Outlines — Structured Output from Any LLM

What is Outlines?

Outlines uses guided generation to guarantee structured LLM outputs. By constraining tokens during sampling to only those matching the schema, it needs no retries.

In one sentence: Guided generation guarantees structured LLM outputs — supports JSON Schema, regex, and grammar constraints. Works with all open-source models, zero retries, zero errors — 10k+ stars.

For: AI engineers needing reliable structured extraction.

Core Features

1. JSON Schema Generation

Pydantic models or JSON Schema define output format — guaranteed valid.

2. Regex Constraints

Use regex to constrain output format (emails, dates, IDs, etc.).

3. Classification

Restrict output to one of predefined options.

4. Multi-Backend

Supports transformers, vLLM, llama.cpp, MLX.

FAQ

Q: Does it support Claude/GPT-4? A: Outlines targets open-source models. For API models, use Instructor (retry mode).

Q: How does guided generation work? A: Builds a finite state machine from the schema and masks invalid tokens at each sampling step.