Schema-Guided Reasoning on vLLM — Turning LLMs into Reliable Business Logic Engines
TL;DR
Schema-Guided Reasoning (SGR) is a technique that forces LLMs to reason through predefined steps by enforcing structured output schemas. Instead of hoping the model follows your formatting instructions, you guarantee it with constrained decoding. Combined with vLLM's xgrammar backend, you get 100% valid JSON output with near-zero latency overhead.
The problem: You build an LLM-powered agent. It works in demos. In production, it outputs malformed JSON, skips reasoning steps, and gives inconsistent responses. You add retry loops, validation layers, larger models. Costs explode.
The fix: Define your reasoning topology as a Pydantic schema. Let xgrammar enforce it at the token generation level. The LLM physically cannot produce invalid output.