Does Probe Conditioning Isolate Composition? An Information-Control Audit of Multi-Hop Evaluation

Conditioning multi-hop evaluation on successful component probes is intended to reduce missing-prerequisite confounding. A FULL-FACT condition that supplies both component facts, however, also reveals the answer-bearing second hop, so its performance gain cannot identify composition by itself. We audit this contrast with an information-control suite in MedCompBench, an ontology-controlled biomedical evaluation containing 18,856 real chains and 4,711 counterfactual twins. On Qwen2.5-7B, providing only the bridge improves multiple-choice accuracy by .097 [.059,.137], whereas providing only the answer-bearing second fact improves it by .338 [.286,.392] and providing both facts improves it by .389 [.335,.443]. Across seven Qwen2.5 and Qwen3 checkpoints, the full-fact support gap ranges from .259 to .392 for control-passing models. Independent endpoint checks with Phi-3.5-mini and BioMistral-7B yield gaps of .317 [.264,.374] and .202 [.145,.257]. On matched items, neither endpoint shows a resolved difference from Qwen2.5-7B. Across five prespecified new composed-question wordings, the gap remains positive for all 15 checkpoint–wording pairs. Separately, independently reworded component probes retain only 40.7–58.7% of originally admitted items, showing that the conditional population is strongly prompt-dependent. The original contrast is therefore mainly an answer-support effect rather than an isolated measure of bridge retrieval or composition. Conditional multi-hop evaluations should separate bridge provision, answer-bearing facts, and gate stability before assigning a compositional interpretation.