Building an AI Stock Agent

In this guide, you're going to create a simple agent that fetches the last day's closing stock price for a given symbol. You'll learn how to create a tool, add it to an agent, and use the agent to fetch stock prices.

Prerequisites

Node.js v20.0 or later installed
An API key from a supported Model Provider
An existing Mastra project (Follow the installation guide to set up a new project)

Creating the Agent

To create an agent in Mastra use the Agent class to define it and then register it with Mastra.

Create a new file src/mastra/agents/stockAgent.ts and define your agent:

src/mastra/agents/stockAgent.ts
import { openai } from "@ai-sdk/openai";
import { Agent } from "@mastra/core/agent";

export const stockAgent = new Agent({
  name: "Stock Agent",
  instructions:
    "You are a helpful assistant that provides current stock prices. When asked about a stock, use the stock price tool to fetch the stock price.",
  model: openai("gpt-4o-mini"),
});

In your src/mastra/index.ts file, register the agent:

src/mastra/index.ts
import { Mastra } from "@mastra/core";
import { stockAgent } from "./agents/stockAgent";

export const mastra = new Mastra({
  agents: { stockAgent },
});

Creating the Stock Price Tool

So far the Stock Agent doesn't know anything about the current stock prices. To change this, create a tool and add it to the agent.

Create a new file src/mastra/tools/stockPrices.ts. Inside, add a stockPrices tool that will fetch the last day's closing stock price for a given symbol:

src/mastra/tools/stockPrices.ts
import { createTool } from "@mastra/core/tools";
import { z } from "zod";

const getStockPrice = async (symbol: string) => {
  const data = await fetch(
    `https://mastra-stock-data.vercel.app/api/stock-data?symbol=${symbol}`,
  ).then((r) => r.json());
  return data.prices["4. close"];
};

export const stockPrices = createTool({
  id: "Get Stock Price",
  inputSchema: z.object({
    symbol: z.string(),
  }),
  description: `Fetches the last day's closing stock price for a given symbol`,
  execute: async ({ context: { symbol } }) => {
    console.log("Using tool to fetch stock price for", symbol);
    return {
      symbol,
      currentPrice: await getStockPrice(symbol),
    };
  },
});

Inside src/mastra/agents/stockAgent.ts import your newly created stockPrices tool and add it to the agent.

src/mastra/agents/stockAgent.ts
import { openai } from "@ai-sdk/openai";
import { Agent } from "@mastra/core/agent";
import { stockPrices } from "../tools/stockPrices";

export const stockAgent = new Agent({
  name: "Stock Agent",
  instructions:
    "You are a helpful assistant that provides current stock prices. When asked about a stock, use the stock price tool to fetch the stock price.",
  model: openai("gpt-4o-mini"),
  tools: {
    stockPrices,
  },
});

Running the Agent Server

Learn how to interact with your agent through Mastra's API.

You can run your agent as a service using the mastra dev command:
```
mastra dev
```
This will start a server exposing endpoints to interact with your registered agents. Within the playground you can test your stockAgent and stockPrices tool through a UI.
By default, mastra dev runs on http://localhost:4111. Your Stock agent will be available at:
```
POST http://localhost:4111/api/agents/stockAgent/generate
```

You can interact with the agent using curl from the command line:

curl -X POST http://localhost:4111/api/agents/stockAgent/generate \
  -H "Content-Type: application/json" \
  -d '{
    "messages": [
      { "role": "user", "content": "What is the current stock price of Apple (AAPL)?" }
    ]
  }'

Expected Response:

You should receive a JSON response similar to:

{
  "text": "The current price of Apple (AAPL) is $174.55.",
  "agent": "Stock Agent"
}

This indicates that your agent successfully processed the request, used the stockPrices tool to fetch the stock price, and returned the result.

Prerequisites​

Creating the Agent​

Creating the Stock Price Tool​

Running the Agent Server​

Prerequisites

Creating the Agent

Creating the Stock Price Tool

Running the Agent Server