---
name: mutation-proof-is-the-house-standard
description: spt-core lanes do not accept green-first test rigs — every new test must be shown to FAIL under a targeted revert of the impl behavior it claims to prove, with the mutation then undone.
metadata:
  type: feedback
---

Every new test written for an spt-core lane must be accompanied by a **mutation proof**: revert
the specific implementation behavior the test claims to cover, show the test goes red on the
assertion that names that behavior, then undo the mutation and re-verify green.

**Why:** the project's stated position is that a green-first rig is not evidence — a test that
cannot fail is worthless, and the traceability gate only checks that a `[unit->REQ-*]` tag
exists, never that the tagged test has teeth. Lane briefs from the orchestrating agents state
this as MANDATORY and name the specific mutations they expect.

**How to apply:** budget for it up front. Aim each mutation at one predicate so exactly one
assertion dies — a mutation that reddens the whole file proves less than one that kills a single
named assertion. A mutation that kills ONLY the new test is also the cleanest possible evidence
that the behavior was previously unproved, which is worth reporting explicitly. Mark temporary
edits with a `// MUTATION (x)` comment and `grep -rn "MUTATION" crates/` before declaring done —
a forgotten mutation is far worse than a missing test. Do not commit unless the lane asked.

Related: [[spt-bin-test-run-recipe]]