TwoChannel: Separating Missing from Corrupted Modalities in Multimodal Fusion

A multimodal system in deployment decides from whatever channels arrived, and a channel that never arrived and one that arrived corrupted call for opposite repairs, a new acquisition or a recalibration. Existing fusion rules write both failures onto one non-negative scalar per channel, whose margin narrows as the damage deepens, and neither filling nor a trust fitted to task loss recovers the distinction. We propose , a fusion rule that carries absence and unreliability on a support mass and a conflict mass rather than on one weight per channel. Prototype decomposition reads the sub-views of each channel onto a shared dictionary and splits what the channel asserts from where its own parts disagree, so each channel enters fusion as an assertion and a contradiction. Provenance-typed transport fills the slots an absent channel leaves along compatibility edges and writes restored evidence into the support mass only, so a restoration can raise support but cannot manufacture conflict. A per-prototype trust product combines a channel's internal dispersion with its agreement with the other present channels and is read from the instance alone, with no label naming the corrupted channel. We prove that neither an absent channel nor restored evidence adds a conflict term, so the separation holds by construction rather than by estimation. On Syn-MOSEI, Syn-IEMOCAP and Syn-ADNI, a probe separating a dropped from a damaged channel reaches $0.9960$ off this readout against a raw-input ceiling of $1.0000$, where mean filling leaves it at $0.4995$, near the $0.5019$ of a random-feature control. On the primary cohort it keeps that distinction at no accuracy cost, matching plain fusion's $0.9383$ at complete input where the scalar gate reads $0.7883$.