Constrained Tagging for Span Labeling
Generative LLMs can label spans without task-specific training, but their output is not tied to positions in the input: a returned span may be absent from the input or occur several times. We present constrained tagging, a training-free method in which the model reproduces the input and wraps each span in labeled inline markup. A logits processor using a byte-level token trie and a finite-state machine admits only tokens that continue the input verbatim or form a valid tag, so the output reproduces the input exactly with labels from the given set, and spans may begin or end at any character boundary, even inside a token. Two-phase decoding lets reasoning models reason before the constraint engages. On six datasets and six open-weight models, the guarantee holds in all but 0.022% of 255,312 constrained generations, each failure a reasoning model exhausting its token budget or never closing its reasoning, and the constraint adds no measurable cost. Unlike context-based prompting, which loses 8.5% of its spans, constrained tagging locates every span it proposes on every dataset; it is as accurate on two datasets, and on the other four its errors lie in which spans the model chooses, not where they are.