Tracking token usage across OpenAI, Anthropic, and Gemini: every streaming gotcha I hit

OpenAI, Anthropic, and Gemini each report token usage differently, and it stops being trivia the moment you track LLM cost. I build Spanlens, an open-source LLM observability tool that sits in front of all three as a proxy and records every call with its model, latency, tokens, and cost. To do the cost part I read the token usage back out of every response, including the streaming ones. I assumed the three providers would report usage in roughly the same way. They send the same kind of data, af...