Tool schemas constrain the structure of a call but need not determine how request evidence becomes argument values. We formalize this layer as a field-level binding contract over presence, evidence, ordered operations, parameters, and validation, and state an identifiability boundary: no schema-only estimator can guarantee recovery of policy distinctions absent from its input. We audit all 1,311 filtered requests in pinned Berkeley Function-Calling Leaderboard (BFCL) V4 Live Simple and Multiple with five open checkpoints and the official abstract-syntax-tree (AST) checker. A partial structural diagnostic and AST acceptance are non-nested: 1,470 of 6,555 calls pass only the diagnostic and 50 pass only AST. A stratified AI-only audit finds coarse concern detection far more consistent across two isolated passes than fine attribution. A pre-specified test of a deterministic schema-derived operation-label encoding gives 67.70% AST acceptance versus 68.27% under the same guidance without field tags (-0.56 points; 95% task-cluster CI -1.17 to +0.05; 186 fixes, 223 breaks). Value-only turnover is nearly balanced; the net imbalance sits in key-set changes. The point estimate reverses sign on one post-hoc out-of-panel checkpoint; thus no improvement is established on the fixed panel, and independently authored contracts remain untested.