Constrained Decoding Isn't Inherently Harmful: Constraint Type Decides LLM Impact
Nokia · hf · 2026-10-27
A Télécom Paris / Nokia Bell Labs paper revisits whether constrained decoding hurts LLM reasoning.
- The common assumption: constraints guarantee format but degrade reasoning. The authors propose a taxonomy of three constraint types: format constraints (output structure), validity constraints (masking invalid answers), and reasoning constraints (restricting reasoning trajectories)
- They theoretically show appropriate constraints can preserve or even improve performance by eliminating invalid paths
- Across 4 model families and 11 open-source models, causal experiments show apparent degradation largely stems from non-equivalent prompting and output formats, not decoder-level masking
- Takeaway: constraint type is what matters — validity and reasoning constraints often improve reliability and accuracy
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