How Layered Architecture Stops LLM Test Suites From Draining API Credits
A developer discovered they were burning through OpenAI API credits rapidly because their test suite made real network calls to the model on every run. The core issue was structural: route handlers had the API call tightly coupled with request parsing and response logic, leaving no clean way to test without hitting the live endpoint. The fix involved separating code into distinct router, service, and schema layers, isolating the AI call inside a single importable service function. Using Python's unittest.mock, the service function can then be patched directly in tests, making them run in milliseconds at zero API cost with fully deterministic results. A key technical note is that the mock must target the function reference where it is imported, not where it is originally defined, to intercept calls correctly.
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