What is LMQL?
LMQL (Language Model Query Language) is a programming language for working with large language models. Instead of building prompts as plain strings and parsing free-form replies, you write query programs that mix ordinary text, Python control flow and typed or constrained variables. A variable can be declared with a type such as int, restricted to a set of values, limited by length, or cut off at a stop phrase, and the runtime enforces these constraints during generation.
With LMQL you can build multi-part prompts, nested queries that reuse prompt components, loops that repeat prompts, and chatbots. Query programs are plain Python functions, so they can be called from Python code and return typed results. The same code runs across several backends, including llama.cpp, OpenAI and Hugging Face Transformers, and switching between them takes a single line of code.
Key features
- Typed variables that guarantee output format
- Hard constraints (where clauses) enforced by the runtime
- Nested queries for modular, reusable prompt components
- Python integration: query programs are regular functions
- Portable code across llama.cpp, OpenAI and Transformers backends
- Execution trace and distribution measurement of outputs
Who is it for
- Developers building LLM-powered applications
- Researchers working on prompting and constrained generation
- Teams needing structured, parseable LLM outputs
Pros and cons
- Constraints are enforced by the runtime, not just requested in the prompt
- Works across several model backends with minimal code changes
- Open source project created by the SRI Lab at ETH Zurich
- Requires learning a new query language syntax
- Page does not describe a pricing plan
LMQL pricing
The vendor does not publish prices in a readable form (often pricing is shown only after signing in).
Is LMQL free?
The vendor does not state this clearly on the homepage or pricing page.
LMQL alternatives
| Tool | Price from | Free plan | Category |
|---|---|---|---|
 Google AI Studio | 0,75 USD | ? | APIs & models |
 Cline | not listed | yes | APIs & models |
 Amazon Bedrock | od 0,20 USD | ? | APIs & models |
 Claude Platform (Anthropic API) | 2 USD input / 10 USD output | no | APIs & models |
 OpenAI Platform | 0,10 USD | ? | APIs & models |
 Langfuse | 29 USD/mo | yes | APIs & models |
FAQ
What is LMQL used for?
Programming language for LLM interaction, letting developers write robust, constrained prompts with types, templates and an optimizing runtime.
Is LMQL free?
The vendor does not state this clearly on the homepage or pricing page.
How much does LMQL cost?
The vendor does not publish prices publicly or requires sign-in.
What are the best LMQL alternatives?
Closest alternatives: Google AI Studio, Cline, Amazon Bedrock, Claude Platform (Anthropic API), OpenAI Platform.
Data checked October 12, 2026 on the vendor website (lmql.ai).