Calculate OpenAI usage for Chat Completion API stream in NodeJS

An VoAn Vo
6 min read

Table of contents

Introduction

OpenAI does have the usage field in its non-stream response.

For example with a non-stream request like this (stream: false):

{
    "model": "gpt-3.5-turbo",
    "messages": [
        {
            "role": "user",
            "content": "say hello world"
        }
    ],
    "temperature": 0.7,
    "stream": false
}

We will get the response mentioned the token usage:

{
    "id": "chatcmpl-84hml7McTvfEihzl7S0Osb1BbfshR",
    "object": "chat.completion",
    "created": 1696132315,
    "model": "gpt-3.5-turbo-0613",
    "choices": [
        {
            "index": 0,
            "message": {
                "role": "assistant",
                "content": "Hello, world!"
            },
            "finish_reason": "stop"
        }
    ],
    "usage": {
        "prompt_tokens": 10,
        "completion_tokens": 4,
        "total_tokens": 14
    }
}

However, when we request for stream response (stream: true), there will be a few chunks in response with no usage field

{
    "id": "chatcmpl-84hoe6uo51KhIEjgkNHkhk58xEUcB",
    "object": "chat.completion.chunk",
    "created": 1696132432,
    "model": "gpt-3.5-turbo-0613",
    "choices": [
        {
            "index": 0,
            "delta": {
                "content": " world"
            },
            "finish_reason": null
        }
    ]
}

In this case, to calculate the token usage, we will need to calculate it in the request prompt and the OpenAI response. We will do it using the tiktoken library

Setup the necessary components

We will set up a basic NodeJS application that will make a stream request to OpenAI and use tiktoken to calculate the token usage

Here is the basic setup:

mkdir openai-usage
cd openai-usage
touch index.js
npm i https

Write this code to the index.js file:

const https = require("https");

const options = {
  hostname: "api.openai.com",
  path: "/v1/chat/completions",
  port: 443,
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
  },
};

const payload = {
  model: "gpt-3.5-turbo",
  messages: [
    {
      role: "user",
      content: "say hello world",
    },
  ],
  temperature: 0.7,
  stream: true,
};

const request = https.request(options, (res) => {
  res.on("data", (chunk) => {
    console.log(chunk.toString());
  });
  res.on("end", () => {
    console.log("done");
  });
});
request.write(JSON.stringify(payload));
request.end();

In this setup, we use the https package to make request to OpenAI API. We put some domain, header information in the options, and put payload information in the payload . Be noted that we set the stream: true in the payload. And we print the chunk of data getting from OpenAI stream response.

Run the command in the project root folder (replace the sk-your-open-ai-key with your OpenAI key):

OPENAI_API_KEY=sk-your-open-ai-key node index.js

We will get the response stream printed in the console as below:

# console.log(chunk.toString());
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null}]}
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]}
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{"content":","},"finish_reason":null}]}
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{"content":" world"},"finish_reason":null}]}
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{"content":"!"},"finish_reason":null}]}
data: {"id":"chatcmpl-84iSdjVbZR88SOfbMRCnuVF4wEuOF","object":"chat.completion.chunk","created":1696134911,"model":"gpt-3.5-turbo-0613","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}
data: [DONE]

# console.log("done");
done

Now we install js-tikoken library and set up the encoder which will help to calculate the token usage from the text string.

In the project root folder run

npm i js-tiktoken

In the index.js file add this to the top:

const tiktoken = require("js-tiktoken");
const encoding = tiktoken.getEncoding("cl100k_base");
const tokenUsage = {
  prompt_tokens: 0,
  completion_tokens: 0,
  total_tokens: 0,
}

We use the cl100k_base model to support GPT-3.5 turbo and GPT-4

We also create an empty tokenUsage object follow the format of OpenAI usage field. The prompt_tokens will calculate the usage in our prompt payload, the completion_tokens will calculate the OpenAI response

Calculate the token usage in the request prompt

Create a helper function to calculate token usage from prompt message

// https://platform.openai.com/docs/guides/gpt/managing-tokens
const numTokensFromPrompt = (messages) => {
  let numTokens = 0;

  for (const message of messages) {
    numTokens += 4; // every message follows <im_start>{role/name}\n{content}<im_end>\n

    for (const [key, value] of Object.entries(message)) {
      numTokens += encoding.encode(value).length;

      if (key === "name") numTokens -= 1; // role is always required and always 1 token
    }
  }

  numTokens += 2; // every reply is primed with <im_start>assistant

  return numTokens;
};

