I'm a bit confused and maybe misunderstanding (and mainly a Retell guy lol), but... if Vapi is making a tool call to your FastAPI server, just return the result to it in a JSONResponse. The format Vapi wants back is here: https://docs.vapi.ai/tools-calling#server-response-format-providing-results-and-context -- set up your dict and reply back to whatever called it. You shouldn't be POSTing a reply back anywhere. I'm pretty certain hookdeck will route it back to Vapi on its own when you return your response.