Sierra Launches Fleming-1 to Detect AI Agents Calling by Phone

Sierra launched fleming-1 on October 7, 2026, a model designed to detect when the caller on a phone call is another AI agent. The company described the release as an update to the constellation of models behind agents built on its platform. Sierra said the question companies face when the phone rings has shifted from who is calling to what is calling.

The company first encountered the issue in 2025, when agents built on Sierra started calling agents built on Sierra in healthcare. With personal agents such as Muse and Instinct taking off, Sierra said, a large share of the calls companies receive could come from AI acting on behalf of consumers. According to Sierra, fleming-1 analyzes a caller’s speech in real time, scoring the audio for signs that it was generated by AI.

The model looks beyond how a voice sounds to a human listener, assessing whether the voice on the other end is synthetic while the call is still happening. Sierra said telling a synthetic voice from a human one over the phone is getting harder. Background noise,…