verifying-external-behaviorlisted

# Verifying External Behavior Most integration bugs are not coding errors. They are a belief about someone else's system — a status code, a parameter name, a response shape, a build backend's scoping rule — that was never checked and turned out to be wrong. The code is written correctly against a contract that does not exist. The fix is not more care. It is a **probe**: a throwaway command that asks the real system the exact question, before the code is written. Probes cost seconds. The bugs they prevent are silent, ship green, and are found months later. ## Probe the exact call you are about to write Not a similar call, not the documented example — the same endpoint form, the same library version, the same argument spelling. ```bash # A library's real defaults / return shapes, with no venv to create or clean up uv run --no-project --with somelib python -c "import somelib; print(somelib.Thing())" # The exact URL, including the collection-vs-item distinction curl -s -o /dev/null -w '%{http_code}\n' https://api.example.com/v1/things curl -s -o /dev/null -w '%{http_code}\n' https://api.example.com/v1/things/1 # What a build backend actually put in the artifact uv build && unzip -l dist/*.whl # A real service instead of a fake, for one minute docker run --rm -p 6390:6379 redis:7-alpine ``` Two rules make probes worth the minute they cost: - **Probe before you design around the answer.** A probe that confirms a parameter tweak "should" fix a discrepancy is worth more t