Delivered but unbilled is the streaming failure where your AI response renders the whole answer to the user, but the terminal usage frame is dropped, zeroed, or malformed, so your client logs 0 output tokens for a call that cost money. stream_billing_gate.py reconciles the delivered text against the logged usage, offline, and blocks when text shipped and nothing was billed.
AI disclosure: I wrote stream_billing_gate.py with an AI assistant and ran it myself, offline, on Python 3.13.5, standard library only, no network. Every number in the output blocks below is pasted from a real local run. I checked the exit codes (0 / 1 / 2), ran each scenario twice to confirm STDOUT is byte-for-byte identical, and had the tool print a sha256 of its own report so you can reproduce the exact bytes. The one external finding I cite (a Dev.to proof of concept) and the one external number (a Hacker News thread's score) are other people's, attributed inline, and kept out of my fixture numbers.
In short:
A streamed response delivers text first and accounting last. If the terminal usage frame never lands, the text is already on the screen and your client logs 0 output tokens. The user got the whole answer. Your bill says it was free.
