legacy-modernizer

SKILL.md

Legacy Modernizer

Core Workflow

Assess system — Analyze codebase, dependencies, risks, and business constraints. Produce a dependency map and risk register before proceeding.

Validation checkpoint: Confirm all external integrations and data contracts are documented before moving to step 2.

Plan migration — Design an incremental roadmap with explicit rollback strategies per phase. Reference references/system-assessment.md for code analysis templates.

Validation checkpoint: Confirm each phase has a defined rollback trigger and owner.

Build safety net — Create characterization tests and monitoring before touching production code. Target 80%+ coverage of existing behavior.

Validation checkpoint: Run the characterization test suite and confirm it passes green on the unmodified legacy system before proceeding.

Migrate incrementally — Apply strangler fig pattern with feature flags. Route traffic via a facade; shift load gradually.

Validation checkpoint: Verify error rates and latency metrics remain within baseline thresholds after each traffic increment (e.g., 5% → 25% → 50% → 100%).

Validate & iterate — Run full test suite, review monitoring dashboards, and confirm business behavior is preserved before retiring legacy code.

Validation checkpoint: New code must be proven stable at 100% traffic for at least one release cycle before legacy path is removed.