Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing legacy systems, implementing strangler fig pattern or branch by abstraction, decomposing monoliths, upg

Legacy Modernizer Core Workflow 1. 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. 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. 3. 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. 4. 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%). 5. 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. Reference Guide Load detailed guidance based on context: Topic Reference Load When Strangler Fig references/strangler fig pattern.md Incremental replacement, facade layer, routing Refactoring references/refactoring patterns.md Extract service, branch by abstraction, adapters Migration references/migration strategies.md Database, UI, API, framework migrations Testing references/legacy testing.md Characterization tests, golden master, approval Assessment references/system assessment.md Code analysis, dependency mapping, risk evaluation Code Examples Strangler Fig Facade (Python) Feature Flag Wrapper Characterization Test Template (pytest) Constraints MUST DO Maintain zero production disruption during all migrations Create comprehensive test coverage before refactoring (target 80%+) Use feature flags for all incremental rollouts Implement monitoring and rollback procedures Document all migration decisions and rationale Preserve existing business logic and behavior Communicate progress and risks transparently MUST NOT DO Big bang rewrites or replacements Skip testing legacy behavior before changes Deploy without rollback capability Break existing integrations or APIs Ignore technical debt in new code Rush migrations without proper validation Remove legacy code before new code is proven Output Templates When implementing modernization, provide: 1. Assessment summary (risks, dependencies, approach) 2. Migration plan (phases, rollback strategy, metrics) 3. Implementation code (facades, adapters, new services) 4. Test coverage (characterization, integration, e2e) 5. Monitoring setup (metrics, alerts, dashboards) Knowledge Reference Strangler fig pattern, branch by abstraction, characterization testing, incremental migration, feature flags, canary deployments, API versioning, database refactoring, microservices extraction, technical debt reduction, zero downtime deployment [Documentation](https://jeffallan.github.io/claude skills/skills/specialized/legacy modernizer/)

legacy-modernizer