientType=GitHubCopilotForAzure header.

Optimization Is a Report, Not a Suggestion

The optimization workflow gates prerequisites before doing any work: the costmanagement and resource-graph CLI extensions must be installed, and Azure Quick Review (azqr) must be present — those three facts are verified with actual commands, not assumed. Step two runs azqr to surface orphaned resources, the instant-savings bucket: unattached disks, unused NICs, idle NAT gateways.

The rest is a fixed cadence of evidence. Costs are queried for the last 30 days, utilization metrics for the last 14, and pricing is re-validated from official Azure pricing pages with a free-tier check — the file explicitly flags that many services carry generous free allowances. Output lands as output/costoptimizereport<YYYYMMDD_HHMMSS>.md with an executive summary, breakdown, and prioritized recommendations, while each query result is saved as JSON for an audit trail. Service-specific branches exist for Redis, Storage (LRS/GRS/ZRS downgrade analysis), and AKS, which gets its own cost-analysis add-on and anomaly investigation references. One rule stands out for small bills: below $10/month, the skill tells the agent to push operational improvements, not SKU gymnastics.

The Name-Twin Trap: Two Skills, One Dead

Search “azure cost skill” and you’ll find azure-cost-optimization wearing 209,527 installs. It doesn’t exist anymore — the raw SKILL.md returns 404 today, and the weekly curve tells the story: [170, 153, 150, 111, 91, 118, 93, 97], the same crash-to-floor pattern we documented for the other removed azure skills. Its name survives as a subfolder inside azure-cost — the cost-optimization/ directory is where the optimization workflow actually lives.

The install numbers carry a second lesson. azure-cost sits at 382,419 while most of its pack siblings lockstep around 578K — it’s one of the few azure skills running on its own counter, and its weekly curve (17,048 → 12,743 across the last eight weeks) still decays in the same shape as theirs. So don’t read rank by rank: read intent by intent.

Install It Before the Next Bill

Run npx skills add https://github.com/microsoft/azure-skills --skill azure-cost and test it on a cheap subscription first. Ask “why did my bill spike last month” and watch whether the agent returns a query-backed breakdown or a hand-wavy paragraph — the workflow demands the former. If capacity limits are your actual bottleneck rather than spend, the azure-quotas procedural guide covers that lane, and azure-compute handles VM sizing and pricing decisions when you’re spinning something up.

Covered alongside: azure-resource-lookup vs azure-resource-visualizer — the inventory and diagram skills that your cost report will keep referencing.