Outlines

Structured text generation from LLMs — guarantee JSON, regex, or grammar-constrained output

Outlines is an open-source Python library that guarantees structured output from LLMs. Instead of hoping an LLM returns valid JSON, Outlines uses constrained generation to mathematically ensure output conforms to a Pydantic model, JSON schema, regex pattern, or custom grammar — no parsing errors, no validation failures. Works with local models and commercial APIs.

Key specs

9,000 GitHub stars source

as of 2026-03-27