TypeLLM Forces LLMs to Return Valid JSON Every Time, 5.8x Faster
TypeLLM constrains open-source LLM decoding to your JSON Schema, delivering guaranteed typed outputs with one-token choices and prefix-cached batching.
- TypeLLM constrains LLM decoding at the token level to guarantee JSON Schema-typed outputs.
- Supports text, integer, number, boolean, and enum fields with single-token choice decoding.
- Batch mode achieved 5.8x throughput over sequential in a Qwen3.8-27B benchmark run.
- Optional thinking mode boosted JevBench accuracy from 84.42% to 98.70%.
- Runs on open models served through SGLang with prefix caching for linear input cost.
- Apache-2.0 licensed, ships probability outputs, sequential dependencies, and per-field thinking budgets.
TypeLLM makes JSON Schema part of LLM decoding
TypeLLM constrains token selection so an LLM returns booleans, numbers, strings, and enumerated values that match a supported subset of JSON Schema. The Apache-2.0 project combines an SGLang server with a Python client and supports optional reasoning before the typed answer.
Malformed JSON, invalid enum values, and inconsistent scalar formats often force applications to add parsing, repair, and retry logic. TypeLLM enforces the declared type during generation and returns Python values that application code can consume directly. The guarantee covers structure and allowed values; factual accuracy still depends on the model and prompt.
Types become decoding rules
Developers provide shared context and describe the fields they need. At each decoding step, the server restricts the available vocabulary to tokens that can continue a valid value for the field's declared type.
| Schema type | Generation behavior | Current limits |
|---|---|---|
string |
Generates free text under string constraints. | Supports optional maxLength. |
integer |
Generates a constrained integer token sequence. | May require multiple tokens. |
number |
Generates a constrained numeric token sequence. | May require multiple tokens. |
boolean |
Resolves a closed choice in one generated token. | Returns one of two values. |
enum |
Resolves a closed choice in one generated token. | Accepts up to 16 string, integer, or numeric candidates. |
Boolean and enum fields cannot produce an undeclared option, which removes an entire class of downstream validation failures. Open-ended strings and numbers retain variable-length generation while remaining constrained by their supported type rules.
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