ContainmentBench: Trace-Based Evaluation of Post-Exposure Containment in Tool-Using LLM Agents

Tool-using large language model (LLM) agents read untrusted content, maintain memory, delegate tasks, and invoke tools with external side effects. Terminal attack-success or policy-violation rates do not show what happens between exposure and commit or whether a defense also suppresses authorized actions. We introduce ContainmentBench, a sandboxed benchmark comprising a 504-scenario specification dataset, a shared rollout-trace schema, and stage-scoped metrics for endpoint violations, logged propagation, and explicitly authorized taint-exposed proposals that commit. The main Qwen2.5-7B-Instruct study evaluates seven policy conditions and five seeds, yielding a 17,640-record trace corpus. Across 600 matched active-tainted rollout pairs, no committed policy violation was observed under either taint-only or intent-ledger enforcement. Their execution records nevertheless differed: 441 pairs (73.5%) had different values in a shared 12-field trace summary that includes commit-related diagnostics, and the mean authorized proposal-commit score was 0.164 under taint-only enforcement and 0.857 under intent-ledger enforcement, compared with 0.923 under tool-boundary enforcement. Logged-propagation rankings changed with stage selection and normalization. In a limited set of custom AgentDojo-native workflows, committed violations were observed without defense and were not observed under either evaluated defense. A separate 6,048-rollout Mistral/common-JSON model-interface configuration retained the v1-to-v2 proposal-commit improvement, but committed violations were observed under intent-ledger v2. Equal terminal outcomes do not imply equal containment. The evaluation uses synthetic workflows. The intended intent-ledger mechanism assumes schema-aligned authorization metadata; one public-status task family violates this assumption and is analyzed separately.