First, I know exactly what I’m talking about. Second, there is a bug in the Patch API related to "cold_transfer_mode": "sip_refer" endpoint: https://api.retellai.com/update-retell-llm/llm_xx Since Saturday, if you patch an LLM without explicitly including this field, it gets silently set to null due to the update. As a result, cold transfers using SIP REFER start failing on the Twilio side because the call ends up in a terminating vs. originating mismatch. This took me an absurd amount of time to debug. My production was down for three days. I eventually found it by running a JSON diff after the update that was rolled out Saturday morning — and that’s where this specific issue showed up. The core problem: If you patch the tools without cold_transfer_mode, you unintentionally break production because the agent tool resets it to null. I just wanted to share this information in case your production broke as well. You don’t need to fix anything on your side if it’s considered expected behavior — but I’ll be honest: this was a really shitty issue to track down. Patch entpoints shouldnt to that only put..... but who am i to judge. I changed nothing, my customer is angry, i need somethink to drink.

Hello @felix2012 , there was a recent deprecation , please check this https://docs.retellai.com/deprecation-notice/1-26-2026

Yeah, I’m pretty sure other people crashed their production because of this as well. Do whatever you want with this information. I’m almost done building my own LiveKit integration anyway, so I honestly don’t really care anymore ^^

Taking that to the team.

thanks, have a good one

Hey @felix2012 I just noticed our production agent that was working perfectly for a couple months broke since this change. Supports been slow/not helping so far. They want me to just have my Retell Agent phone number come up instead of the caller's number for the transfer. Do you have any idea since you seem to be quite knowledgeable here? This is a big deal for my client that I likely will lose now.