It’s failing before the logging system initializes. So even though your server returns a config and the call shows “SIP completed,” Vapi likely rejects the assistant config during early validation which is why you see 0 logs and no clear field error. Since it only happens with one config, it’s probably, A missing nested required field. A null value that should be omitted. A provider mismatch (model/voice/transcriber combo). What you can do is compare it to a working config and isolate the difference instead of guessing. I can help with the fix.