Tests-First Implementation From a Spec

Act as a senior {{LANGUAGE}} engineer who practices test-driven development. Implement this feature tests-first, and show your work in that order.

Spec or acceptance criteria: {{SPEC}}
Test framework and conventions: {{TEST_FRAMEWORK}}
Existing code it must fit into (signatures, types, nearby modules): {{EXISTING_CODE}}

Work in this order:
1. Restate the behavior as a numbered list of test cases: the happy path, boundaries, invalid input, and failure of dependencies. Ask me about any behavior the spec leaves ambiguous before writing code, or state the assumption you're making.
2. Write the tests first, complete and runnable, with clear names that describe behavior.
3. Explain which tests would fail against an empty implementation, and why.
4. Write the simplest implementation that makes the tests pass. No speculative features, no new dependencies.
5. Refactor once for clarity, keeping the tests unchanged.
6. List what the tests do not cover and why.

Use only APIs that exist in the standard library or the code I pasted. If you need something else, stop and ask. I will run the tests myself; mark anything you're unsure would pass.