How to integrate Brex MCP with Mastra AI

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Introduction

This guide walks you through connecting Brex to Mastra AI using the Composio tool router. By the end, you'll have a working Brex agent that can create a new card for our intern, get the latest account statement pdf, add an expense for last week's conference through natural language commands.

This guide will help you understand how to give your Mastra AI agent real control over a Brex account through Composio's Brex MCP server.

Before we dive in, let's take a quick look at the key ideas and tools involved.

Also integrate Brex with

TL;DR

Here's what you'll learn:
  • Set up your environment so Mastra, OpenAI, and Composio work together
  • Create a Tool Router session in Composio that exposes Brex tools
  • Connect Mastra's MCP client to the Composio generated MCP URL
  • Fetch Brex tool definitions and attach them as a toolset
  • Build a Mastra agent that can reason, call tools, and return structured results
  • Run an interactive CLI where you can chat with your Brex agent

What is Mastra AI?

Mastra AI is a TypeScript framework for building AI agents with tool support. It provides a clean API for creating agents that can use external services through MCP.

Key features include:

  • MCP Client: Built-in support for Model Context Protocol servers
  • Toolsets: Organize tools into logical groups
  • Step Callbacks: Monitor and debug agent execution
  • OpenAI Integration: Works with OpenAI models via @ai-sdk/openai

What is the Brex MCP server, and what's possible with it?

The Brex MCP server is an implementation of the Model Context Protocol that connects your AI agent and assistants like Claude, Cursor, etc directly to your Brex account. It provides structured and secure access to your company's financial tools, so your agent can perform actions like managing cards, tracking expenses, creating budgets, and retrieving account details on your behalf.

  • Card creation and management: Instantly issue new cards, assign them to users or vendors, and control spending limits via your agent.
  • Automated expense tracking: Let your agent create and log expenses, helping you keep company finances organized and up to date.
  • Budget and department setup: Quickly create budgets for teams or projects and set up new departments without manual effort.
  • Account and statement retrieval: Fetch detailed account information and download statements whenever you need financial insights or records.
  • User and location management: Add new users or locations to your Brex account, making it easy to scale as your business grows.

