Semantic Quota Constraints for Specification-Faithful Structured Generation

Grammar-constrained generation enforces structural validity but leaves an important class of semantic constraints unenforced: a well-formed object can still miss required fields, undershoot a count, or omit a required operator. We introduce semantic quota constraints, a parameter-free decode-time mechanism that tracks quantitative requirements in parser (or container) state: stopping is blocked while any requirement is unmet, quota-saturated actions are removed, and generation is steered toward unmet requirements. A controlled program-generation environment lets us prove soundness (Theorem 1: every terminating rollout satisfies every parsed requirement) and isolate the mechanism: grammar-only decoding reaches 0.5% All-req, quota blocking—counting validity—92.7%, and the full layer—validity plus steering for timely completion—100.0% across three seeds. The same failure mode appears in deployed structured generation. In a pinned SchemaStore checkout, 691/952 documents (72.6%) contain a cardinality keyword; a pinned constrained decoder exhibits four isolated required/minProperties failures, and a quota adapter closes all four, moving matched full-schema validity from 53/60 to 57/60. At twenty draws the same adapter records 16 repairs and 0 breaks (268/300 to 284/300), a mechanism probe rather than a population estimate. The same masks wrap three frozen instruction hosts; terminating host rollouts satisfy parsed requirements, while residual misses are budget hits whose rate depends on the host.