ATBench: A Diverse and Realistic Agent Trajectory Benchmark for Safety Evaluation and Diagnosis

Evaluating the safety of LLM-based agents is an increasingly important challenge, as risks in realistic deployments often emerge gradually over multi-step interactions rather than from isolated prompts or final responses. Existing trajectory-level benchmarks, however, remain limited in three key dimensions: (1) insufficient interaction diversity, due to restricted tool ecosystems and narrow scenario coverage; (2) limited observability of safety failures, as coarse labels fail to capture how risks arise and evolve; and (3) lack of long-horizon realism, where short or simplified trajectories underestimate delayed and context-dependent risks. To address these limitations, we introduce ATBench, a trajectory-level benchmark that enables structured, diverse, and realistic evaluation of agent safety. ATBench formulates agentic safety risks along three orthogonal dimensions---risk source, failure mode, and real-world harm---providing a controllable framework for capturing diverse risk patterns. Building on this formulation, we construct trajectories using a taxonomy-guided generation engine with heterogeneous tool pools and incorporate a long-context delayed-trigger protocol that models realistic risk emergence through multi-stage interactions. The benchmark contains 1,000 trajectories (503 safe, 497 unsafe), averaging 9.01 turns and 3.95k tokens, with 1,954 invoked tools drawn from pools spanning 2,084 available tools. Data quality is ensured through rule-based & LLM-based filtering and a human full audit. Experiments across frontier LLMs, open-source models, and specialized guard systems show that ATBench remains challenging even for the strongest evaluators---GPT-5.4 achieves only 76.7\% F1 on binary safety classification and 33.6\% on fine-grained risk-source diagnosis---while enabling taxonomy-stratified analysis, cross-benchmark comparison, and diagnosis of long-horizon failure patterns. Data and code will be released upon acceptance.