2 min lesson
Rollback and blast-radius control
Walk through the important items in "Rollback and blast-radius control" and give the practical point of each.
Step 1 of 3
Rollback and blast-radius controlbe able to undo fast
- Keep each batch a separate, revertible commit or PR so a bad slice is one
git revertaway. - Where feasible, gate the new path behind a flag so you can disable it in production without a redeploy.
- Define the blast radiusHow much breaks if a change goes wrong; the scope of potential damage. Press Enter for the full definition. before you start: how many files, which services, what's the worst case if a batch is wrong.
A migration that merges failing changes with a TODO is how you hand the customer a slow-motion outage.
The gate is the product. If a batch can't pass, it stays out until it can.
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Measuring success
Measuring successthe metrics a customer will trust
- Metric
- % migrated
- What it tells the customer
- Concrete progress against the total, the burn-down they can watch.
- Metric
- Defects introduced
- What it tells the customer
- Bugs or regressions traced to the migration, the honest cost.
- Metric
- Reviewer time saved
- What it tells the customer
- Hours the automation removed vs. doing it by hand, the ROIReturn on Investment. The value gained versus what it cost, the language an economic buyer funds deals in. Press Enter for the full definition..
- Metric
- Rollbacks / reverts
- What it tells the customer
- How often a batch had to be undone, the stability signal.
| Metric | What it tells the customer |
|---|---|
| % migrated | Concrete progress against the total, the burn-down they can watch. |
| Defects introduced | Bugs or regressions traced to the migration, the honest cost. |
| Reviewer time saved | Hours the automation removed vs. doing it by hand, the ROIReturn on Investment. The value gained versus what it cost, the language an economic buyer funds deals in. Press Enter for the full definition.. |
| Rollbacks / reverts | How often a batch had to be undone, the stability signal. |
Report these proactively. A customer trusts the engineer who shows defects introduced, not the one who hides them.
"I'd pilot the migration on one module to nail the recipe, encode it as agent instructions plus a rules entry, then roll out in 20-file batches. Every batch has to type-check, pass the affected tests and get a human diff review before merge and each is a separate revertible PR. I'd report percent migrated, defects introduced, reviewer hours saved and rollbacks each week, so the customer sees both the progress and the cost."
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What this looks like at real scale
What this looks like at real scalegrind mode, Projects and cloud agents on big repos
For the longest, hardest transformations, Cursor's grind-until-done toggle lets a cloud agent work for hours, or as long as needed, on a big migration or a from-scratch build. Most jobs don't need it: toggling a cloud agent on and asking "implement this feature" already does well. Reserve it for the genuinely long horizon. For larger bodies of work, Projects give the work a coordinator agent that keeps context over months and delegates to many cloud agents in parallel, and /goal sets a long-lived objective for an agent.
A Cursor demo ran against Grafana, a roughly ten-year-old repo with 13K-plus contributors and potentially gigabytes of files. Semantic search plus efficient grep inside the agent harness lets the agent find the slice that matters instead of reading the whole tree, and sub-agents split the work.
For slow-to-build repos like this, Builds prepare the environment ahead of time, so new agents start from it instead of rebuilding.