Codebase Migration Skill

1.3 — Function & Class Inventory

Save to migration/03_inventory.md:

• List every public function/class/module with its signature and purpose
• Note internal vs. external interfaces
• Flag anything with no clear analog in the target language

Phase 2: Establish Success Criteria

If the codebase has unit tests:

• Copy all test files to migration/tests_original/
• Shuffle and split into 70% train, 30% held-out test
• Save the split manifest to migration/test_split.json
• You will use train tests to validate progress; held-out tests only run at the very end

If the codebase has NO unit tests:

• Stop and ask the user: "What is the deterministic success criteria for this migration? For example: identical outputs for the same inputs, all API endpoints returning the same responses, specific benchmark thresholds."
• Document the agreed criteria in migration/success_criteria.md
• Write synthetic train/test cases based on the flow map from Phase 1

Phase 3: Incremental Migration (Test-Driven)

Migrate module by module, not all at once.

For each module:

1. Write the target-language implementation
2. Run the relevant train tests against it
3. Fix failures before moving to the next module
4. Log progress in migration/04_progress.md

Rules:

• Never skip a failing train test — fix it or flag it explicitly
• If a language-idiom gap makes a direct port impossible, document the design decision in migration/05_decisions.md
• Do not run held-out tests during this phase

Phase 4: Held-Out Test Validation

Only proceed here after all train tests pass.

Run the held-out test suite. This validates that:

• Nothing was hardcoded to match specific training examples
• The migration generalizes correctly

If held-out tests fail:

• Diagnose whether it's a logic error or a data-path gap from Phase 1
• Fix, re-run train tests to confirm no regression, then re-run held-out

Phase 5: Migration Complete

Produce migration/06_summary.md containing:

• Final pass/fail counts for train and held-out tests
• List of known behavioral differences (intentional or unavoidable)
• Dependency mapping (original → target language equivalents)
• Any follow-up items for the user

Deliver the migrated codebase and present the summary to the user.