How a Lightweight JSON Gateway Stops CI Pipelines Breaking on Drifting Model APIs

A developer discovered that a free AI model endpoint silently changed its response shape, returning an unexpected field name that broke a GitLab CI pipeline without any code or config changes. The incident highlighted how free model routes can drift in field names, enum casing, or response size, causing downstream failures that are hard to trace. Rather than relying on retries or fallback defaults — which mask the real problem — the author built a small gateway server that sits between the CI job and the model endpoint. The gateway enforces a strict JSON contract, checking required fields, allowed fields, data types, and enum values before passing any response downstream. On a contract violation, it returns a 502 with a named problem list and logs a one-line diagnostic, failing the pipeline early and cleanly before wasted compute time accumulates.