How to integrate Resend MCP with Mastra AI

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Introduction

This guide walks you through connecting Resend to Mastra AI using the Composio tool router. By the end, you'll have a working Resend agent that can create a new contact for your newsletter, list all domains connected to your account, cancel a scheduled marketing email through natural language commands.

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

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

Also integrate Resend 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 Resend tools
  • Connect Mastra's MCP client to the Composio generated MCP URL
  • Fetch Resend 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 Resend 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 Resend MCP server, and what's possible with it?

The Resend MCP server is an implementation of the Model Context Protocol that connects your AI agent and assistants like Claude, Cursor, etc directly to your Resend account. It provides structured and secure access to your email sending and contact management capabilities, so your agent can perform actions like sending emails, managing audiences, creating domains, and handling contacts on your behalf.

  • Automated email delivery and scheduling: Ask your agent to send emails instantly or schedule them for later, and even cancel scheduled messages if needed.
  • Audience and contact management: Let your agent create, list, or delete audiences and individual contacts, keeping your recipient lists organized without manual effort.
  • Domain administration: Easily have your agent set up, list, or remove email sending domains, streamlining onboarding or maintenance for your email infrastructure.
  • Seamless recipient segmentation: Direct your agent to group contacts into audiences for targeted campaigns or communications, improving personalization and reach.
  • Bulk operations and cleanups: Enable your agent to quickly list, update, or delete multiple contacts, audiences, or domains to maintain a tidy and up-to-date email system.

Supported Tools & Triggers

Tools
Add Contact To SegmentAdd an existing contact to a segment in Resend.
Cancel EmailCancel a scheduled email.
Create API KeyCreate a new API key to authenticate communications with Resend.
Create AudienceCreate a list of contacts.
Create ContactCreate a contact in Resend.
Create Contact PropertyTool to create a new contact property in Resend.
Create Contact V2Tool to create a new contact in Resend.
Create DomainCreate a domain through the Resend Email API.
Create TemplateTool to create a new email template in Resend.
Create TopicTool to create a new topic to segment your audience.
Create WebhookTool to create a webhook to receive real-time notifications about email events.
Delete API KeyRemove an existing API key from Resend.
Delete AudienceRemove an existing audience.
Delete ContactDelete a contact in Resend.
Delete Contact By IDTool to remove an existing contact by its ID.
Delete Contact PropertyRemove an existing contact property from Resend.
Delete DomainDelete a domain through the Resend Email API.
Delete SegmentRemove an existing segment.
Delete TemplateRemove an existing template.
Delete TopicTool to remove an existing topic in Resend.
Delete WebhookRemove an existing webhook.
Duplicate TemplateDuplicate an existing template through the Resend Email API.
Get ContactTool to retrieve a single contact from Resend by ID or email.
Get Contact PropertyTool to retrieve a single contact property from Resend.
Get Email AttachmentRetrieve a single attachment from a sent email.
Get SegmentRetrieve a single segment by its ID.
Get TemplateRetrieve a single template by ID or alias from Resend.
Get TopicTool to retrieve a single topic by its ID in Resend.
Get WebhookRetrieve a single webhook for the authenticated user.
List All ContactsTool to retrieve a list of all contacts from Resend.
List API KeysTool to retrieve a list of API keys for the authenticated user.
List AudiencesList all audiences.
List BroadcastsTool to retrieve a list of broadcasts.
List Contact PropertiesTool to retrieve a list of contact properties from Resend.
List ContactsList contacts in Resend.
List Contact SegmentsRetrieve a list of segments that a contact is part of.
List Contact TopicsRetrieve a list of topic subscriptions for a contact in Resend.
List DomainsList all domains.
List Email AttachmentsTool to retrieve a list of attachments from a sent email.
List EmailsTool to retrieve a list of emails sent by your team.
List Received EmailsTool to retrieve a list of received emails for the authenticated user.
List SegmentsTool to retrieve a list of segments from Resend.
List TemplatesTool to retrieve a list of templates from Resend.
List TopicsTool to retrieve a list of topics for the authenticated user.
List WebhooksRetrieve a list of webhooks for the authenticated user.
Publish TemplatePublish a template through the Resend Email API.
Remove Contact From SegmentRemove an existing contact from a segment.
Retrieve AudienceRetrieve a single audience.
Retrieve ContactRetrieve a contact in Resend.
Retrieve DomainRetrieve a single domain.
Retrieve EmailRetrieve a single email.
Send Batch EmailsTrigger up to 100 batch emails at once.
Send EmailSend an email using Resend.
Update BroadcastUpdate an existing broadcast in Resend.
Update ContactTool to update an existing contact in Resend by ID or email.
Update Contact PropertyUpdate an existing contact property in Resend.
Update DomainUpdate an existing domain.
Update EmailUpdate a scheduled email.
Update TemplateTool to update an existing email template in Resend.
Update TopicTool to update an existing topic in Resend.
Update WebhookTool to update an existing webhook configuration.
Verify DomainVerify a domain through the Resend Email API.

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 Resend 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 Resend

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

  const composioMCPUrl = session.mcp.url;
  console.log("Resend MCP URL:", composioMCPUrl);
What's happening:
  • create spins up a short-lived MCP HTTP endpoint for this user
  • The toolkits array contains "resend" for Resend 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 Resend toolkit

Create the Mastra agent

typescript
const agent = new Agent({
    name: "resend-mastra-agent",
    instructions: "You are an AI agent with Resend 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: {
        resend: 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 Resend 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 Resend 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: ["resend"],
  });

  const composioMCPUrl = session.mcp.url;

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

  const composioTools = await mcpClient.getTools();

  const agent = new Agent({
    name: "resend-mastra-agent",
    instructions: "You are an AI agent with Resend 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: { resend: 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 Resend 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 Resend MCP?

With a standalone Resend MCP server, the agents and LLMs can only access a fixed set of Resend tools tied to that server. However, with the Composio Tool Router, agents can dynamically load tools from Resend 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 Resend tools.

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

Yes, absolutely. You can configure which Resend 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 Resend data and credentials are handled as safely as possible.

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