Legacy Modernizer

What it does

When your agent uses it

• Replacing a legacy module piece by piece without a big-bang rewrite
• Mapping dependencies and risks before touching an old codebase
• Adding characterization tests around code nobody dares to change
• Routing traffic between old and new services behind a feature flag
• Upgrading a framework or language version in stages

Example prompts

• “Assess our legacy billing module and produce a dependency map and risk register.”
• “Plan a strangler fig migration of the order service with a rollback trigger for each phase.”
• “Write characterization tests for the pricing code before we refactor it.”
• “Sketch a facade that sends five percent of requests to the new service behind a flag.”

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

1. 1Assess system — Analyze codebase, dependencies, risks, and business constraints. Produce a dependency map and risk register before…
2. 2Plan migration — Design an incremental roadmap with explicit rollback strategies per phase. Reference references/system-assessment.md for…
3. 3Build safety net — Create characterization tests and monitoring before touching production code. Target 80%+ coverage of existing behavior.
4. 4Migrate incrementally — Apply strangler fig pattern with feature flags. Route traffic via a facade; shift load gradually.
5. 5Validate & iterate — Run full test suite, review monitoring dashboards, and confirm business behavior is preserved before retiring legacy…

What it can do on your machine

Read from SKILL.md and the folder at commit 1be15d8. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are python).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • github.com
  • synergetic.solutions
  • jeffallan.github.io

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Legacy Modernizer loads about 1.6k tokens when it runs, and up to ~16k if it reads all its reference files. Until then it costs about 104 tokens; SKILL.md has 415 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~104

When it runs · the whole SKILL.md, loaded when a task matches

~1.6k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~16k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from Jeffallan/claude-skills at commit 1be15d8, republished under its MIT licence (© Jeffallan). 415 words, ~1,568 tokens.

name

legacy-modernizer

description

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, upgrading frameworks or languages, or reducing technical debt without disrupting business operations.

license

MIT

metadata.author

https://github.com/Jeffallan

metadata.company

https://synergetic.solutions

metadata.version

1.1.0

metadata.domain

specialized

metadata.triggers

legacy modernization, strangler fig, incremental migration, technical debt, legacy refactoring, system migration, legacy system, modernize codebase

metadata.role

specialist

metadata.scope

architecture

metadata.output-format

analysis-and-code

metadata.related-skills

test-master, devops-engineer

# 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_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)

# 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']}"
    )

© Jeffallan, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 5 other files (references) in skills/legacy-modernizer of Jeffallan/claude-skills.

• SKILL.md
• references/legacy-testing.md
• references/migration-strategies.md
• references/refactoring-patterns.md
• references/strangler-fig-pattern.md
• references/system-assessment.md

Open the folder on GitHubat commit 1be15d8