ModelRefs / Tiktoken — AI Glossary
Tiktoken — AI Glossary
OpenAI's fast BPE tokenizer library powering GPT-2 through GPT-4o; the reference for counting tokens and estimating costs.
Overview
Tiktoken implements cl100k_base (GPT-3.5/4), o200k_base (GPT-4o), and legacy encodings. Its Rust core makes it 3–6× faster than HuggingFace tokenizers for the same vocabulary. Developers use it to accurately count tokens before sending requests to measure context usage and predict cost.
Reference details
| Topic | architecture |
|---|---|
| Last reviewed | 2026-06-24 |
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Frequently asked questions
What is Tiktoken?
OpenAI's fast BPE tokenizer library powering GPT-2 through GPT-4o; the reference for counting tokens and estimating costs.
What concepts are related to Tiktoken?
Closely related concepts include byte pair encoding, vocabulary size, context window.