The Bill and the Counter Disagreed

The chat said a managed model had used a handful of ordinary input and output tokens. The account balance said something else. Both numbers came from the same request, yet the line beside the answer could not explain the charge. Cached input made the mismatch worse: a warm request could be much cheaper without the client being able to say which input was new, read from cache, or written into cache for the next turn.

The first instinct was to improve the client's pricing table. That would have made the arithmetic look more complete while leaving the authority in the wrong place. OpenAI, Google, and Anthropic report different usage shapes; reasoning tokens, cache writes, long-context tiers, and provider pricing can all change the amount settled by the managed backend. Reconstructing that charge again on the user's machine creates two bills for one call.

We moved the boundary instead. The backend now normalizes provider usage before settlement and records the exact token classes and pricing snapshot used for the debit. Core carries that contract through its trace, and the React client preserves it without translating it back into a provider-specific approximation. Older Hosts and direct-provider sessions still keep their existing, smaller usage shape.

The useful UI is deliberately modest. A completed turn can say how much input was new, how much came from cache, whether the provider charged for a cache write, how much output was billed, and the final server cost. It does not expose the command or wire details that produced those numbers. A historical event that only knows 42 tokens still says exactly that; it does not invent a zero-valued breakdown to fill the new layout.

The measurement that mattered was not whether one total could be made to add up locally. It was whether the same normalized fields survived the backend, Core, React, and browser without loss. The focused Core suite passed 93 checks, the React suite passed 141, and desktop and phone browser flows showed the same new-versus-cached accounting without horizontal overflow. One charge now has one source of truth, and the client can finally explain it.