What's the difference between Langchain Agents and OpenAI Functions?

self.functions = [
{
    "type": "function",
    "function": {
        "name": "call_rest_api",
        "description": "Sends a request to the REST API",
        "parameters": {
            "type": "object",
            "properties": {
                "method": {
                    "type": "string",
                    "description": "The HTTP method to be used",
                    "enum": ["GET", "POST", "PUT", "DELETE"],
                },
                "url": {
                    "type": "string",
                    "description": "The URL of the endpoint. Value placeholders must be replaced with actual values.",
                },
                "body": {
                    "type": "string",
                    "description": "A string representation of the JSON that should be sent as the request body.",
                },

            },
            "required": ["method", "url"],
        },
    }
}
]

def call_rest_api(self, arguments):
    # reques.in is a hosted, fake REST API that we can use for testing
    url = 'https://reqres.in' + arguments['url']
    body = arguments.get('body', {})
    response = None
    if arguments['method'] == 'GET':
        response = requests.get(url)
    elif arguments['method'] == 'POST':
        response = requests.post(url, json=body)
    elif arguments['method'] == 'PUT':
        response = requests.put(url, json=body)
    elif arguments['method'] == 'DELETE':
        response = requests.delete(url)
    else:
        raise ValueError(arguments)

    if response.status_code == 200:
        return json.dumps(response.json())
    else:
        return f"Status code: {response.status_code}"

self.available_apis = [
        {'method': 'GET', 'url': '/api/users?page=[page_id]', 'description': 'Lists employees. The response is paginated. You may need to request more than one to get them all. For example,/api/users?page=2.'},
        {'method': 'GET', 'url': '/api/users/[user_id]', 'description': 'Returns information about the employee identified by the given id. For example,/api/users/2'},
        {'method': 'POST', 'url': '/api/users', 'description': 'Creates a new employee profile. This function accepts JSON body containing two fields: name and job'},
        {'method': 'PUT', 'url': '/api/users/[user_id]', 'description': 'Updates employee information. This function accepts JSON body containing two fields: name and job. The user_id in the URL must be a valid identifier of an existing employee.'},
        {'method': 'DELETE', 'url': '/api/users/[user_id]', 'description': 'Removes the employee identified by the given id. Before you call this function, find the employee information and make sure the id is correct. Do NOT call this function if you didn\'t retrieve user info. Iterate over all pages until you find it or make sure it doesn\'t exist'}
    ]

self.messages = [
    {"role": "user", "content": "You are an HR helper who makes API calls on behalf of an HR representative"},
    {"role": "user", "content": "You have access to the following APIs: " + json.dumps(self.available_apis)},
    {"role": "user", "content": "If a function requires an identifier, list all employees first to find the proper value. You may need to list more than one page"},
    {"role": "user", "content": "If you were asked to create, update, or delete a user, perform the action and reply with a confirmation telling what you have done."}
]

def call_ai(self, new_message):
    if new_message:
        self.messages.append({"role": "user", "content": new_message})

    response = self.client.chat.completions.create(
        model="gpt-3.5-turbo-1106",
        messages=self.messages,
        tools=self.functions,
        tool_choice="auto",
    )

    msg = response.choices[0].message
    self.messages.append(msg)
    if msg.content:
        logging.debug(msg.content)
    if msg.tool_calls:
        for tool_call in tool_calls:
            # we may get a request to call more than one function(!)
            function_name = tool_call.function.name
            function_args = json.loads(tool_call.function.arguments)
            if function_name == 'call_rest_api':
                # ['function_call']['arguments'] contains the arguments of the function
                logging.debug(function_args)
                # Here, we call the requested function and get a response as a string
                function_response = self.call_rest_api(function_args)
                logging.debug(function_response)
                # We add the response to messages
                self.messages.append({
                    "tool_call_id": tool_call.id,
                    "role": "tool",
                    "name": function_name,
                    "content": function_response
                })

        self.call_ai(new_message=None) # pass the function response back to AI
    else:
      # return the final response
      return msg

import json
import requests
from langchain import OpenAI
from langchain.agents import initialize_agent, Tool
from langchain.agents import AgentType
from langchain.chat_models import ChatOpenAI


def call_rest_api(method_and_url):
    method, url = method_and_url.split(' ')
    url = 'https://reqres.in' + url
    response = None
    if method == 'GET':
        response = requests.get(url)
    elif method == 'DELETE':
        response = requests.delete(url)
    else:
        raise ValueError(method)

    if response.status_code == 200:
        return response.json()
    else:
        return f"Status code: {response.status_code}"

available_apis = [
        {'api ': 'GET /api/users?page=[page_id]', 'description': 'Lists employees. The response is paginated. You may need to request more than one to get them all. For example,/api/users?page=2.'},
        {'api': 'DELETE /api/users/[numeric user_id]', 'description': 'Removes the employee identified by the given id. Before you call this function, find the employee information and make sure the id is correct. Do NOT call this function if you didn\'t retrieve user info. Iterate over all pages until you find it or make sure it doesn\'t exist'}
    ]

api_description = json.dumps(available_apis)

function_description = f"""Sends a request to the REST API.

Accepts a single parameter containing two parts:
* method - The HTTP method to be used (GET, POST, PUT, or DELETE)
* url - The URL of the endpoint. Value placeholders must be replaced with actual values.

For example: GET /api/users?page=1 or DELETE /api/users/13

To find users by name, use the GET method first.

Available endpoints:
{api_description}"""

llm = ChatOpenAI(temperature=0, model="gpt-4", openai_api_key='sk-...')
tools = [
    Tool(
        name = "REST",
        func=call_rest_api,
        description=function_description
    )
]

agent = initialize_agent(tools, llm, agent=AgentType.OPENAI_FUNCTIONS, verbose=True)
agent.run("Fire Lawson")