Principle:Sgl project Sglang Constrained Text Generation
| Knowledge Sources | |
|---|---|
| Domains | NLP, Structured_Generation, Frontend_DSL |
| Last Updated | 2026-02-10 00:00 GMT |
Overview
A generation primitive that produces text constrained by regex patterns, JSON schemas, or discrete choices within the SGLang frontend DSL.
Description
Constrained text generation extends standard LLM generation by enforcing output structure at the token level. Unlike post-hoc validation (generate then check), constraints are applied during generation via logit masking, guaranteeing valid output. SGLang supports three constraint types: regex patterns (arbitrary structure), JSON schemas (typed objects), and discrete choices (selection from a fixed list). These constraints integrate with the frontend DSL's sgl.gen primitive and the Engine API's sampling parameters.
Usage
Use constrained generation when you need reliable structured output — extracting entities, generating configuration files, producing typed API responses, or implementing tool calls. This eliminates retry loops and parsing failures.
Theoretical Basis
Constrained generation works by maintaining a constraint state machine that tracks valid continuations:
- At each generation step, query the constraint for valid next tokens
- Mask all invalid token logits to negative infinity
- Sample from the remaining valid tokens
- Update the constraint state
Constraint types:
- Regex — Finite-state machine tracking position in regex pattern
- JSON Schema — Compiled to regex, then FSM (via outlines)
- Choices — Trie-based matching against a fixed set of strings