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, after all: input tokens, output tokens, maybe a cached count. How different could it be.
Pretty different, it turns out. Here is the whole thing in one table, then each gotcha in detail with the real parser code from the repo.
Provider
Where usage lives (streaming)
