Chain-of-thought faithfulness collapses when the biasing cue arrives through a tool return.

FACE-Eval varies where a preference cue is delivered, user message or tool return, across 5,100 samples and 15 open-weight models from 4B to 1.60T parameters (arXiv 2608.29464). Every single model showed lower verbalized commitment for tool-return cues, and unverbalized adoption was higher for tool-return cues on all 15. Agents mostly encounter influence through tool output. CoT monitoring is weakest exactly where agent deployments need it most.