Code for article "How LLMs Actually Call Functions (They Don't)"

llm-tools.js

import OpenAI from 'openai'

process.loadEnvFile('.env')

const client = new OpenAI({
  baseURL: 'https://api.openai.com/v1',
  apiKey: process.env.OPENAI_API_KEY,
})

const send_email = ({ email, message }) => {
  // code to send email
  // ...
  return {
    status: 'success',
    content: `Email sent to ${email}`,
  }
}

const toolSchemas = [
  {
    type: 'function',
    function: {
      name: 'send_email',
      description: 'Send an email to a given email address.',
      parameters: {
        type: 'object',
        properties: {
          email: { type: 'string' },
          message: { type: 'string' },
        },
        required: ['email', 'message'],
      },
    },
  },
]

const messages = [
  {
    role: 'user',
    content: 'Send an email to test@example.com and say Hi there!',
  },
]

const response = await client.chat.completions.create({
  model: 'gpt-5.4-mini',
  messages,
  tools: toolSchemas,
})

const output = response.choices[0].message

messages.push(output)

if (output.tool_calls?.length) {
  for (const call of output.tool_calls) {
    // LLM is asking you to call a tool for it
    const functionName = call.function.name
    const args = JSON.parse(call.function.arguments)

    if (functionName === 'send_email') {
      const result = send_email({
        email: args.email,
        message: args.message,
      })
      // Tell the LLM about the result
      messages.push({
        role: 'tool',
        tool_call_id: call.id,
        content: JSON.stringify(result),
      })
      const responseAfterToolCall = await client.chat.completions.create({
        model: 'gpt-5.4-mini',
        messages,
        tools: toolSchemas,
      })
      console.log(responseAfterToolCall.choices[0].message.content)
    } else {
      console.log(`Tool with name ${functionName} does not exist!`)
    }
  }
} else {
  // The LLM does not need tool call. It just responded.
  console.log(output.content)
}