# 3. Understand before editing

Use `how` for mechanism and `why` for rationale. Mechanism comes from current code/runtime evidence. Rationale comes from contemporaneous history such as commits, PRs, issues, docs, or chat when the host can access it.

Keep facts and inference separate. When an explanation hinges on one fragile assumption, test that assumption before editing. For a defect, the bug-fix playbook requires a reproduction or an explicit statement that the reproduction surface is unavailable.