Trustworthy JSON: schema-validate your model's structured output
· 14 min read
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"Return only JSON" is one of the most common instructions in production LLM apps — and one
of the least reliable. Models wrap JSON in markdown fences, add a friendly sentence, or (if
they're reasoning models) narrate their entire thought process around it. Any of those
breaks a strict JSON.parse, and your pipeline falls over.
In this guide we build a Promptfoo eval that makes WEC
Inference API models classify support tickets into schema-validated JSON, then harden
it against real-world messiness — and use it to pick a model you can actually trust. This is
part 2 of the evals series (see part 1
for first-run setup). Everything here was run live against https://inference.wiline.com.