Supported Tools & Triggers

Tools
Add Webhook Group MembersAdd webhook subscription members to a webhook group.
Archive BudgetArchive a budget to mark it as inactive and prevent future expenses.
Archive Spend LimitArchive a spend limit in Brex.
Create BudgetCreate a new budget for departments or projects.
Create Spend Limit (Budget V1)Tool to create a new Spend Limit (Budget) in Brex.
Create CardCreate a new card and assign it to a user.
Create DepartmentCreate a new department.
Create Document Upload for ReferralCreate a document upload URL for a referral and return a pre-signed S3 upload URL.
Create ExpenseCreate a new expense.
Create Custom FieldCreate a new custom field in Brex.
Create Field ValuesCreate custom field values for a specific Brex field.
Create LocationCreate a new location.
Create Referral RequestTool to create a new referral to Brex for onboarding.
Create Spend LimitTool to create a spend limit in Brex.
Create Job TitleCreate a new job title.
Create UserCreate a new user in the Brex account.
Create VendorCreate a new vendor in Brex for payment operations.
Create Webhook GroupTool to create a webhook group for targeting webhook subscriptions.
Create Webhook SubscriptionRegister a new webhook subscription to receive real-time notifications for Brex events.
Delete FieldTool to delete a custom field by Brex ID.
Delete Field ValuesDelete custom field values for a specific field.
Delete VendorDelete a vendor by ID.
Get Account StatementsGet finalized statements for the primary cash account.
Get Budget DetailsGet detailed information about a specific budget.
Get Card DetailsGet detailed information about a specific card.
Get Card ExpenseTool to retrieve detailed information about a specific card expense by its ID.
Get Card NumberGet card number, CVV, and expiration date for a specific card.
Get Card TransactionsGet settled card transactions for the primary card account.
Get Company Cash AccountsRetrieve all cash (deposit) accounts for the company.
Get Company DetailsGet company information associated with the OAuth2 access token.
Get Department By IDGet detailed information about a specific department by ID.
Get ExpenseTool to get details of a specific expense by ID.
Get Field By IDTool to retrieve a custom field by its Brex ID.
Get Field Value By IDTool to retrieve a specific field value by field ID and field value ID.
Get Legal EntityTool to retrieve a legal entity by its ID from Brex.
Get Location By IDGet detailed information about a specific location by ID.
Get Referral By IDTool to retrieve a specific referral by its ID from Brex.
Get Spend Limit By IDTool to get detailed information about a specific spend limit by its ID.
Get Spend LimitsGet spend limits configuration.
Get Title by IDGet detailed information about a specific job title by ID.
Get Transaction by IDGet details of a specific transaction by ID.
Get Card TransactionsGet card transactions from the primary Brex account.
Get Transactions by Amount RangeGet card transactions filtered by amount range and date period.
Search Transactions by DescriptionSearch and filter card transactions by description text.
Get User LimitRetrieves the monthly spending limit and available balance for a specific Brex user.
Get User ProfileGet user profile information from Brex Team API.
Get Vendor By IDTool to get vendor details by ID.
Get Webhook GroupTool to retrieve details of a specific webhook group by ID.
Get Webhook SubscriptionTool to retrieve details of a specific webhook subscription by ID.
List Budget ProgramsTool to list all budget programs in the organization.
List BudgetsList all budgets and show available amounts across all cards.
List Card AccountsTool to list all card accounts for the company.
List CardsList all cards associated with the account.
List Card StatementsTool to list finalized statements for primary card accounts.
List DepartmentsList all departments in the organization.
List ExpensesTool to list expenses from the Brex platform.
List Field ValuesTool to list values under a custom field.
List Legal EntitiesTool to list all legal entities in the Brex account with pagination support.
List LocationsList all locations in the organization.
List ReferralsTool to list all referrals created in the Brex account.
List Job TitlesList all job titles in the organization.
List TransfersLists transfers for the account.
List TripsLists trips according to the filters passed in the query string.
List UsersList all users in the Brex account.
List VendorsList all vendors.
List Webhook Group MembersTool to list all members of a webhook group.
List Webhook GroupsTool to list all webhook groups.
List Webhook SecretsTool to retrieve webhook signing secrets for validating incoming webhook messages from Brex.
List Webhook SubscriptionsTool to list all registered webhook subscriptions.
Create Receipt MatchCreates a receipt match request and returns a pre-signed S3 upload URI.
Create Receipt Upload for ExpenseCreates a receipt upload request for a specific card expense and returns a pre-signed S3 upload URL.
Set User LimitSet or update the monthly spending limit for a Brex user.
Transfer CardTransfer a card to a different user.
Update BudgetUpdate an existing budget's configuration in Brex.
Update Spend Limit (V1)Tool to update a Spend Limit (budget) using the v1 API.
Update Card LimitsUpdate spending limits and controls for a Brex card.
Update Card StatusUpdate the status of a Brex card by locking, unlocking, or terminating it.
Update ExpenseTool to update an expense by its ID.
Update FieldTool to update a custom field in Brex.
Update Field ValuesTool to update custom field values in Brex for a specific field.
Update Spend LimitUpdates an existing Brex spend limit by its ID.
Update UserUpdate user details.
Update VendorTool to update vendor information in Brex.
Update Webhook SubscriptionUpdates an existing Brex webhook subscription's configuration.

What is the Composio tool router, and how does it fit here?

What is Composio SDK?

Composio's Composio SDK helps agents find the right tools for a task at runtime. You can plug in multiple toolkits (like Gmail, HubSpot, and GitHub), and the agent will identify the relevant app and action to complete multi-step workflows. This can reduce token usage and improve the reliability of tool calls. Read more here: Getting started with Composio SDK

The tool router generates a secure MCP URL that your agents can access to perform actions.

How the Composio SDK works

The Composio SDK follows a three-phase workflow:

  1. Discovery: Searches for tools matching your task and returns relevant toolkits with their details.
  2. Authentication: Checks for active connections. If missing, creates an auth config and returns a connection URL via Auth Link.
  3. Execution: Executes the action using the authenticated connection.

Step-by-step Guide

Prerequisites

Before starting, make sure you have:
  • Node.js 18 or higher
  • A Composio account with an active API key
  • An OpenAI API key
  • Basic familiarity with TypeScript

Getting API Keys for OpenAI and Composio

OpenAI API Key
  • Go to the OpenAI dashboard and create an API key.
  • You need credits or a connected billing setup to use the models.
  • Store the key somewhere safe.
Composio API Key
  • Log in to the Composio dashboard.
  • Go to Settings and copy your API key.
  • This key lets your Mastra agent talk to Composio and reach Brex through MCP.

