Derivation Depth Exposes Reasoning Failures in Tool-Augmented Music Analysis
Audio-language models remain unreliable at extracting basic musical properties from audio, motivating pipelines that place a music information retrieval (MIR) tool between the audio and the model. The tool reports measurements such as keys, chords, and structural similarity, and the model is expected to derive the answer. We test whether it can. Holding the musical information fixed, we vary derivation depth: the number of reasoning steps between the tool output and the answer. When the report states the conclusion, all five models score near 100%, and they follow it even when it is wrong. One step away, accuracy drops. On modulation, open models fall to 57-81%, although they answer that single step correctly when asked directly, and reach 0% at greater depths. Frontier models degrade less, but still degrade. Adding the audio does not recover the loss. The bottleneck in tool-augmented music analysis is not access to musical information but reasoning from it, so tool design must account for how much derivation it leaves to the model.