How to integrate Retellai MCP with Mastra AI

This guide walks you through connecting Retellai to Mastra AI using the Composio tool router. By the end, you'll have a working Retellai agent that can list all phone numbers linked to your account, retrieve call details for a specific agent this week, buy a new phone number with area code 415 through natural language commands. This guide will help you understand how to give your Mastra AI agent real control over a Retellai account through Composio's Retellai MCP server. Before we dive in, let's take a quick look at the key ideas and tools involved.

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RetellAI is a platform for capturing calls and transcripts, centralizing business conversations. It helps teams analyze interactions and extract insights to improve customer experience.

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Introduction

This guide walks you through connecting Retellai to Mastra AI using the Composio tool router. By the end, you'll have a working Retellai agent that can list all phone numbers linked to your account, retrieve call details for a specific agent this week, buy a new phone number with area code 415 through natural language commands.

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

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

Also integrate Retellai with

OpenAI Agents SDKClaude Agent SDKClaude CodeClaude CoworkCodexOpenClawHermesCLIGoogle ADKLangChainVercel AI SDKLlamaIndexCrewAI

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

The Retellai MCP server is an implementation of the Model Context Protocol that connects your AI agent and assistants like Claude, Cursor, etc directly to your Retellai account. It provides structured and secure access to your call records, phone numbers, and conversation transcripts, so your agent can perform actions like retrieving call details, managing phone numbers, initiating outbound calls, and analyzing voice data on your behalf.

  • Retrieve and analyze call records: Your agent can fetch detailed call logs, filter by agent or time, and surface insights from past conversations.
  • Initiate outbound and web-based calls: Easily direct your agent to start new phone or web calls between specific numbers or agents, supporting various business workflows.
  • Manage phone numbers and assignments: Buy, update, or delete phone numbers, and bind them to agents for streamlined inbound and outbound call handling.
  • Access and review call transcripts and details: Let your agent drill down into specific calls, pulling transcripts and metadata for compliance, training, or analytics.
  • Explore and configure voice settings: Fetch detailed information about available voice options, including provider, accent, gender, and preview audio for customization of call experiences.

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

Step by step09 STEPS
1

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
2

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 Retellai through MCP.
3

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
4

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
5

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
6

Create a Tool Router session for Retellai

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

  const composioMCPUrl = session.mcp.url;
  console.log("Retellai MCP URL:", composioMCPUrl);
What's happening:
  • create spins up a short-lived MCP HTTP endpoint for this user
  • The toolkits array contains "retellai" for Retellai access
  • session.mcp.url is the MCP URL that Mastra's MCPClient will connect to
7

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 Retellai toolkit
8

Create the Mastra agent

typescript
const agent = new Agent({
    name: "retellai-mastra-agent",
    instructions: "You are an AI agent with Retellai 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
9

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: {
        retellai: 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 Retellai 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 Retellai 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: ["retellai"],
  });

  const composioMCPUrl = session.mcp.url;

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

  const composioTools = await mcpClient.getTools();

  const agent = new Agent({
    name: "retellai-mastra-agent",
    instructions: "You are an AI agent with Retellai 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: { retellai: 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 Retellai 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
TOOLS

Supported Tools

Every Retellai action and event your agent gets out of the box.

Add community voice

Add a community voice from ElevenLabs to your Retell voice library.

Add sources to knowledge base

Tool to add sources (documents, URLs, text) to an existing knowledge base in Retell AI.

Buy a new phone number bind agents

This endpoint allows purchasing a new phone number with a specified area code and binding it to designated agents for inbound and outbound calls.

Create Voice AI Agent

Create a new voice AI agent with specified configuration.

Create a new outbound phone call

Initiate an outbound call by POST to '/v2/create-phone-call'.

Create a new web call

The /v2/create-web-call endpoint creates a web call with a unique agent ID, returning call details like type, token, call ID, and status in JSON format, with a 201 response.

Create Batch Test

Tool to create a batch test job that runs multiple test cases against an agent.

Create a new chat session

Tool to create a new chat session with a chat agent.

Create a new chat agent

Create a new chat agent with specified configuration.

Create chat completion

Tool to create a chat completion for an existing chat session, generating the agent's response to a user message.

Create conversation flow

Create a new Conversation Flow that can be attached to an agent for response generation.

Create conversation flow component

Creates a new shared conversation flow component at POST '/create-conversation-flow-component'.

Create a new knowledge base

Tool to create a new knowledge base in Retell AI with texts, files, and URLs.

Create Retell LLM Response Engine

Create a new Retell LLM Response Engine that can be attached to an agent.

Create Test Case Definition

Tool to create a test case definition for agent QA testing in Retell AI.

Delete agent

Deletes an existing agent by its unique identifier.

Delete call

Delete a specific call and its associated data by call ID.

Delete chat agent

Delete an existing chat agent by its unique identifier.

Delete conversation flow

Delete a conversation flow and all its versions.

Delete conversation flow component

Delete a shared conversation flow component.

Delete knowledge base

Delete an existing knowledge base by its unique identifier.

Delete knowledge base source

Delete an existing source from a knowledge base.

Delete phone number

Tool to delete an existing phone number from Retell AI.

Delete Retell LLM

Delete an existing Retell LLM Response Engine by its unique identifier.

Delete test case definition

Delete a test case definition by its unique identifier.

End chat

Tool to end an active chat session.

