Outlines

Outlines is a Python library from .txt (dottxt) that pioneered grammar-based constrained generation for language models. Instead of validating output after the fact, Outlines uses a finite state machine to mask invalid tokens during generation, so the model can only produce schema-compliant output. It supports JSON Schema, regex, and full context-free grammar (CFG/EBNF) constraints. The same code runs across OpenAI, Ollama, vLLM, and Hugging Face models. The outlines-core Rust port (in collaboration with Hugging Face) delivers a 2x improvement in index compilation speed. For developers running local models who need guaranteed schema compliance with zero retries, Outlines is the tool. It excels at research, custom grammars, and self-hosted LLM prototyping where deterministic output is non-negotiable.