# LastLetter-Extended

Constrained decoding is often assumed to degrade LLM reasoning, but once the prompt and expected output format are aligned, causal ablations show the apparent gap comes mainly from prompt-level differences rather than decoder masking. The work splits constraints into format, validity and reasoning types and tests them across four model families and eleven open-source models; validity constraints, which remove invalid answers, give the most consistent gains, up to 17.4%, while reasoning constraints help only when the enforced trajectory is itself beneficial.

Constrained decoding is often assumed to degrade LLM reasoning, but once the prompt and expected output format are aligned, causal ablations show the apparent gap comes mainly from prompt-level differences rather than decoder masking. The work splits constraints into format, validity and reasoning types and tests them across four model families and eleven open-source models; validity constraints, which remove invalid answers, give the most consistent gains, up to 17.4%, while reasoning constraints help only when the enforced trajectory is itself beneficial.