# What Is an AI Agent, Really?

**May 26, 2026**

An AI agent = LLM + Tools + Loop. If any of the three is missing, it is not an agent.

## The Three Components

**LLM** — The reasoning engine. Given context and available tools, it decides what to do next. The LLM is NOT the agent. It is the brain of the agent.

**Tools** — The capability surface. Functions the agent can call: read files, run commands, search the web, query databases. Each tool has a name, description, and input schema.

**Loop** — The autonomous decision cycle. Think (LLM decides) leads to Act (tool executes) leads to Observe (result comes back) leads to Repeat.

## What Each Combination Gives You

| Combination | Result |
|-------------|--------|
| LLM only | Chatbot |
| LLM + Tools (no loop) | Augmented inference |
| LLM + Loop (no tools) | Talking to itself |
| LLM + Tools + Loop | Agent |

## The Loop in Pseudocode

```python
messages = [system_prompt, user_request]
while True:
    response = llm.invoke(messages, tools=tool_definitions)
    if response.is_final_answer:
        print(response.content)
        break
    tool_name = response.tool_call.name
    tool_args = response.tool_call.args
    result = execute_tool(tool_name, tool_args)
    messages.append(response)
    messages.append(result)
```

## Common Misconception

"The model is the agent." No. The model is the brain. The agent is the whole system: brain plus tools plus loop. A better brain helps, but better tools and a better loop help more.

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*From Module 1 of the [Agentic Engineering Course](/). The full module includes your first lab: building a single-tool agent from scratch.*