ReLLM

Overview

ReLLM is a Python library that constrains language model completions to produce exact structured output, such as JSON or other formal grammars. It works by filtering the model's token probabilities at each step to only allow tokens that conform to a user-defined structure.

Best for

Best for
Developers who need deterministic, structured outputs from any language model

Use cases

• Generating valid JSON objects from free-form LLM responses
• Enforcing a specific schema for data extraction tasks
• Building reliable pipelines that require parseable outputs

Notes

ReLLM is a Python library that constrains language model completions to produce exact structured output, such as JSON or other formal grammars. It works by filtering the model’s token probabilities at each step to only allow tokens that conform to a user-defined structure.

513 stars on GitHub. Last updated 2023-08-10. Licensed MIT.

Use cases

• Generating valid JSON objects from free-form LLM responses
• Enforcing a specific schema for data extraction tasks
• Building reliable pipelines that require parseable outputs

Pros

• Lightweight and model-agnostic, works with any completion API
• Open source with a simple Python interface
• Guarantees structural validity without post-processing

Cons

• Limited to exact structure constraints, not suitable for fuzzy or probabilistic outputs
• May slow down generation due to per-token filtering
• Community-maintained with no official support or extensive documentation

Indexed from awesome-langchain and enriched against its public facts.