claude-plugins-official/plugins/code-modernization/README.md

Code Modernization Plugin

A structured workflow and set of specialist agents for modernizing legacy codebases — COBOL, legacy Java/C++, monolith web apps — into current stacks while preserving behavior.

Overview

Legacy modernization fails most often not because the target technology is wrong, but because teams skip steps: they transform code before understanding it, reimagine architecture before extracting business rules, or ship without a harness that would catch behavior drift. This plugin enforces a sequence:

assess → map → extract-rules → brief → reimagine | transform → harden

The discovery commands (assess, map, extract-rules) build artifacts under analysis/<system>/. The brief command synthesizes them into an approval gate. The build commands (reimagine, transform) write new code under modernized/. The harden command audits and patches the legacy system. Each step has a dedicated slash command, and specialist agents (legacy analyst, business rules extractor, architecture critic, security auditor, test engineer) are invoked from within those commands — or directly — to keep the work honest.

Expected layout

Commands take a <system-dir> argument and assume the system being modernized lives at legacy/<system-dir>/. Discovery artifacts go to analysis/<system-dir>/, transformed code to modernized/<system-dir>/…. If your codebase lives elsewhere, symlink it in:

mkdir -p legacy && ln -s /path/to/your/legacy/codebase legacy/billing

Optional tooling

/modernize-assess works best with scc (LOC + complexity + COCOMO) or cloc, and falls back to find/wc if neither is installed. Portfolio mode also benefits from lizard (cyclomatic complexity). The commands degrade gracefully without them, but the metrics will be coarser.

Commands

The commands are designed to be run in order, but each produces a standalone artifact so you can stop, review, and resume.

/modernize-assess <system-dir> — or — /modernize-assess --portfolio <parent-dir>

Inventory the legacy codebase: languages, line counts, complexity, build system, integrations, technical debt, security posture, documentation gaps, and a COCOMO-derived effort estimate. Produces analysis/<system>/ASSESSMENT.md and analysis/<system>/ARCHITECTURE.mmd. Spawns legacy-analyst (×2) and security-auditor in parallel for deep reads. With --portfolio, sweeps every subdirectory of a parent directory and writes a sequencing heat-map to analysis/portfolio.html.

/modernize-map <system-dir>

Build a dependency and topology map of the legacy system: program/module call graph, data lineage (programs ↔ data stores), entry points, dead-end candidates, and one traced critical-path business flow. Writes a re-runnable extraction script and produces analysis/<system>/topology.json (machine-readable), analysis/<system>/TOPOLOGY.html (rendered Mermaid + architect observations), and standalone call-graph.mmd, data-lineage.mmd, and critical-path.mmd.

/modernize-extract-rules <system-dir> [module-pattern]

Mine the business rules embedded in the legacy code — calculations, validations, eligibility, state transitions, policies — into Given/When/Then "Rule Cards" with file:line citations and confidence ratings. Spawns three business-rules-extractor agents in parallel (calculations, validations, lifecycle). Produces analysis/<system>/BUSINESS_RULES.md and analysis/<system>/DATA_OBJECTS.md.

/modernize-brief <system-dir> [target-stack]

Synthesize the discovery artifacts into a phased Modernization Brief — the single document a steering committee approves and engineering executes: target architecture, strangler-fig phase plan with entry/exit criteria, behavior contract, validation strategy, open questions, and an approval block. Reads ASSESSMENT.md, TOPOLOGY.html, and BUSINESS_RULES.md and stops if any are missing — run the discovery commands first. Produces analysis/<system>/MODERNIZATION_BRIEF.md and enters plan mode as a human-in-the-loop gate.

/modernize-reimagine <system-dir> <target-vision>

Greenfield rebuild from extracted intent rather than a structural port. Mines a spec (analysis/<system>/AI_NATIVE_SPEC.md), designs a target architecture and has it adversarially reviewed (analysis/<system>/REIMAGINED_ARCHITECTURE.md), then scaffolds services with executable acceptance tests under modernized/<system>-reimagined/ and writes a CLAUDE.md knowledge handoff for the new system. Two human-in-the-loop checkpoints. Spawns business-rules-extractor, legacy-analyst (×2), architecture-critic, and general-purpose scaffolding agents.

