Refactor — one structural experiment

Refactor changes structure while preserving observable behavior. It performs one caller-selected transformation and reports the result. "Behavior-preserving" is a claim to prove with before/after checks, never to assert.

What counts as behavior

Unless the caller explicitly excluded a surface, all of these must survive:

• Messages and exit codes. Error and output text compares byte-for-byte; exit codes, error types and which input raises which error stay the same. Scripts and callers parse them. Preserve an inconsistent message and report it; normalizing it is a behavior change.
• Differences between near-duplicates. When merging duplicated branches, carry every difference (constants, comparisons, messages, extra steps) as a parameter or a branch. Do not unify a difference the caller has not declared accidental.
• Interfaces. Public signatures, defaults, return types, persisted field names, protocol values and CLI flags. Renaming one is a compatibility change unless the accepted scope provides for it.
• Order and coverage. Branch priority, default handling, evaluation count, side-effect order, and the set of tests that run. A pre-existing red that vanishes, or a test that stops running, is a behavior change.
• Protective checks. Trust-boundary validation, data-loss guards, security controls and accessibility affordances stay, even when another layer, upstream or downstream, duplicates one or no test reaches it. The neutrality gates cannot see their removal, so looking redundant is no reason to delete one.

Procedure

1. Name the preserved behavior, the focused acceptance surface and the concrete structural problem for its callers, in the caller's domain terms.
2. Run the focused check and the smallest regression check the changed surface justifies, error paths included, and record that honest baseline, including reproducible ambient failures. Add any characterization test the transformation needs now, before the edit, so it is part of the baseline. For output-producing surfaces (generators, serializers, formatters, reports), hash the output over fixed inputs. When the caller's evaluation compares the built program before and after the change, pin the starting source, build its baseline before edits and keep that binary and the comparison inputs.
3. Apply one bounded transformation: extract, move, rename, inline, simplify a conditional, reduce parameters, encapsulate, substitute existing code, or delete dead code; the refactoring catalog holds per-pattern safety checks. Judge it by what callers must understand and where a domain rule must change, not by file size.
4. A bug or suspicious inconsistency found on the way is reported separately (location, why it looks wrong) and left unfixed. Fixing it inside the refactor hides a behavior change the caller did not authorize.
5. Rerun exactly the step-2 checks over the same inputs and compare any output hashes byte-for-byte.
6. Report, then stop. A red result is evidence for the caller; this skill does not revert, narrow, retry, commit, validate, or route subsequent work.

When nothing can be executed (no runtime, no tests, code supplied in a message), neutrality is unproven: give the exact before/after commands and inputs the caller must run, error paths included, and list every surface under not checked.

transformation: <the one change>
preserved:      <behavior and surfaces from step 1>
checks:         <command>: before -> <result>; after -> <result>   (or "not run")
outputs:        <before/after hashes when the surface produces output>
abstraction:    <a new one and the second use or contract that justifies it, or "none">
diff:           <files touched>; only those the transformation names, plus tests added before the change
suspected bugs: <file:line, why>; reported, not fixed
not checked:    <surfaces no check covered>; present even when empty

Neutrality gates

The checks from step 2 give the same result after as before: no new red, no vanished red, identical output hashes. A mismatch is a behavior diff to surface and explain, never to shrug at; the caller decides whether to keep, narrow, or reverse it. A gate 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 under not checked.

Boundaries and tests

Choosing where to restructure, or a new module boundary, is Architecture's job; this skill executes one transformation in the area the caller selected. Reject a wrapper that only adds another name or pushes the same coordination into its callers. Keep internal test seams private when callers need no substitution. Exercise the preserved behavior through the caller-facing boundary; retain existing tests until equivalent coverage of their distinct cases is demonstrated. Moving a test is not permission to delete the behavior it protected.

Use the caller's vocabulary for extracted operations and types. A naming ambiguity that changes behavior belongs with the existing domain definition; consult Domain only when that distinction needs work.

When the caller-chosen transformation can be cut more than one way (which lines an extraction takes, where a chosen interface sits), probe before you cut. A choice between module boundaries the caller has not made goes to Architecture first. 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.

References

• Behavior-preserving simplification — refactoring catalog and per-pattern safety checks
• Codebase design — Matt Pocock; the wrapper and test-seam rules under Boundaries and tests adapt it.