How do I catch a hallucination mid-stream before my agent acts on it?

Hallucination detection today happens after the fact. The model outputs a full response, a separate judge model scores it, and a human or downstream check decides what to do. In agentic pipelines with tool calls, web searches, or code execution, the agent may have already acted on a fabricated entity or misattributed fact by the time any check runs. A January 2026 paper on streaming hallucination detection in long chain-of-thought reasoning shows that detecting fabrication mid-generation is feasible using internal representations, but the technique is research grade and requires access to hidden states not available through any public API. The gap is a streaming, API-compatible hallucination sensor that can flag a generation before the agent takes an irreversible action.