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Interface-Induced Trajectory Censoring

arXiv · AI, language, vision and robotics · article · Sep 3, 2026 · UTC

Agent evaluations report a tool-call rate read off the serving stack. That number can be zero while the model is emitting well-formed calls: the interface censors the trajectory before anything downstream sees it. On BFCL v4's own data, executor and scorer, holding weights, cases, decoding and seeds fixed and changing only the serving adapter, the same model scores 0.00 or 0.96 / 0.19. A 2x2 over chat template and parser locates the effect exactly: both main effects are exactly zero and all of it sits in the interaction -- no component is defective, and repairing one side of the contract buys

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Evidence & attribution

First collected: 2026-09-21T04:51:57.792Z. This is not the publication date.