xgrammar

The model never sees the grammar; the sampler does, as a 16 KB bitmask per request per step. The answer builds that mask from an automaton, prices the naive version at ten milliseconds, and shows how precompilation removes almost all of it.

A per-request schema compile stalls the scheduler, a per-step mask must beat the forward pass, and skipping forced characters can push the model off its own tokenization. The answer handles all three with numbers from one realistic schema.

The mask is a 16 KB bitmap per request. Whether it costs nothing or stalls every step depends on which thread computes it and what it is allowed to overlap with.