What is llmjson? llmjson is a constrained decoding library and Ollama-compatible proxy that guarantees 100% valid JSON from local LLMs on Apple Silicon (MLX).

The model physically cannot produce invalid tokens. Why it belongs on this list:

It fills a gap no other listed library covers: native MLX constrained decoding with drop-in Ollama proxy compatibility. No cloud. No retries. No post-processing.

Feature	llmjson	Outlines	Instructor	lm-format-enforcer
Local / Offline	✅	✅	❌	✅
Ollama Drop-in proxy	✅	❌	❌	❌
Apple Silicon MLX	✅	❌	❌	❌
Zero retries (mathematical guarantee)	✅	✅	❌	✅
n8n / no-code compatible	✅	❌	❌	❌

Benchmark: 80/80 trials passed, zero failures, 95% CI [95.5%, 100%] (Clopper-Pearson).

Links: https://github.com/fatdinhero/llmjson

Find the Python Libraries section and add this entry: llmjson https://github.com/fatdinhero/llmjson

(2024, Apache 2.0, Fatih Dinc / OMNI_GENESIS) Constrained decoding for local LLMs on Apple Silicon via MLX. Drop-in Ollama proxy — only a port change required. Guarantees 100% schema-valid JSON through VocabScanner + JSONContextTracker (stack FSM) + AND Gate (logit masking). Zero retries. Zero post-processing. n8n compatible.

pip install jsongate

from llmjson import generate

result = generate( model_id="mlx-community/Qwen2.5-14B-Instruct-4bit", prompt="Profile for Alice, senior engineer", schema={"type": "object", "properties": {"name": {"type": "string"}, "age": {"type": "integer"}}} )

result.is_valid is always True

Feature Local / Offline ✅ Ollama Drop-in ✅ (unique in this list) Apple Silicon MLX ✅ Zero retries ✅ n8n compatible ✅

Benchmark: 80/80 trials, 0 failures across Qwen2.5-14B and Llama-3.1-8B.