Cost Drift

Cost Drift

Compare actual vs projected infrastructure costs, flag drifting resources, and explain why drift is happening.

Core principle: Wrap finfocus JSON output — do not compute drift independently or query cloud billing APIs directly.

Step 1: Detect Pulumi project

Scan the current directory for a Pulumi project:

ls Pulumi.yaml 2>/dev/null && echo "pulumi: detected"

If no Pulumi.yaml is found, stop and report: "No Pulumi project detected — cost-drift requires a Pulumi project with finfocus."

Step 2: Detect finfocus

Check if finfocus is installed:

command -v finfocus 2>/dev/null && echo "finfocus: available"

If finfocus is not found, soft-fail: report that the Pulumi project was detected but finfocus is needed for drift analysis. Link to https://github.com/rshade/finfocus for installation. Do not prescribe install commands.

Step 3: Load thresholds

Check for .cost-check.yml in the project root. If present, read the drift: section for threshold overrides. If absent or no drift: section exists, use defaults from references/default-thresholds.md:

• drift_pct: 10% (flag resources drifting above)
• drift_absolute: $50/mo (flag resources drifting above)
• total_drift_pct: 15% (flag if overall drift exceeds)

Step 4: Read baseline

If a previous COST_DRIFT.md exists in the project root, read the overall drift percentage and total actual spend. This serves as the baseline for trend comparison. If no previous report exists, skip trend display.

Step 5: Run finfocus

Run finfocus with JSON output:

finfocus overview --output json --yes

Parse the JSON output. Key fields per resource in the resources array:

• costDrift.extrapolatedMonthly — extrapolated monthly actual
• costDrift.projected — projected monthly cost
• costDrift.delta — extrapolated minus projected
• costDrift.percentDrift — drift as percentage
• costDrift.isWarning — finfocus built-in warning flag
• actualCost.mtdCost — month-to-date actual spend
• projectedCost.monthlyCost — projected monthly cost

From summary:

• totalActualMTD — total month-to-date actual
• projectedMonthly — total projected monthly

See references/drift-explanations.md for per-resource-type context.

Step 6: Flag drifting resources

Apply thresholds to each resource:

1. Drift percentage — flag if absolute value of costDrift.percentDrift exceeds the drift_pct threshold
2. Drift absolute — flag if absolute value of costDrift.delta exceeds the drift_absolute threshold
3. Total drift — flag if overall portfolio drift percentage exceeds the total_drift_pct threshold

Flag in both directions — overspend and underspend are both signals.

Step 7: Generate explanations

For each flagged resource, look up its resource type in references/drift-explanations.md. Determine drift direction (overspend if delta > 0, underspend if delta < 0) and select the matching explanation. Each explanation has:

• A likely cause hint
• An investigation prompt with an actionable next step

If the resource type is not in the reference file, use the fallback explanations.

Step 8: Terminal summary

Print the results immediately:

• Overall actual vs projected with drift percentage
• Trend from previous check (if baseline exists)
• Each flagged resource with actual, projected, delta, drift%, likely cause, and investigation prompt
• Total flagged resource count

Step 9: Generate COST_DRIFT.md

Write the report to the project root using the template in references/report-template.md. Include all sections: summary, drifting resources with explanations, non-drifting resources, thresholds used, trend, and tool info.

Run markdownlint on the generated file.

Step 10: Present results

Show the user:

1. Overall drift percentage and direction
2. Trend vs previous check (better, worse, or stable)
3. Number of drifting resources
4. Top drifting resources with explanations
5. Ask if they want to investigate specific resources or adjust thresholds