Developer finds streams ending without a completion marker still reported as success

A developer building a small summarizer discovered that an HTTP 200 status does not confirm a streamed model response completed. Their output stopped mid-sentence without any error, because the client only checked the status code and the presence of output. Streaming success and failure paths share the same initial data, so simple checks miss truncation. To address this, they wrote a stdlib-only probe that records time to first byte, finish reasons, the [DONE] sentinel, and how the connection ended. The probe is meant to be run before trusting any streaming endpoint.