/modernize-transform <system-dir> <module> <target-stack>

Surgical, single-module strangler-fig rewrite. Plans first (HITL gate), then writes characterization tests via test-engineer, then an idiomatic target implementation under modernized/<system>/<module>/, proves equivalence by running the tests, and produces TRANSFORMATION_NOTES.md mapping legacy → modern with deliberate deviations called out. Reviewed by architecture-critic.

/modernize-harden <system-dir>

Security hardening pass on the legacy system: OWASP/CWE scan, dependency CVEs, secrets, injection. Spawns security-auditor. Produces analysis/<system>/SECURITY_FINDINGS.md ranked Critical / High / Medium / Low, then patches Critical and High findings directly in legacy/<system>/ and re-scans to verify. Useful as a pre-modernization step when the legacy system will keep running in production during the migration.

Note: /modernize-harden is the one command that edits legacy/. If you adopt the deny: Edit(legacy/**) workspace setting below, relax it for this command — or run hardening as a separate workstream against its own checkout.

Agents

• legacy-analyst — Reads legacy code (COBOL, legacy Java/C++, procedural PHP, classic ASP) and produces structured summaries. Good at spotting implicit dependencies, copybook inheritance, and "JOBOL" patterns (procedural code wearing a modern syntax). Used by assess and reimagine.
• business-rules-extractor — Extracts business rules from procedural code with source citations. Each rule includes: what, where it's implemented, which conditions fire it, and any corner cases hidden in data. Used by extract-rules and reimagine.
• architecture-critic — Adversarial reviewer for target architectures and transformed code. Default stance is skeptical: asks "do we actually need this?" Flags microservices-for-the-resume, ceremonial error handling, abstractions with one implementation. Used by reimagine and transform.
• security-auditor — Reviews code for auth, input validation, secret handling, and dependency CVEs. Tuned for the kinds of issues that appear when translating security primitives across stacks (e.g., session handling from servlet to stateless JWT). Used by assess and harden.
• test-engineer — Writes characterization, contract, and equivalence tests that pin legacy behavior so transformation can be proven correct. Flags tests that exercise code paths without asserting outcomes. Used by transform.

Installation

/plugin install code-modernization@claude-plugins-official

Recommended Workspace Setup

This plugin ships commands and agents, but modernization projects benefit from a workspace permission layout that enforces the "never touch legacy, freely edit modernized" rule. A starting-point .claude/settings.json for the project directory you're modernizing:

{
  "permissions": {
    "allow": [
      "Bash(git diff:*)",
      "Bash(git log:*)",
      "Bash(git status:*)",
      "Read(**)",
      "Write(analysis/**)",
      "Write(modernized/**)",
      "Edit(analysis/**)",
      "Edit(modernized/**)"
    ],
    "deny": [
      "Edit(legacy/**)"
    ]
  }
}

Adjust legacy/ and modernized/ to match your actual layout. The key invariants: Edit under legacy/ is denied, and writes are scoped to analysis/ (for documents) and modernized/ (for the new code). The exception is /modernize-harden, which intentionally patches legacy/ — see its note above.

Typical Workflow

# 1. Inventory the legacy system (or sweep a portfolio of them)
/modernize-assess billing

# 2. Map call graph, data lineage, and the critical path
/modernize-map billing

# 3. Extract business rules into testable Rule Cards
/modernize-extract-rules billing

# 4. Synthesize the approved Modernization Brief (human-in-the-loop gate)
/modernize-brief billing java-spring

# 5a. Greenfield rebuild from the extracted spec…
/modernize-reimagine billing "event-driven services on Java 21 / Spring Boot"

# 5b. …or transform module by module (strangler fig)
/modernize-transform billing interest-calc java-spring

# 6. Security-harden the legacy system that's still in production
/modernize-harden billing

License

Apache 2.0. See LICENSE.