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What is a Planner?

  • Article

Once you have multiple plugins, you then need a way for your AI agent to use them together to solve a user’s need. This is where planning comes in.

Early on, Semantic Kernel introduced the concept of planners that used prompts to request the AI to choose which functions to invoke. Since Semantic Kernel was introduced, however, OpenAI introduced a native way for the model to invoke or “call” a function: function calling. Other AI models like Gemini, Claude, and Mistral have since adopted function calling as a core capability, making it a cross-model supported feature.

Because of these advancements, Semantic Kernel has evolved to use function calling as the primary way to plan and execute tasks.

Important

Function calling is only available in OpenAI models that are 0613 or newer. If you use an older model (e.g., 0314), this functionality will return an error. We recommend using the latest OpenAI models to take advantage of this feature.

How does function calling create a "plan"?

At its simplest, function calling is merely a way for an AI to invoke a function with the right parameters. Take for example a user wants to turn on a light bulb. Assuming the AI has the right plugin, it can call the function to turn on the light.

Role Message
🔵 User Please turn on light #1
🔴 Assistant (function call) Lights.change_state(1, { "isOn": true })
🟢 Tool { "id": 1, "name": "Table Lamp", "isOn": true, "brightness": 100, "hex": "FF0000" }
🔴 Assistant The lamp is now on

But what if the user doesn't know the ID of the light? Or what if the user wants to turn on all the lights? This is where planning comes in. Today's LLM models are capable of iteratively calling functions to solve a user's need. This is accomplished by creating a feedback loop where the AI can call a function, check the result, and then decide what to do next.

For example, a user may ask the AI to "toggle" a light bulb. The AI would first need to check the state of the light bulb before deciding whether to turn it on or off.

Role Message
🔵 User Please toggle all the lights
🔴 Assistant (function call) Lights.get_lights()
🟢 Tool { "lights": [ { "id": 1, "name": "Table Lamp", "isOn": true, "brightness": 100, "hex": "FF0000" }, { "id": 2, "name": "Ceiling Light", "isOn": false, "brightness": 0, "hex": "FFFFFF" } ] }
🔴 Assistant (function call) Lights.change_state(1, { "isOn": false }) Lights.change_state(2, { "isOn": true })
🟢 Tool { "id": 1, "name": "Table Lamp", "isOn": false, "brightness": 0, "hex": "FFFFFF" }
🟢 Tool { "id": 2, "name": "Ceiling Light", "isOn": true, "brightness": 100, "hex": "FF0000" }
🔴 Assistant The lights have been toggled

Note

In this example, you also saw parallel function calling. This is where the AI can call multiple functions at the same time. This is a powerful feature that can help the AI solve complex tasks more quickly. It was added to the OpenAI models in 1106.

The automatic planning loop

Supporting function calling without Semantic Kernel is relatively complex. You would need to write a loop that would accomplish the following:

  1. Create JSON schemas for each of your functions
  2. Provide the LLM with the previous chat history and function schemas
  3. Parse the LLM's response to determine if it wants to reply with a message or call a function
  4. If the LLM wants to call a function, you would need to parse the function name and parameters from the LLM's response
  5. Invoke the function with the right parameters
  6. Return the results of the function so that the LLM can determine what it should do next
  7. Repeat steps 2-6 until the LLM decides it has completed the task or needs help from the user

In Semantic Kernel, we make it easy to use function calling by automating this loop for you. This allows you to focus on building the plugins needed to solve your user's needs.

Note

Understanding how the function calling loop works is essential for building performant and reliable AI agents. For an in-depth look at how the loop works, see the function calling article.

