While chains allow us to execute predefined sequences of LLM calls and other actions, many real-world tasks require more dynamic behavior. Imagine needing to answer a question like, "What was the weather like in London yesterday, and what is the square root of the number of rainy days reported there last month?" A simple chain might struggle because the exact steps depend on intermediate results (finding the weather report, extracting the rainy days, then calculating). This is where agents come into play.

What Exactly is an Agent?

In the context of LLM frameworks like LangChain, an agent uses a Large Language Model not just to generate text, but as a reasoning engine to determine a sequence of actions to take. Think of it less like following a recipe (a chain) and more like a helper who can figure out how to accomplish a goal using a set of available tools.

The core idea is that the agent:

Receives an input or objective.
Uses the LLM to "think" about what step to take next.
This step might involve using a specific tool (like a web search, a calculator, or a custom function accessing your database) or deciding that it has enough information to formulate the final response.
If a tool is used, the agent executes it and observes the result.
The result (or "observation") is fed back into the LLM for the next round of "thinking".
This cycle repeats until the objective is met.

The Agent Loop

This decision-making process operates within a loop often called the "Agent Executor" or "Runtime". At each step, the LLM is prompted not just with the original goal, but also with the history of actions taken and observations received so far, along with a description of the tools it could use.

Here's a simplified view of the agent execution flow:

A conceptual diagram illustrating the agent execution cycle, where the LLM decides between using a tool or generating a final response based on the input and previous steps.

The prompt used to guide the LLM's reasoning is absolutely essential. It typically instructs the LLM on how to think step-by-step, lists the available tools with descriptions of what they do and what inputs they expect, and specifies the format the LLM should use to indicate its chosen action and the input for that action. Techniques like ReAct (Reason+Act) often underpin these prompts, encouraging the LLM to explicitly state its reasoning before choosing an action.

Why Use Agents?

Agents offer significant advantages over simpler chains for certain types of problems:

Flexibility: They can adapt their approach based on intermediate results, choosing different tools or sequences of actions as needed.
Complex Problem Solving: They can break down complex goals into smaller, manageable steps, using tools to gather information or perform calculations along the way.
Interactivity: They enable LLMs to interact with external systems (databases, APIs, file systems) in a structured way, going beyond their pre-trained knowledge.

However, this dynamic nature also means agents can be less predictable than chains. Their behavior heavily depends on the quality of the LLM, the clarity of the prompt, and the reliability of the tools. Debugging can sometimes involve tracing the agent's thought process through multiple steps. There's also the potential for the agent to get stuck in loops or fail to complete the task if the reasoning isn't guided properly or if the tools produce unexpected errors. Additionally, since agents often make multiple calls to the LLM within their execution loop, they can incur higher operational costs compared to a single chain execution.

In the next section, we will look closely at how to define and integrate these "tools" that give agents their power to act.