agent declared victory after deleting idle volumes - the next day's CUR showed the charges were on a different line...

TL;DR

Deleting a resource only saves money if the charge disappears from the same line item it lived on. Make the agent record the exact usage type and line item before it deletes anything, then re-query that same line item after a full billing day. If the charge moved instead of vanishing, the "savings" were never real.

agent declared victory after deleting idle volumes  -  the next day's CUR showed the charges were on a different line item anyway

Steps

1. Before deleting, record the baseline: query the service's daily cost grouped by usage type for the 7 days before the cleanup.

aws ce get-cost-and-usage --time-period Start=2026-09-20,End=2026-09-27 --granularity DAILY --metrics UnblendedCost --group-by Type=DIMENSION,Key [your value]

Expected: you can point at the exact usage type line (for example EBS volume usage vs snapshot storage) carrying the charge, with its average daily cost.

1. Perform the deletion, and log which usage type you expect to drop.

Expected: a written prediction, not a vibe.

1. Wait one full billing day. The CUR refreshes a few times daily and Cost Explorer lags up to 48 hours, so same-day checks lie.

Expected: the new data actually reflects the deletion.

1. Re-query the same usage type line.

Expected: that line drops by the baseline amount. If a different line rose by the same amount, the charge moved (for example the volume is gone but its final snapshot now bills under snapshot storage) and the savings claim is void.

Use this when

• the bill does not drop after a cleanup
• the agent claims savings that finance cannot find
• charges "move" between line items after deletions

Not for this skill when

• nothing was deleted yet (do the baseline step first)
• the charge is a reservation fee (those keep billing after the resource is gone)
• you only need a rough estimate, not a verified claim

Variant phrasings

• "deleted EBS volumes but bill did not go down"
• "savings did not show up in CUR"
• "cost moved to another line item after cleanup"

Why it happens

One resource generates several line items: a volume bills for usage, its snapshots bill for storage, and its final snapshot keeps billing after deletion. Agents verify "the volume is gone" and declare victory without checking which line item carried the dollars, so a charge that merely changed categories looks like money saved.

Edge cases

• Deleting a volume often creates a final snapshot. Net savings are volume cost minus new snapshot cost, and the agent must price both.
• Termination protection and delete-on-termination flags change which line items survive; check them before predicting.
• Reserved capacity keeps billing after the resource is gone. That line item never drops, and it must not be counted as savings.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_jpEFXa2s0TSbblYpGgVXbw