Start by naming the behavior surface: public APIs, CLI flags, wire formats, database effects, files, environment variables, logs that tests assert, and user-visible UI states.
Establish characterization tests before changing unclear or under-tested behavior. Prefer narrow tests that lock down current outputs, side effects, ordering, and error cases.
Write down invariants that must remain true after the refactor. Include both domain invariants and mechanical invariants such as idempotence, ordering, resource cleanup, and concurrency assumptions.
Make one reversible transformation at a time. Keep each diff small enough to review and roll back without losing unrelated work.
Verify after each meaningful step with the smallest relevant test command. Broaden verification when the refactor crosses module boundaries.
Do not combine behavior changes with simplification. If a bug is discovered, capture it separately unless the user explicitly asks to fix it now.

Workflow

Inspect the target code and its call sites. Identify the observable contract, risky dependencies, and the smallest safe scope for the first change.
Build a behavior baseline. Run existing tests and add characterization tests for unprotected behavior before structural edits.
State invariants in the work notes or final report. Make the intended preservation target explicit enough for another engineer to audit.
Apply a single isomorphic transformation, such as extracting a pure helper, replacing duplicated branches with a shared path, simplifying conditionals, renaming for clarity, or moving code behind an equivalent interface.
Compare the diff against the invariants. Remove incidental churn, formatting noise, and unrelated cleanup.
Run targeted verification. If verification fails, rollback the latest transformation or isolate the behavioral difference before continuing.
Repeat only while the next simplification remains clearly valuable and low risk. Stop when additional cleanup would outgrow the evidence available.

Characterization Tests

Good characterization tests describe what the current system does, not what the system should ideally do. Cover representative success paths, boundary inputs, known odd behavior, failure modes, and side effects. Use fixtures or golden outputs only when they make behavioral drift easier to detect than assertions.

When tests are expensive, add a narrow harness around the refactor target and keep the full suite for final confirmation. When tests cannot be added, document the manual or command-line probes used as the baseline and keep the refactor smaller.

Diff Discipline

Avoid broad reformatting unless the formatter is already the local standard and the target file is expected to be formatted.
Preserve names, exported symbols, error text, and ordering unless the baseline proves they are not part of the contract.
Prefer mechanical moves and extraction before semantic rewrites.
Keep generated files, lockfiles, and unrelated dependency changes out of the refactor unless they are required for verification.
Re-read the final diff as a reviewer looking for hidden behavior changes.

Rollback

Every step needs a clear rollback point. If a transformation becomes tangled, revert that step and choose a smaller path. If the baseline is too weak to prove equivalence, stop and report the missing evidence instead of pushing through on intuition.

Output

Report the behavior baseline, invariants, changed files, verification commands, and any residual risk. If rollback was needed, say what was rolled back and why.

behavior-preserving-simplification

Simplify And Refactor Code Isomorphically

Operating Rules