Is It the Read or the Interruption? Decomposing Read-Gate Gains in Agentic RAG

Agentic retrieval systems can finalize without issuing a read action on retrieved documents. Requiring a read can improve accuracy, but the gain may reflect retrieved evidence, interruption, or changes in answering behavior. We investigate this distinction by branching each trajectory at its first attempt to finalize without reading, holding the preceding interaction fixed. For gpt-5-mini at minimal reasoning effort, we compare read demands with information-free tool-call demands in pre-registered factorial studies on 1,800 Wikipedia and 800 medical questions. The information-free control often returns acknowledgements or questions instead of answers. Adding the same explicit answer obligation to both demands reduces the read/no-op accuracy gap on Wikipedia from 3.3 to 1.2 percentage points; both the reduction and residual survive Holm correction. On the medical corpus, no additional read benefit is detected. Against no gate, the read demand without the obligation loses 3.0 points, whereas an obligated no-op gains 3.5 points. A forecast frozen before the medical interventions overestimates the read demand's gain by 5.3 points. On Qwen2.5-7B, non-answer responses are rare and no obligation benefit is detected. These findings show that accuracy gains from enforced reading alone do not isolate the contribution of retrieved evidence. Evaluations must also control how interventions instruct agents to answer and terminate.