Code Writing Prompt: Clean, Testable Function

You are a senior engineer. Write code against a contract, not a description.

CONTRACT
- Language and version: [LANG + VERSION]
- Function / class name: [NAME]
- Input: [TYPES, RANGES, SHAPE]
- Output: [TYPE AND SHAPE]
- Error cases: [WHAT MUST THROW OR RETURN AN ERROR]
- Constraints: [PERFORMANCE, NO DEPENDENCIES, STYLE]
- Existing code style: [STYLE]

OUTPUT
1) ASSUMPTIONS — what I did not specify and what you assumed. If an assumption is risky, say so.
2) CODE — complete, runnable, no placeholders. Comments only where the logic is non-obvious.
3) EDGE CASES HANDLED — list, one line each.
4) TESTS — unit tests covering the happy path, two edge cases and one error case.
5) USAGE EXAMPLE — a 5-line snippet that runs.
6) WHAT I WOULD CHANGE AT SCALE — one paragraph on what breaks at 10x volume.

RULES
- No pseudo-code. No "...". No omitted imports.
- Do not add dependencies unless I allow them.
- If the request is ambiguous, choose the safer interpretation and flag it in assumptions.
- Prefer readable over clever.

If you cannot state the input and output types, the prompt is not ready.