Refactor — one structural experiment

Refactor changes structure while preserving observable behavior. It performs one caller-selected transformation and reports the result.

Procedure

1. Name the preserved behavior and the focused acceptance surface.
2. Record an honest baseline, including any reproducible ambient failures.
3. Apply one bounded transformation: extract, rename, inline, simplify, encapsulate, move, or delete dead code.
4. Run the focused check and the smallest package-level regression check justified by the changed surface.
5. Return the diff summary, commands, results, and behavior not checked.

Do not combine a newly discovered behavior fix with the structural change. A red result is evidence for the caller; this skill does not revert, narrow, retry, commit, validate, or route subsequent work automatically.

Seam experiments before commitment

When the transformation needs a seam — an extraction boundary, interface, or module split — and more than one candidate seam exists, probe before you cut. Run the probe in disposable isolation (a scratch branch, worktree, or copied tree the caller's policy allows): rough in the seam, see what it forces — signature churn, import cycles, test rewrites — then discard the probe and keep only the knowledge. Stop condition: at most two probes; if the second candidate seam also fights back, report both findings to the caller instead of trying a third. Cutting the first imaginable seam directly into the working tree is the premature seam failure mode: the wrong boundary calcifies because reverting it now costs more than living with it.

Neutrality gates

"Behavior-preserving" is a claim to execute, not assert. Gate the transformation on behavior-identical proof:

• The focused check and the package-level regression check pass both before and after, with the same set of pre-existing failures — no new red, and no quietly vanished red either (a test that stops running is a behavior change).
• For output-producing surfaces (generators, serializers, formatters, reports), hash the outputs: capture golden-output hashes over identical inputs before the change and compare byte-for-byte after. A hash mismatch is a behavior diff to surface and explain, never to shrug at; the caller decides whether to keep, narrow, or reverse the change.
• Observable error messages, exit codes, and public signatures on the changed surface are part of behavior unless the caller excluded them.

A neutrality gate that was skipped or narrowed after the fact is the post-hoc neutrality failure mode — the diff decides what got tested. Name any surface the gates did not cover in the report's behavior-not-checked list.

References

• Behavior-preserving simplification
• Behavior scenarios