Module usage 

Real provider-reported token usage extraction.

The proxy already rewrites requests; this module reads the response so the dashboard/terminal can show measured cost (the user’s real provider bill) instead of an estimate. All three providers report the exact model and the billed token breakdown — including prompt-cache reads/writes — in the final event of a stream (or the body of a non-streaming response):

• Anthropic: message_start carries model + input/cache tokens, the final message_delta carries output_tokens. Non-streaming: one usage object.
• OpenAI Responses: the response.completed event nests response.usage.
• OpenAI Chat Completions: the final chunk carries usage (needs stream_options.include_usage, which the proxy injects).
• Gemini: every chunk carries usageMetadata; the last one has the totals.

RealUsage normalizes every provider onto the four billable buckets that crate::core::gain::model_pricing::ModelCost::estimate_usd prices: uncached input, output (incl. reasoning/thoughts), cache-read, cache-write.

Structs

RealUsage

One LLM turn’s real, provider-reported usage, normalized to billable buckets.

Scanner

Incrementally extracts RealUsage from a response stream (or a full body).

Enums

Provider

LLM provider whose response shape a Scanner understands.

Functions

gemini_model_from_path

Extracts the model from a Gemini request path (/v1beta/models/{model}:generateContent). Returns None for other paths.

tee_stream

Wraps a response byte stream so every chunk is forwarded byte-for-byte while a Scanner observes it; on stream end the merged usage is recorded. Memory overhead is one buffered SSE line, never the whole response.