Reasoning Is More Than the Model: Harness-Aware Evaluation of Agents on Verifiable Reasoning Tasks

Open agents are no longer just models: they are model-harness systems whose behavior depends on tool access, control loops, execution feedback, memory, etc. Yet existing evaluations either test models without a harness or fix a single harness to compare models within a domain, obscuring how harness design shapes reasoning capability. To fill this gap, we introduce AlphaDiana, a unified system for harness-aware evaluation of open agents on verifiable reasoning tasks. AlphaDiana standardizes models, harnesses, benchmarks, execution environments, scorers, budgets, and trajectory logging, enabling controlled comparisons across both models and harnesses. We use AlphaDiana to evaluate open agents across mathematical, scientific, coding, terminal, and multimodal reasoning tasks, combining macro-level comparisons of direct inference and agentic execution with micro-level ablations of core harness capabilities. Trajectory-level analysis attributes successes and failures to reasoning, tool use, execution, state, budget, recovery, and verification, revealing that harnesses can both enable iterative problem-solving and introduce systematic errors. AlphaDiana moves agent evaluation beyond asking whether a system succeeds toward explaining why a model-harness system succeeds or fails.