Testing is Your Migration Insurance
The single biggest predictor of migration success is test coverage. If you can’t verify that behavior is preserved, you’re flying blind. AI can dramatically accelerate test creation for legacy code that was never properly tested.
Characterization Tests
When migrating legacy code without tests, start by writing characterization tests — tests that document current behavior, whether that behavior is correct or not. AI assistants can generate these by analyzing code paths and generating test cases for each branch.
Snapshot Testing for Migrations
Snapshot tests capture the output of a function or component and fail if the output changes. They’re perfect for migrations because they verify behavior preservation without needing to understand the implementation. AI can generate snapshot tests across your entire codebase.
Contract Testing for API Migrations
Pact-style contract tests verify that API consumers and providers agree on the API contract. During migration, these tests catch breaking changes that unit tests miss. AI can generate contract tests from API documentation or OpenAPI specs.
AI-Generated Test Suites
Modern AI tools can analyze function signatures, docstrings, and usage patterns to generate comprehensive test suites. While these tests need human review, they provide 80% coverage in a fraction of the time.
Hands-On Exercise
Choose a module in your codebase with low test coverage. Use an AI assistant to generate characterization tests for its key functions. Run the tests to establish a baseline. Now attempt a small refactoring and verify the tests catch any behavior changes.
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