Using automatic function calling

To use automatic function calling in Semantic Kernel, you need to do the following:

  1. Register the plugin with the kernel
  2. Create an execution settings object that tells the AI to automatically call functions
  3. Invoke the chat completion service with the chat history and the kernel
using System.ComponentModel;
using Microsoft.SemanticKernel;
using Microsoft.SemanticKernel.ChatCompletion;
using Microsoft.SemanticKernel.Connectors.OpenAI;

// 1. Create the kernel with the Lights plugin
var builder = Kernel.CreateBuilder().AddAzureOpenAIChatCompletion(modelId, endpoint, apiKey);
builder.Plugins.AddFromType<LightsPlugin>("Lights");
Kernel kernel = builder.Build();

var chatCompletionService = kernel.GetRequiredService<IChatCompletionService>();

// 2. Enable automatic function calling
OpenAIPromptExecutionSettings openAIPromptExecutionSettings = new() 
{
    ToolCallBehavior = ToolCallBehavior.AutoInvokeKernelFunctions
};

var history = new ChatHistory();

string? userInput;
do {
    // Collect user input
    Console.Write("User > ");
    userInput = Console.ReadLine();

    // Add user input
    history.AddUserMessage(userInput);

    // 3. Get the response from the AI with automatic function calling
    var result = await chatCompletionService.GetChatMessageContentAsync(
        history,
        executionSettings: openAIPromptExecutionSettings,
        kernel: kernel);

    // Print the results
    Console.WriteLine("Assistant > " + result);

    // Add the message from the agent to the chat history
    history.AddMessage(result.Role, result.Content ?? string.Empty);
} while (userInput is not null)

import asyncio

from semantic_kernel import Kernel
from semantic_kernel.functions import kernel_function
from semantic_kernel.connectors.ai.open_ai import AzureChatCompletion
from semantic_kernel.connectors.ai.function_call_behavior import FunctionCallBehavior
from semantic_kernel.connectors.ai.chat_completion_client_base import ChatCompletionClientBase
from semantic_kernel.contents.chat_history import ChatHistory
from semantic_kernel.functions.kernel_arguments import KernelArguments

from semantic_kernel.connectors.ai.open_ai.prompt_execution_settings.azure_chat_prompt_execution_settings import (
    AzureChatPromptExecutionSettings,
)

async def main():
    # 1. Create the kernel with the Lights plugin
    kernel = Kernel()
    kernel.add_service(AzureChatCompletion(
        deployment_name="your_models_deployment_name",
        api_key="your_api_key",
        base_url="your_base_url",
    ))
    kernel.add_plugin(
        LightsPlugin(),
        plugin_name="Lights",
    )

    chat_completion : AzureChatCompletion = kernel.get_service(type=ChatCompletionClientBase)

    # 2. Enable automatic function calling
    execution_settings = AzureChatPromptExecutionSettings(tool_choice="auto")
    execution_settings.function_call_behavior = FunctionCallBehavior.EnableFunctions(auto_invoke=True, filters={})

    # Create a history of the conversation
    history = ChatHistory()

    userInput = None
    while True:
        # Collect user input
        userInput = input("User > ")

        # Terminate the loop if the user says "exit"
        if userInput == "exit":
            break

        # Add user input to the history
        history.add_user_message(userInput)

        # 3. Get the response from the AI with automatic function calling
        result = (await chat_completion.get_chat_message_contents(
            chat_history=history,
            settings=execution_settings,
            kernel=kernel,
            arguments=KernelArguments(),
        ))[0]

        # Print the results
        print("Assistant > " + str(result))

        # Add the message from the agent to the chat history
        history.add_message(result)

# Run the main function
if __name__ == "__main__":
    asyncio.run(main())

When you use automatic function calling, all of the steps in the automatic planning loop are handled for you and added to the ChatHistory object. After the function calling loop is complete, you can inspect the ChatHistory object to see all of the function calls made and results provided by Semantic Kernel.

What about the Function Calling Stepwise and Handlebars planners?

The Stepwise and Handlebars planners are still available in Semantic Kernel. However, we recommend using function calling for most tasks as it is more powerful and easier to use. Both the Stepwise and Handlebars planners will be deprecated in a future release of Semantic Kernel.

Before we deprecate these planners, we will provide guidance on how to migrate your existing planners to function calling. If you have any questions about this process, please reach out to us on the discussions board in the Semantic Kernel GitHub repository.

Caution

If you are building a new AI agent, we recommend that you not use the Stepwise or Handlebars planners. Instead, use function calling as it is more powerful and easier to use.

Next steps

Now that you understand how planners work in Semantic Kernel, you can learn more about how influence your AI agent so that they best plan and execute tasks on behalf of your users.

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