Safe Incremental Coding

Choose the branch before selecting a model. Coordinator work and deterministic commands proceed within invocation authority. Apply this method in the current role. It does not dispatch a worker. For a worker dispatch, use shared/references/model-preview.md and its one approved route snapshot. Nested skills and runners consume it without another selection or worker.

Protect the existing behavior needed for a requested change, using the legacy change method from Dave Farley's The Software Developers' Guidebook. The aim is ease of change: the affected code can be changed safely.

Build a characterization test net before refactoring. Keep the work within the area needed for the requested change.

Build the net

1. Identify the affected behavior and an observable boundary that can exercise it.
2. Capture current outputs or side effects with representative inputs, including relevant boundary and failure cases. Pin existing behavior even when it contains a known defect; an intentional behavior change is a separate step.
3. Control or normalize timestamps, identifiers, seeds, and other variable values without hiding behavior that matters to the change.
4. Prefer tests at an acceptance or integration boundary when unit tests would depend on the current internal structure.
5. If no existing boundary exposes the behavior, limit the initial production edit to the minimum observation seam. Keep its business logic unchanged and capture the baseline before further production changes.
6. Check that the test passes against the baseline and would detect a relevant change in behavior.

A passing baseline protects only the cases it exercises. Add coverage for a missing case when the requested change depends on it.