MasterMind: Deprecate

Every other skill adds or fixes. This one deletes, migrates and retires: the half of engineering where the reward (a smaller system) arrives long after the risk, and where what breaks is whatever you couldn't see was still reading it.

The shape: expand → migrate → contract

Any removal that can't land in one atomic commit takes three phases, in this order, each shipped on its own:

• Expand: add the new thing beside the old. Both work. Nothing has moved yet, and stopping here breaks nothing.
• Migrate: move consumers in batches sized by blast radius: one caller at a time where a mistake pages someone, a whole service where it doesn't. Verify each batch before starting the next.
• Contract: remove the old, once nothing reads it.

Renaming users.email_addr to users.email on a live system, as five deploys:

1  add `email` · backfill · write both · still read `email_addr`   ← expand, fully reversible
2  read `email` · keep writing both                                ← the only behavior change; watch it
3  move every caller off `email_addr`, batch by service            ← migrate
4  stop writing `email_addr` · leave the column for one rollback window
5  drop the column                                                 ← contract

Each line deploys and reverts on its own; the sequence is the safety. Collapse any two and you get the classic outage: during a rolling deploy both versions run at once, so the instance that hasn't restarted yet is still writing to the column you just dropped.

Prove it's unused: never assert it

Never contract on a search you didn't run, and never treat one symbol search as the answer. "Nothing references it" is a claim about the whole system, and the places that break are the ones a symbol search can't see: config and environment, infra and CI, docs, runbooks and dashboards, other repos and clients you don't build, and every dynamic reference: a name assembled from a string, reflection, a container registration, a route table, a serialized payload, a stored default, an analytics event name. Search for the spelling, everywhere it could be written, not the identifier inside one tree.

The excuses to catch in yourself: "it's obviously dead": obvious to you, not to the caller in someone else's repo · "the search came back empty": empty for that spelling, in that tree · "we can always put it back": not for data, and not for whoever broke in between.

When you can't prove it from the outside, measure it from the inside. Ship a counter or a log line on the old path, wait a real interval: one full business cycle, including the monthly job and the quarterly report, then delete against recorded evidence instead of against absence of evidence.

Advisory or compulsory: pick one out loud

A deprecation with no removal date is advisory, and advisory deprecations get ignored, correctly: nothing happens to anyone who skips them. A date makes it compulsory: and the date is a promise you now owe. Choose, and say which:

• No date → say plainly that it's a preference, and budget to maintain both paths indefinitely.
• A date → name it, name who moves each consumer, and hold it. A removal date that slips twice teaches everyone the next one is fiction.

The churn rule

If you own the thing being deprecated, you own migrating its users. A deprecation notice doesn't remove the work, it multiplies it by the number of teams and bills them for a decision they didn't make. Where you can reach the code, do the migration. Where you can't, ship the codemod, the adapter or the script: a tool, not an announcement.

Zombie code: no owner, live consumers

The state worth naming, because most rot lives in it: code nobody maintains that something still calls. It isn't dead (the traffic proves it) and it isn't alive (nobody will change it), and limbo protects it, nobody dares delete it, nobody dares touch it, and every incident inside it opens with an archaeology dig.

A zombie gets an owner or a removal date. There is no third state. Write down which one it got.

Deleting is a verified change like any other

The check proving nothing broke is the work: ~/.mastermind/engineering/core/rigor.md governs the verification, the evidence-backed report, and the "say first, then do" tier that already covers data migrations and deleting files that aren't yours. Two things belong to removal specifically:

• Keep the deletion its own commit. Bundled with a refactor, the revert stops being one action.
• The old path is the rollback path. Contract after the rollback window closes, not before.

Gotchas

• Data outlives code. Rows the old path wrote, queued jobs carrying the old shape, cached payloads, a mobile build from eight versions ago nobody can force-update. Consumers with no call site to find.
• Removing a flag is a deprecation. Delete the losing branch and the flag together; a flag deleted on its own leaves a branch nothing can reach and nobody will read.
• Non-code consumers break too. A metric name in a dashboard, an alert query, a runbook step, a documented example people copy-paste.
• Deleting a feature's tests is right; deleting the coverage isn't. If a test reached shared behavior through the deleted path, that behavior is now untested, move the test rather than dropping it.