Install dependencies

bash
npm install @composio/core @mastra/core @mastra/mcp @ai-sdk/openai dotenv

Install the required packages.

What's happening:

  • @composio/core is the Composio SDK for creating MCP sessions
  • @mastra/core provides the Agent class
  • @mastra/mcp is Mastra's MCP client
  • @ai-sdk/openai is the model wrapper for OpenAI
  • dotenv loads environment variables from .env

Set up environment variables

bash
COMPOSIO_API_KEY=your_composio_api_key_here
COMPOSIO_USER_ID=your_user_id_here
OPENAI_API_KEY=your_openai_api_key_here

Create a .env file in your project root.

What's happening:

  • COMPOSIO_API_KEY authenticates your requests to Composio
  • COMPOSIO_USER_ID tells Composio which user this session belongs to
  • OPENAI_API_KEY lets the Mastra agent call OpenAI models

Import libraries and validate environment

typescript
import "dotenv/config";
import { openai } from "@ai-sdk/openai";
import { Agent } from "@mastra/core/agent";
import { MCPClient } from "@mastra/mcp";
import { Composio } from "@composio/core";
import * as readline from "readline";

import type { AiMessageType } from "@mastra/core/agent";

const openaiAPIKey = process.env.OPENAI_API_KEY;
const composioAPIKey = process.env.COMPOSIO_API_KEY;
const composioUserID = process.env.COMPOSIO_USER_ID;

if (!openaiAPIKey) throw new Error("OPENAI_API_KEY is not set");
if (!composioAPIKey) throw new Error("COMPOSIO_API_KEY is not set");
if (!composioUserID) throw new Error("COMPOSIO_USER_ID is not set");

const composio = new Composio({
  apiKey: composioAPIKey as string,
});
What's happening:
  • dotenv/config auto loads your .env so process.env.* is available
  • openai gives you a Mastra compatible model wrapper
  • Agent is the Mastra agent that will call tools and produce answers
  • MCPClient connects Mastra to your Composio MCP server
  • Composio is used to create a Tool Router session

Create a Tool Router session for Brex

typescript
async function main() {
  const session = await composio.create(
    composioUserID as string,
    {
      toolkits: ["brex"],
    },
  );

  const composioMCPUrl = session.mcp.url;
  console.log("Brex MCP URL:", composioMCPUrl);
What's happening:
  • create spins up a short-lived MCP HTTP endpoint for this user
  • The toolkits array contains "brex" for Brex access
  • session.mcp.url is the MCP URL that Mastra's MCPClient will connect to

Configure Mastra MCP client and fetch tools

typescript
const mcpClient = new MCPClient({
    id: composioUserID as string,
    servers: {
      nasdaq: {
        url: new URL(composioMCPUrl),
        requestInit: {
          headers: session.mcp.headers,
        },
      },
    },
    timeout: 30_000,
  });

console.log("Fetching MCP tools from Composio...");
const composioTools = await mcpClient.getTools();
console.log("Number of tools:", Object.keys(composioTools).length);
What's happening:
  • MCPClient takes an id for this client and a list of MCP servers
  • The headers property includes the x-api-key for authentication
  • getTools fetches the tool definitions exposed by the Brex toolkit

Create the Mastra agent

typescript
const agent = new Agent({
    name: "brex-mastra-agent",
    instructions: "You are an AI agent with Brex tools via Composio.",
    model: "openai/gpt-5",
  });
What's happening:
  • Agent is the core Mastra agent
  • name is just an identifier for logging and debugging
  • instructions guide the agent to use tools instead of only answering in natural language
  • model uses openai("gpt-5") to configure the underlying LLM

Set up interactive chat interface

typescript
let messages: AiMessageType[] = [];

console.log("Chat started! Type 'exit' or 'quit' to end.\n");

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: "> ",
});

rl.prompt();

rl.on("line", async (userInput: string) => {
  const trimmedInput = userInput.trim();

  if (["exit", "quit", "bye"].includes(trimmedInput.toLowerCase())) {
    console.log("\nGoodbye!");
    rl.close();
    process.exit(0);
  }

  if (!trimmedInput) {
    rl.prompt();
    return;
  }

  messages.push({
    id: crypto.randomUUID(),
    role: "user",
    content: trimmedInput,
  });

  console.log("\nAgent is thinking...\n");

  try {
    const response = await agent.generate(messages, {
      toolsets: {
        brex: composioTools,
      },
      maxSteps: 8,
    });

    const { text } = response;

