The Structured Output Pattern: How to Get LLMs to Return Clean JSON Every Time

Most developers struggle to get LLMs to return clean, parseable JSON. Here's the pattern that works every time in production. Why LLMs Fight Structured Output By default, LLMs are chat models. They produce conversational text. Asking them for JSON is asking them to behave like APIs — and they'll resist if you don't guide them properly. The naive approach: "Return JSON" in your prompt. This works maybe 60% of the time. Not good enough. The Pattern That Works: Constraint + Example + Validation Loop Step 1: System prompt constraint You are a JSON API. You respond ONLY with valid JSON matching this schema. Never include explanation, markdown, or anything outside the JSON object. Schema: { "field" : "type" , ... } The key phrase: "respond ONLY with valid JSON." This primes the model to suppress its conversational instincts. Step 2: Few-shot example Show, don't tell: // Request: "What's the weather in London?" // Response: { "city" : "London" , "temp_c" : 14 , "condition" : "partly_cloudy" ,