Valid Per-Field Selective Risk Control for Document Extraction: Three Failure Modes, a Validity Ladder, and When Conditioning Pays

Document extraction systems often violate per-field selective risk constraints due to document clustering, score-refit leakage, and tie-mass pathology. Implementing a fit/val split protocol restores expected-selective-risk control, achieving 0.318 coverage at 0.096 risk on Claude-Sonnet-5 fields extracted from CORD receipts.