legacy-modernizer

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

jeffallan/claude-skills|Updated Apr 9, 2026

Works with

Claude CodeCursorClineWindsurfCodexGoose

What it does

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
E

Repository

jeffallan/claude-skills

Last updated

Apr 9, 2026

Installation Guide

Select your AI agent

How to use legacy-modernizer on Cursor

AI-first code editor with Composer

Prerequisites

Before installing skills in Cursor, ensure your development environment meets these requirements:

›Cursor installed and configured on your machine
›Node.js 16+ with npm — verify with node --version
›Active project directory where you want to add legacy-modernizer

Run the install command

Execute the skills CLI command in your project's root directory to begin installation:

$npx skills add https://github.com/jeffallan/claude-skills --skill legacy-modernizer

Fetches legacy-modernizer from jeffallan/claude-skills and configures it for Cursor.

Select Cursor when prompted

The CLI shows a list of agents. Use arrow keys and space to select Cursor:

◆ Which agents do you want to install to?

│

│ ── Universal (.agents/skills) ────────────────

│ · Cline · Codex · Goose · Windsurf

│ ●Cursor(selected)

│ · Cursor · Aider · Continue

Verify installation

Confirm successful installation by checking the skill directory location:

.cursor/skills/legacy-modernizer

Restart Cursor to activate legacy-modernizer. Access via /legacy-modernizer in your agent's command palette.

⚠

Security Notice

We perform automated surface-level scans (Gen AI Scanner, Socket, Snyk) during installation. These checks detect common vulnerabilities but do not guarantee complete security. Always review skill source code and verify the publisher's reputation before production use.

Skills execute code in your environment. Always review source, verify the publisher, and test in isolation before production.

›View source on GitHub ›Skills CLI docs ›About Cursor ›What are agent skills?

Documentation

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Use Cases

User Story & Requirements Generation

Create detailed user stories, acceptance criteria, and feature specs

Example

Generate user stories for 'password reset feature' with acceptance criteria, edge cases, and test scenarios

✓

Reduce spec writing time by 50%, ensure comprehensive coverage

Competitive Analysis

Research competitors, compare features, identify gaps

Example

Analyze 5 competitor products, create feature comparison matrix, suggest differentiation opportunities

✓

Complete competitive research in 2 hours instead of 2 days

Roadmap Prioritization

Evaluate features using frameworks (RICE, ICE, Kano) and create prioritized backlogs

Example

Score 20 feature ideas using RICE framework, generate prioritized roadmap with rationale

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.

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)

# facade.py — routes requests to legacy or new service based on a feature flag import os from legacy_service import LegacyOrderService from new_service import NewOrderService class OrderServiceFacade: def __init__(self): self._legacy = LegacyOrderService() self._new = NewOrderService() def get_order(self, order_id: str): if os.getenv("USE_NEW_ORDER_SERVICE", "false").lower() == "true": return self._new.fetch(order_id) return self._legacy.get(order_id)

Feature Flag Wrapper

# feature_flags.py — thin wrapper around an environment or config-based flag store import os def flag_enabled(flag_name: str, default: bool = False) -> bool: """Check whether a migration feature flag is active.""" return os.getenv(flag_name, str(default)).lower() == "true" # Usage if flag_enabled("USE_NEW_PAYMENT_GATEWAY"): result = new_gateway.charge(order) else: result = legacy_gateway.charge(order)

Characterization Test Template (pytest)

# test_characterization_orders.py # Captures existing legacy behavior as a golden-master safety net. import pytest from legacy_service import LegacyOrderService service = LegacyOrderService() @pytest.mark.parametrize("order_id,expected_status", [ ("ORD-001", "SHIPPED"), ("ORD-002", "PENDING"), ("ORD-003", "CANCELLED"), ]) def test_order_status_golden_master(order_id, expected_status): """Fail loudly if legacy behavior changes unexpectedly.""" result = service.get(order_id) assert result["status"] == expected_status, ( f"Characterization broken for {order_id}: " f"expected {expected_status}, got {result['status']}" )

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

legacy-modernizer

What it does

Category

Repository

Last updated

Installation Guide

How to use legacy-modernizer on Cursor

Prerequisites

Run the install command

Select Cursor when prompted

Verify installation

Security Notice

Documentation

List & Monetize Your Skill

Use Cases

User Story & Requirements Generation

Competitive Analysis

Roadmap Prioritization

Install Skill

Legacy Modernizer

Core Workflow

Reference Guide

Code Examples

Strangler Fig Facade (Python)

Feature Flag Wrapper

Characterization Test Template (pytest)

Constraints

MUST DO

MUST NOT DO

Output Templates

Knowledge Reference

Stakeholder Communication

Implementation Guide

Best Practices

When to Use This

Learning Path

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Reviews

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