Structured Outputs in vLLM 2026: Grammar-Constrained JSON at Scale
vLLM structured outputs guarantee valid JSON at scale with grammar-constrained decoding. Learn schema enforcement, backends, and throughput tuning in 2026.
Chapter 1: Why JSON Parsing Still Breaks Your Agents in 2026 Every team that ships an LLM agent eventually writes the same function. It has a name like safe_parse_json or extract_json_from_response, it is forty lines long, and it is a monument to a design mistake. It strips markdown fences. It hunts for the first { and the last }. It runs a regex to kill trailing commas. It tries json.loads, catches the exception, calls the model again with "your output was invalid JSON, please try again," and prays. That function exists because the model was asked to follow a schema...