claude-plugins/plugins/code-modernization/commands/modernize-transform.md

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description: Transform one legacy module to the target stack — idiomatic rewrite with behavior-equivalence tests

argument-hint:

Transform legacy/$1 module $2 into $3, with proof of behavioral equivalence.

This is a surgical, single-module transformation — one vertical slice of the strangler fig. Output goes to modernized/$1/$2/.

Step 0 — Plan (HITL gate)

Read the source module and any business rules in analysis/$1/BUSINESS_RULES.md that reference it. Then enter plan mode and present:

• Which source files are in scope
• The target module structure (packages/classes/files you'll create)
• Which business rules / behaviors this module implements
• How you'll prove equivalence (test strategy)
• Anything ambiguous that needs a human decision NOW

Wait for approval before writing any code.

Step 1 — Characterization tests FIRST

Before writing target code, spawn the test-engineer subagent:

"Write characterization tests for legacy/$1 module $2. Read the source, identify every observable behavior, and encode each as a test case with concrete input → expected output pairs derived from the legacy logic. Target framework: . Write to modernized/$1/$2/src/test/. These tests define 'done' — the new code must pass all of them."

Show the user the test file. Get a 👍 before proceeding.

Step 2 — Idiomatic transformation

Write the target implementation in modernized/$1/$2/src/main/.

Critical: Write code a senior $3 engineer would write from the specification, not from the legacy structure. Do NOT mirror COBOL paragraphs as methods, do NOT preserve legacy variable names like WS-TEMP-AMT-X. Use the target language's idioms: records/dataclasses, streams, dependency injection, proper error types, etc.

Include: domain model, service logic, API surface (REST controller or equivalent), and configuration. Add concise Javadoc/docstrings linking each class back to the rule IDs it implements.

Step 3 — Prove it

Run the characterization tests:

cd modernized/$1/$2 && <appropriate test command for $3>

Show the output. If anything fails, fix and re-run until green.

Step 4 — Side-by-side review

Generate modernized/$1/$2/TRANSFORMATION_NOTES.md:

• Mapping table: legacy file:lines → target file:lines, per behavior
• Deliberate deviations from legacy behavior (with rationale)
• What was NOT migrated (dead code, unreachable branches) and why
• Follow-ups for the next module that depends on this one

Then show a visual diff of one representative behavior, legacy vs modern:

delta --side-by-side <(sed -n '<lines>p' legacy/$1/<file>) modernized/$1/$2/src/main/<file>

Step 5 — Architecture review

Spawn the architecture-critic subagent to review the transformed code against $3 best practices. Apply any HIGH-severity feedback; list the rest in TRANSFORMATION_NOTES.md.

Report: tests passing, lines of legacy retired, location of artifacts.