Explain Change

Give the user an evidence-backed mental model. Stay read-only for explanation- only work; a walkthrough is not permission to fix or publish.

Resolve the base, head, and scope. Refresh evidence for a named PR; include relevant committed and working-tree changes for a local request. Ask only when competing scopes would materially change the answer.

Read the diff and enough entrypoints, callers, tests, and decision records to explain old and new behavior. Consult intent when rationale or a boundary matters, not as a mandatory repository tour. Accepted decisions govern within host permissions and explicit safety constraints; verify PR descriptions and handoffs.

Lead with changes for the user, developer, or operator. Group files conceptually and use an example when it clarifies behavior. Explain material contract, data, ownership, failure, compatibility, and operational consequences; omit irrelevant surfaces unless asked about them.

For material primitives/boundaries, name what was reused, extended, added, replaced, moved, or removed; its owner and source of truth; and the consumers or contracts affected. Explain the implementation rather than re-running design selection. Do not manufacture formal ledgers for a small change.

Distinguish accepted tradeoffs from inferred consequences. State the benefit, cost, documented alternatives, and revisit conditions where known. For complexity questions, map churn to behavior, compatibility, tests, docs, and incidental complexity, then support a proportional, mixed, or disproportionate verdict.

Use adequate recorded verification. Run a narrow non-destructive check only for an unresolved material claim. Return the smallest explanation that answers the question with anchors and meaningful unknowns. Omit empty sections and repeated summaries. Safety review belongs to an available reviewer; do not present an explanation as merge approval.