Agents gain their power by moving beyond simple text generation. They interact with the world through Tools. Think of tools as specialized functions or services that an agent can choose to use to accomplish parts of its task. If the LLM is the agent's brain, deciding what to do, tools are its hands, allowing it to perform actions. LangChain provides a collection of pre-built tools and makes it straightforward to define your own.

Giving an agent access to tools dramatically expands its capabilities. An LLM alone cannot browse the current internet, perform precise calculations, or look up information in your private database. Tools bridge this gap.

Common Tool Categories

LangChain offers a variety of tools, often falling into several useful categories:

Search Tools: These allow agents to query search engines like Google, DuckDuckGo, Bing, or specialized search APIs (e.g., Wikipedia, ArXiv). This is indispensable when the agent needs up-to-date information or facts not present in its training data. For example, an agent asked about today's weather would use a search tool. Setting up these tools usually requires obtaining API keys from the respective search providers and configuring them securely (as discussed in Chapter 2).
Calculation Tools: While LLMs understand mathematical ideas, they often struggle with precise arithmetic or complex calculations. Calculation tools address this.
- LLMMathChain: This tool uses an LLM to figure out what calculation needs to be done and then uses a separate, reliable calculator (like Python's numexpr library) to get the exact answer.
- Python REPL Tool: This provides the agent with access to a Python Read-Eval-Print Loop. The agent can write and execute Python code to perform calculations, manipulate data, or do anything else Python allows. Use this tool with extreme caution, as it allows arbitrary code execution.
Database Tools: Agents can interact with structured data stored in SQL databases. The SQLDatabaseToolkit (or specific tools like QuerySQLDataBaseTool) allows an agent to inspect database schemas, construct SQL queries based on a natural language request, execute those queries, and interpret the results. This enables applications like asking questions about business metrics stored in a company database.
API Interaction Tools: Many modern applications rely on APIs. Agents can be equipped with tools to interact with specific APIs. This could involve:
- Generic HTTP request tools (like wrappers around the requests library) where the agent formulates the API call.
- Custom tools built specifically to interact with a particular API endpoint (e.g., a tool to look up a customer's order history via an internal company API).
File System Tools: Agents can be given tools to read, write, and list files on the local filesystem. Like the Python REPL tool, this requires careful consideration of security implications, as it grants potentially broad access.
Web Browsing Tools: Beyond simple search, some tools allow agents to load the content of web pages, potentially extracting specific information or even navigating websites. This uses libraries like requests or BeautifulSoup under the hood.

How Agents Use Tools

When you initialize an agent, you provide it with a list of available Tool objects. Each Tool typically has a name (e.g., "search", "calculator") and a description. The agent's LLM uses these descriptions to decide which tool, if any, is appropriate for fulfilling the user's request or advancing towards its goal. The agent's internal logic determines the sequence: receive input, decide on an action (use a tool or respond directly), execute the action, observe the result, and repeat until the task is complete.

An agent receiving user input, reasoning about the necessary action, selecting and executing an appropriate tool (like Search), observing the result, and formulating a final response.

Custom Tools

You are not limited to the tools provided by LangChain. A significant strength of the framework is the ability to easily create custom tools from your own Python functions. By adding a specific docstring describing what the function does, you can wrap it in a Tool object, give it a name and description, and make it available to your agent. This allows you to integrate proprietary logic, internal services, or any specialized functionality your application requires.

Understanding the available tools and how agents select and use them is fundamental to building sophisticated agentic workflows that can effectively solve complex, multi-step problems. The next sections will guide you through implementing agents and utilizing these tools in practice.