A Four-Condition Diagnostic Protocol for Evidence Utilization in Long-Context and Retrieval-Augmented Language Models

Abstract: Final-answer accuracy, retrieval recall, and citation overlap do not by themselves identify whether a long-context or retrieval-augmented LLM used the evidence it was given. A model can answer from parametric memory, fail despite receiving the right passages, or cite evidence without converting it into the requested answer. This paper proposes a matched four-condition evidence-availability protocol--no evidence, full context, retrieved evidence, and oracle-evidence reference--for diagnosing evidence utilization under fixed examples, prompts, score fields, retrieval settings, and validity checks. ONCU is used as a protocol-bound estimator of recovered oracle-reference evidence advantage and is computed only for denominator-valid groups; denominator-free answer, evidence, retrieval, and failure-audit metrics are reported separately. The empirical study evaluates five local open-weight models from the Qwen, Gemma, Llama, and Mistral families across Controlled-ONCU-safe16K, HotpotQA-ONCU, and 2WikiMultiHopQA-ONCU, with 18,000 ONCU-compatible predictions. The main finding is a task-dependent bottleneck split: controlled synthetic settings primarily expose full-context utilization failures, whereas the tested realistic multi-hop settings primarily expose retrieval-chain coverage failures in denominator-free answer and evidence metrics, with ONCU supporting the same direction on oracle-improving groups. The contribution is a diagnostic protocol for separating no-evidence answerability, oracle-evidence recoverability, full-context utilization, and retrieval-conditioned utilization, rather than a single-score leaderboard for long-context or retrieval-augmented systems.