Add this code below the payload

tokenUsage.prompt_tokens = numTokensFromPrompt(payload.messages);

Calculate the token usage in the OpenAI response

Modify the request to calculate the token usage in the response data stream

const request = https.request(options, (res) => {
  let content = "";
  res.on("data", (chunk) => {
    const jsonString = chunk.toString();
    const prefixToRemove = "data: ";
    const trimmedString = jsonString.slice(prefixToRemove.length);
    try {
      const jsonObject = JSON.parse(trimmedString);
      const deltaContent = jsonObject.choices[0]?.delta.content || "";
      content += deltaContent;
    } catch (error) {
      console.error("Error counting tokens from OpenAI response:", error);
    }
  });
  res.on("end", () => {
    const tokens = encoding.encode(content);
    tokenUsage.completion_tokens = tokens.length;
    if (tokens.length > 0) tokenUsage.completion_tokens += 1; // +1 missing token from first chunk: https://community.openai.com/t/stream-response-from-v1-chat-completions-endpoint-is-missing-the-first-token/187835/7?u=zenbuidler
    tokenUsage.total_tokens = tokenUsage.prompt_tokens + tokenUsage.completion_tokens;
    console.log("tokenUsage", tokenUsage);
  });
});

In this code, we listen to the data event from the response stream, remove the data: prefix in each chunk, parse to JSON, get the message and append it to the content variable for later using.

In the end event of the response stream, we get full message in the content variable. We then use tiktoken to calculate the usage.

Note: We use 1 hack here: We + 1 token because the first 2 chunks of data are sent together to we miss 1 token when parsing it. Later we can calculate data chunk by byte for more accuracy.

Full code in the index.js file:

const https = require("https");
const tiktoken = require("js-tiktoken");
const encoding = tiktoken.getEncoding("cl100k_base");
const tokenUsage = {
  prompt_tokens: 0,
  completion_tokens: 0,
  total_tokens: 0,
};

// https://platform.openai.com/docs/guides/gpt/managing-tokens
const numTokensFromPrompt = (messages) => {
  let numTokens = 0;

  for (const message of messages) {
    numTokens += 4; // every message follows <im_start>{role/name}\n{content}<im_end>\n

    for (const [key, value] of Object.entries(message)) {
      numTokens += encoding.encode(value).length;

      if (key === "name") numTokens -= 1; // role is always required and always 1 token
    }
  }

  numTokens += 2; // every reply is primed with <im_start>assistant

  return numTokens;
};

const options = {
  hostname: "api.openai.com",
  path: "/v1/chat/completions",
  port: 443,
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
  },
};

const payload = {
  model: "gpt-3.5-turbo",
  messages: [
    {
      role: "user",
      content: "say hello world",
    },
  ],
  temperature: 0.7,
  stream: true,
};

tokenUsage.prompt_tokens = numTokensFromPrompt(payload.messages);

const request = https.request(options, (res) => {
  let content = "";
  res.on("data", (chunk) => {
    const jsonString = chunk.toString();
    const prefixToRemove = "data: ";
    const trimmedString = jsonString.slice(prefixToRemove.length);
    try {
      const jsonObject = JSON.parse(trimmedString);
      const deltaContent = jsonObject.choices[0]?.delta.content || "";
      content += deltaContent;
    } catch (error) {
      console.error("Error counting tokens from OpenAI response:", error);
    }
  });
  res.on("end", () => {
    const tokens = encoding.encode(content);
    tokenUsage.completion_tokens = tokens.length;
    if (tokens.length > 0) tokenUsage.completion_tokens += 1; // +1 missing token from first chunk: https://community.openai.com/t/stream-response-from-v1-chat-completions-endpoint-is-missing-the-first-token/187835/7?u=zenbuidler
    tokenUsage.total_tokens = tokenUsage.prompt_tokens + tokenUsage.completion_tokens;
    console.log("tokenUsage", tokenUsage);
  });
});
request.write(JSON.stringify(payload));
request.end();

Now run the program again

OPENAI_API_KEY=sk-your-open-ai-key node index.js

We will see the result

tokenUsage { prompt_tokens: 10, completion_tokens: 4, total_tokens: 1

Recap

We already finish the code to calculate the OpenAI stream response usage using js-tiktoken library.

This code is useful when we want to actively calculate OpenAI token usage by ourselves.

Source code can be found here: https://github.com/votanlean/openai-usage

Thank you for reading ^^

See you in the next post!

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