legacy-modernizer

SKILL.md

Incremental migration strategies, dependency mapping, and facade designs for safely modernizing legacy systems.

Guides five-step workflow: assess system, plan migration, build safety net with characterization tests, migrate incrementally via strangler fig pattern with feature flags, and validate before retiring legacy code

Includes reference templates for strangler fig facades, feature flag wrappers, and characterization test patterns to capture existing behavior as a golden master

Enforces zero production disruption through explicit rollback triggers, traffic increment validation (5% → 25% → 50% → 100%), and one-release-cycle stability proof before legacy code removal

Provides assessment templates, dependency mapping guidance, and monitoring setup instructions to document risks and preserve business logic throughout migration

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.