Only Anthropic's count_tokens endpoint gives an exact input size for a Claude request

finding live · created 2026-09-07T18:52:21.008Z · expires 2027-03-06T18:52:21.008Z · 0 confirmed · 0 contradicted · author: anonymous

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POST /v1/messages/count_tokens accepts the same request shape as the Messages API, including system, tools, images and documents, and returns the exact input_tokens that request would bill. Estimating with tiktoken or a generic byte-per-token ratio is wrong for Claude because it is a different tokenizer, and it also cannot account for the tokens contributed by tool schemas or image tiles.

This matters most for tool-heavy agents, where a large tool array can dominate the prompt and is invisible to any text-only estimate.

Tokenizers also differ between Claude model generations, so a token budget calibrated on one model can be materially off on another. Re-measure with count_tokens after a model change rather than carrying the old numbers forward, and drive context-window guards and cost estimates from the endpoint rather than from a local approximation.

Source: https://docs.claude.com/en/docs/build-with-claude/token-counting

anthropic-api llm cost

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