    if (text && text.trim().length > 0) {
      console.log(`Agent: ${text}\n`);
        messages.push({
          id: crypto.randomUUID(),
          role: "assistant",
          content: text,
        });
      }
    } catch (error) {
      console.error("\nError:", error);
    }

    rl.prompt();
  });

  rl.on("close", async () => {
    console.log("\nSession ended.");
    await mcpClient.disconnect();
    process.exit(0);
  });
}

main().catch((err) => {
  console.error("Fatal error:", err);
  process.exit(1);
});
What's happening:
  • messages keeps the full conversation history in Mastra's expected format
  • agent.generate runs the agent with conversation history and Brex toolsets
  • maxSteps limits how many tool calls the agent can take in a single run
  • onStepFinish is a hook that prints intermediate steps for debugging

Complete Code

Here's the complete code to get you started with Brex and Mastra AI:

typescript
import "dotenv/config";
import { openai } from "@ai-sdk/openai";
import { Agent } from "@mastra/core/agent";
import { MCPClient } from "@mastra/mcp";
import { Composio } from "@composio/core";
import * as readline from "readline";

import type { AiMessageType } from "@mastra/core/agent";

const openaiAPIKey = process.env.OPENAI_API_KEY;
const composioAPIKey = process.env.COMPOSIO_API_KEY;
const composioUserID = process.env.COMPOSIO_USER_ID;

if (!openaiAPIKey) throw new Error("OPENAI_API_KEY is not set");
if (!composioAPIKey) throw new Error("COMPOSIO_API_KEY is not set");
if (!composioUserID) throw new Error("COMPOSIO_USER_ID is not set");

const composio = new Composio({ apiKey: composioAPIKey as string });

async function main() {
  const session = await composio.create(composioUserID as string, {
    toolkits: ["brex"],
  });

  const composioMCPUrl = session.mcp.url;

  const mcpClient = new MCPClient({
    id: composioUserID as string,
    servers: {
      brex: {
        url: new URL(composioMCPUrl),
        requestInit: {
          headers: session.mcp.headers,
        },
      },
    },
    timeout: 30_000,
  });

  const composioTools = await mcpClient.getTools();

  const agent = new Agent({
    name: "brex-mastra-agent",
    instructions: "You are an AI agent with Brex tools via Composio.",
    model: "openai/gpt-5",
  });

  let messages: AiMessageType[] = [];

  const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout,
    prompt: "> ",
  });

  rl.prompt();

  rl.on("line", async (input: string) => {
    const trimmed = input.trim();
    if (["exit", "quit"].includes(trimmed.toLowerCase())) {
      rl.close();
      return;
    }

    messages.push({ id: crypto.randomUUID(), role: "user", content: trimmed });

    const { text } = await agent.generate(messages, {
      toolsets: { brex: composioTools },
      maxSteps: 8,
    });

    if (text) {
      console.log(`Agent: ${text}\n`);
      messages.push({ id: crypto.randomUUID(), role: "assistant", content: text });
    }

    rl.prompt();
  });

  rl.on("close", async () => {
    await mcpClient.disconnect();
    process.exit(0);
  });
}

main();

Conclusion

You've built a Mastra AI agent that can interact with Brex through Composio's Tool Router. You can extend this further by:
  • Adding other toolkits like Gmail, Slack, or GitHub
  • Building a web-based chat interface around this agent
  • Using multiple MCP endpoints to enable cross-app workflows

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FAQ

What are the differences in Tool Router MCP and Brex MCP?

With a standalone Brex MCP server, the agents and LLMs can only access a fixed set of Brex tools tied to that server. However, with the Composio Tool Router, agents can dynamically load tools from Brex and many other apps based on the task at hand, all through a single MCP endpoint.

Can I use Tool Router MCP with Mastra AI?

Yes, you can. Mastra AI fully supports MCP integration. You get structured tool calling, message history handling, and model orchestration while Tool Router takes care of discovering and serving the right Brex tools.

Can I manage the permissions and scopes for Brex while using Tool Router?

Yes, absolutely. You can configure which Brex scopes and actions are allowed when connecting your account to Composio. You can also bring your own OAuth credentials or API configuration so you keep full control over what the agent can do.

How safe is my data with Composio Tool Router?

All sensitive data such as tokens, keys, and configuration is fully encrypted at rest and in transit. Composio is SOC 2 Type 2 compliant and follows strict security practices so your Brex data and credentials are handled as safely as possible.

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