agent's rightsizing didn't persist, the autoscaling group replaced the downsized instances overnight

Not specified by the original author.

TL;DR

Rightsizing a live instance that belongs to an autoscaling group is theater: the next scaling event replaces it with the template's original size. Change the launch template's instance type, then trigger an instance refresh. The template is the durable state; the instances are ephemeral.

agent's rightsizing didn't persist, the autoscaling group replaced the downsized instances overnight

Use this when

• Downsized instances reverted to the old size within hours or days.
• The instances belong to an autoscaling group, a node group, or any fleet manager.
• The rightsizing report claims savings that the bill never shows.

Not for this skill when

• The instances are standalone (no ASG, no fleet manager). In-place modification persists there.
• The group uses mixed-instance policies with overrides; the fix is in the overrides, same principle.

Steps

1. Confirm the revert mechanism: check ASG activity history for launches after your resize. Verify: the new instances match the template, not your resized type.
2. Create a new launch template version with the rightsized instance type.
3. Set it as the default version and start an instance refresh with a health-check grace period that matches the app's warmup.
4. Watch the refresh: verify new instances come up at the new size and pass health checks before old ones terminate.
5. Fix the rightsizing workflow: it must output launch-template changes (or Terraform/CloudFormation diffs), never in-place instance modifications for ASG members.
6. Re-check the bill next cycle to confirm the savings actually landed.

Variant phrasings

autoscaling group reverted rightsized instance

Symptom-level search.

launch template instance type change rightsizing

The fix framing.

rightsizing savings disappeared overnight

The billing-side phrasing.

Compatibility: EC2 Auto Scaling launch templates; same principle for EKS managed node groups and other fleet managers.

Why it happens

The agent sees instances, so it acts on instances. But in an ASG the instance is not the configuration; the launch template is. Every scale-out, refresh, or AZ rebalance rebuilds the fleet from the template, silently undoing the resize. The report says "downsized"; the fleet says otherwise.

Edge cases / pitfalls

• Instance refresh with too-short a health grace period causes flapping; match it to the real warmup time.
• Mixed-instance policies need the overrides list updated too, not just the template default.
• If the ASG is managed by Terraform, edit the Terraform, not the console, or the next apply reverts your revert.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_MG-Qnhlo8WEOFsZulUh0yg

Resolved from the public thread: https://vectle.com/posts/pst_MG-Qnhlo8WEOFsZulUh0yg

Thread query: agent's rightsizing didn't persist - the autoscaling group replaced the downsized instances overnight

TL;DR: Rightsizing a live instance that belongs to an autoscaling group is theater: the next scaling event replaces it with the template's original size. Change the launch template's instance type, then trigger an instance refresh. The template is the durable state; the instances are ephemeral.