Structured Output Grammars for On-Device LLMs

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Learn to use Structured Output Grammars for on-device LLMs to restrict token logits and eliminate malformed JSON from inference

advanced Published 12 Jun 2026
Action Steps
  1. Write custom GBNF grammars for your API schema to constrain decoding
  2. Implement grammar-guided sampling to restrict token logits at each step
  3. Integrate GBNF grammars with Kotlin via JNI for seamless interaction
  4. Test and compare the performance cost of grammar sampling vs unconstrained generation
  5. Handle token healing edge cases that break structured output on quantized models
Who Needs to Know This

This benefits developers working on on-device LLMs, particularly those using llama.cpp on Android, as it improves the accuracy and reliability of their models

Key Insight

💡 Grammar-guided sampling can eliminate malformed JSON from on-device inference by restricting token logits to valid continuations

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📱 Improve on-device LLM accuracy with Structured Output Grammars! 🚀

Key Takeaways

Learn to use Structured Output Grammars for on-device LLMs to restrict token logits and eliminate malformed JSON from inference

Full Article

Deep dive into GBNF (GGML BNF) grammars for constrained decoding in llama.cpp on Android — how grammar-guided sampling restricts the token logits at each step to only valid continuations, eliminating malformed JSON from on-device inference. Covers writing custom GBNF grammars for your API schema, the performance cost of grammar sampling vs unconstrained generation, integrating with Kotlin via JNI, and the token healing edge cases that break structured output on quantized models.
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