In this article, you will learn how to use the tiktoken package to tokenize text for OpenAI LLMs. We will look at:

What is tiktoken?
tiktoken usage examples in Tiktokenizer app.

What is tiktoken?

If you need a programmatic interface for tokenizing text, check out tiktoken package for Python. For JavaScript, the community-supported @dbdq/tiktoken package works with most GPT models.

This @dbdq/tiktoken is officially recommended by the OpenAI and it is community supported.

tiktoken is a BPE tokeniser for use with OpenAI’s models, forked from the original tiktoken library to provide JS/WASM bindings for NodeJS and other JS runtimes.

This repository contains the following packages:

tiktoken (formally hosted at @dqbd/tiktoken): WASM bindings for the original Python library, providing full 1-to-1 feature parity.
js-tiktoken: Pure JavaScript port of the original library with the core functionality, suitable for environments where WASM is not well supported or not desired (such as edge runtimes).

Documentation for js-tiktoken can be found in here. Documentation for the tiktoken can be found here.

Install

The WASM version of tiktoken can be installed from NPM:

npm install tiktoken

Usage

Basic usage follows, which includes all the OpenAI encoders and ranks:

import assert from "node:assert";
import { get_encoding, encoding_for_model } from "tiktoken";

const enc = get_encoding("gpt2");
assert(
  new TextDecoder().decode(enc.decode(enc.encode("hello world"))) ===
    "hello world"
);

// To get the tokeniser corresponding to a specific model in the OpenAI API:
const enc = encoding_for_model("text-davinci-003");

// Extend existing encoding with custom special tokens
const enc = encoding_for_model("gpt2", {
  "<|im_start|>": 100264,
  "<|im_end|>": 100265,
});

// don't forget to free the encoder after it is not used
enc.free();

Now that we understand what tiktoken is, lets look at some usage example in tiktokenizer app.

tiktoken usage examples in Tiktokenizer app.

In tiktokenizer/src/models/tokenizer.ts, at line 2, you will find the following code

import { get_encoding, encoding_for_model, type Tiktoken } from "tiktoken";

get_encoding

You will find the following code in tiktokenizer/src/models/tokenizer.ts at line 44.

   const enc =
        model === "gpt-3.5-turbo" || model === "gpt-4" || model === "gpt-4-32k"
          ? get_encoding("cl100k_base", {
              "<|im_start|>": 100264,
              "<|im_end|>": 100265,
              "<|im_sep|>": 100266,
            })
          : model === "gpt-4o"
          ? get_encoding("o200k_base", {
              "<|im_start|>": 200264,
              "<|im_end|>": 200265,
              "<|im_sep|>": 200266,
            })

encoding_for_model

Below is an example invoking encoding_for_model function.

 encoding_for_model(model);

You will find this above code in tiktokenizer/src/models/tokenizer.ts at line 56.

About me:

Hey, my name is Ramu Narasinga. I study codebase architecture in large open-source projects.

Email: ramu.narasinga@gmail.com

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Use the tiktoken package to tokenize text for OpenAI LLMs.