Using OpenAI's tiktoken for Claude Token Counts Can Skew Cost Estimates by 20%
Developers using OpenAI's tiktoken tokenizer to estimate token counts for Anthropic's Claude models risk cost and context budget errors of 15–20%, and more on code or non-English text. This happens because Claude uses its own tokenizer, which splits text differently than tiktoken, causing systematic undercounting. Anthropic provides a dedicated countTokens API endpoint in its SDK that returns accurate, model-specific token counts before inference is run. Token counts also vary across Claude model versions, meaning cached counts from older models should not be reused when switching versions. The recommended fix is to always call countTokens against the specific Claude model being used, and never apply a blanket multiplier to convert counts between models.
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