Diagnoses, implements, and verifies non-trivial or risk-sensitive changes in an existing repository while preserving the user's uncommitted work, following repository instructions (AGENTS.md or CLAUDE.md), keeping edits within authorized scope, and handing off with runtime evidence. Use when the user asks to fix a bug, add or change a feature, refactor, migrate, edit configuration, chase down a regression, or make a change safely amid overlapping edits or worktrees where verification and change control matter. Not for isolated code snippets or simple explanations; for diagnosis-only requests, prove and explain the cause without fixing unless asked.
