LM Format Enforcer constrains generation by filtering the tokens a model is allowed to emit so the output conforms to a JSON Schema or a regular expression. It is designed to plug into existing inference stacks, with integrations for Transformers, vLLM, and llama.cpp, so a team can add schema enforcement to a serving setup they already run rather than adopting a new engine. It belongs to the same enforcement family as Outlines, XGrammar, and llguidance: all of them mask the sampler to keep output valid. Its distinguishing emphasis is breadth of integration across stacks rather than being the fastest core or the most ergonomic API. That makes it a pragmatic choice when the goal is to bolt structured output onto a heterogeneous set of runtimes. For open-source AI it is one of the libraries that made schema-constrained output portable across the open serving ecosystem before the engines absorbed the capability natively.
The Stack · Runtime · Open source
LM Format Enforcer
Token-filtering library that enforces a JSON Schema or regex during generation; integrates with Transformers, vLLM, and llama.cpp.
Sources
- LM Format Enforcer on GitHub https://github.com/noamgat/lm-format-enforcer
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Other projects at the Runtime layer
12 siblings · ordered open first
- vLLM Open source
Dominant open production inference engine; PagedAttention and continuous batching; NVIDIA / AMD / Intel / TPU support.
- SGLang Open source
RadixAttention plus structured generation; from the LMSYS team; gains for shared-prefix and agent workloads.
- llama.cpp Open source
Georgi Gerganov's local-first inference engine; defines the GGUF format; the on-device standard.
- Ollama Open source
Local model runner; Docker-style UX over llama.cpp; the easiest way to run open weights on your machine.
- Text Generation Inference (TGI) Open source
HuggingFace's production inference server; maintenance mode in 2026 as vLLM became the standard.
- MLC-LLM Open source
Cross-platform compilation (TVM-based); the 'LLM in your browser' or 'on your phone' standard.
- Outlines Open source
Structured-output library using finite-state-machine guided decoding to force model output to match a regex, JSON Schema, or grammar.
- XGrammar Open source
Fast grammar-based constrained-decoding engine, used as a structured-output backend by several open serving engines.
- llguidance Open source
Low-level Rust constrained-decoding engine that enforces grammars and JSON Schema at high throughput; powers Guidance.
- Guidance Open source
A programming model for constrained generation that interleaves control flow with model calls and enforces regex, grammars, and JSON Schema.
- Instructor Open source
Library for getting Pydantic-typed structured output from LLMs across many providers, built on function calling and JSON mode with automatic retries.
- TensorRT-LLM Source available
NVIDIA's closed-runtime counterpart; fastest on NVIDIA hardware; depends on closed CUDA kernels and the proprietary TensorRT compiler.