Function Calling in Fastify with OpenAI Functions

Introduction

OpenAI function calling allows AI to invoke predefined functions. Integrate with Fastify for AI-driven operations.

Prerequisites

• Fastify v4+
• OpenAI API key

Step 1: Install Dependencies

npm install fastify openai

Step 2: Define Functions

In openai/functions.ts:

export const functions = [
  {
    name: 'get_current_weather',
    description: 'Get current weather for a location',
    parameters: {
      type: 'object',
      properties: {
        location: { type: 'string', description: 'City name' },
        unit: { type: 'string', enum: ['celsius','fahrenheit'] },
      },
      required: ['location'],
    },
  },
];

export async function get_current_weather({ location, unit }: any) {
  // Call real weather API here
  return { location, temperature: 22, unit };
}

Step 3: Fastify Route

In server.js:

import Fastify from 'fastify';
import OpenAI from 'openai';
import { functions, get_current_weather } from './openai/functions';

const fastify = Fastify({ logger: true });
const openai = new OpenAI({ apiKey: process.env.OPENAI_API_KEY });

fastify.post('/chat', async (req, reply) => {
  const { messages } = req.body;

  const response = await openai.chat.completions.create({
    model: 'gpt-4-0613',
    messages,
    functions,
    function_call: 'auto',
  });

  const message = response.choices[0].message;

  if (message.function_call) {
    const { name, arguments: args } = message.function_call;
    const func = { get_current_weather }[name];
    const result = await func(JSON.parse(args));

    return reply.send({ 
      role: 'function',
      name,
      content: JSON.stringify(result)
    });
  }

  reply.send(message);
});

fastify.listen({ port: 3000 });

Summary

Fastify + OpenAI function calling allows AI to perform actions by invoking defined functions, enabling dynamic, AI-driven endpoints.