Retrieve details of a specific agent

Retrieve details of a specific agent by its unique identifier.

Get agent versions

Tool to retrieve all versions of a specific agent.

Get batch test

Retrieve details and results of a specific batch test job.

Get chat details

Tool to retrieve details of a specific chat session by chat ID.

Retrieve details of a specific chat agent

Retrieve details of a specific chat agent by its unique identifier.

Get all versions of a chat agent

Retrieve all versions of a specific chat agent by its unique identifier.

Get concurrency

Retrieves the current concurrency and concurrency limits for the organization.

Get Conversation Flow

Retrieve details of a specific Conversation Flow by its ID.

Get conversation flow component

Retrieves a shared conversation flow component by its unique identifier.

Get knowledge base

Retrieve details of a specific knowledge base by its unique identifier.

Retrieve details of a specific Retell LLM

Retrieve details of a specific Retell LLM Response Engine by its unique identifier.

Import phone number

Tool to import a phone number from custom telephony and bind agents to it.

List agents

Retrieves a list of all agents associated with the account.

List all chats

Tool to retrieve a list of all chats associated with the account.

List all phone numbers

Retrieves a list of all phone numbers associated with the account.

List batch tests

Tool to list batch test jobs for a response engine.

List chat agents

Tool to retrieve a list of all chat agents associated with the account.

List conversation flow components

Retrieves a list of all shared conversation flow components.

List conversation flows

Tool to list all conversation flows that can be attached to an agent.

List knowledge bases

Tool to retrieve all knowledge bases associated with the account.

List Retell LLMs

Tool to list all Retell LLM Response Engines that can be attached to an agent.

List test case definitions

Tool to list test case definitions for a response engine (Retell LLM or Conversation Flow).

List test runs

Tool to list all test case jobs (test runs) for a batch test job.

List voices

List all voices available to the user.

Publish agent

Publishes the latest version of the agent and creates a new draft agent with a newer version.

Publish chat agent

Publishes the latest version of the chat agent and creates a new draft chat agent with a newer version.

Register phone call

Register a phone call for custom telephony integration with Retell AI.

Retrieve call details

Tool to retrieve call details with filtering options.

Retrieve call details by id

Retrieve call details by ID for web/phone calls, including type, agent ID, status, timestamps, and web access token; covering responses from success to server errors.

Retrieve phone number details

Tool to retrieve details of a specific phone number from Retell AI.

Retrieve details of a specific voice

Tool to retrieve details of a specific voice by its voice_id.

Search community voice

Search for community voices from voice providers.

Update agent

Update an existing agent's latest draft version.

Update call

Update an active call's parameters such as metadata, dynamic variables, or data storage settings.

Update chat agent

Update an existing chat agent configuration.

Update chat metadata

Tool to update metadata and sensitive data storage settings for an existing chat.

Update conversation flow

Update an existing conversation flow configuration.

Update conversation flow component

Update an existing shared conversation flow component by its ID.

Update phone number configuration

Update agent bound to a purchased phone number.

Update Retell LLM Response Engine

Update an existing Retell LLM Response Engine by its unique identifier.

Update test case definition

Update a test case definition for agent testing.

FRAMEWORKS

How to build Retellai MCP Agent with another framework

OpenAI Agents SDK logo

OpenAI Agents SDK

Use Retellai MCP with OpenAI Agents SDK

Claude Agent SDK logo

Claude Agent SDK

Use Retellai MCP with Claude Agent SDK

Claude Code logo

Claude Code

Use Retellai MCP with Claude Code

Claude Cowork logo

Claude Cowork

Use Retellai MCP with Claude Cowork

Codex logo

Codex

Use Retellai MCP with Codex

OpenClaw logo

OpenClaw

Use Retellai MCP with OpenClaw

Hermes logo

Hermes

Use Retellai MCP with Hermes

CLI logo

CLI

Use Retellai MCP with CLI

Google ADK logo

Google ADK

Use Retellai MCP with Google ADK

LangChain logo

LangChain

Use Retellai MCP with LangChain

Vercel AI SDK logo

Vercel AI SDK

Use Retellai MCP with Vercel AI SDK

LlamaIndex logo

LlamaIndex

Use Retellai MCP with LlamaIndex

CrewAI logo

CrewAI

Use Retellai MCP with CrewAI

FAQ

Frequently asked questions

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

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 Retellai tools.

Yes, absolutely. You can configure which Retellai 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.

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 Retellai data and credentials are handled as safely as possible.

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Karan skipped his own birthday party to fix our critical issue. It was 10pm and he diverted his Waymo to help us instead. This really sets the bar — it shows the commitment you need when users rely on your software.

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Co-founder, Slashy

A lot of students tell us that the moment their connected tools start talking to each other inside Opennote feels almost magical. The agent just knows them, and it's immensely helped keep new users on the platform.

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Co-founder, Opennote

We chose Composio over Pipedream because it delivered depth where it mattered — niche tools and tricky edge cases other platforms simply ignored. That gave us confidence to scale without compromising.

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CEO, Zams

As a solo builder, shipping fast is life or death. The only way I can outcompete incumbents is by outmanoeuvring them. Getting bogged down managing agent auth would have been a death sentence.

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Founder, Extra Thursday

Before Composio, adding tool integrations was slow and resource-intensive. Each one could take weeks of engineering time, and maintaining them meant constantly keeping up with API changes.

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Founder & CEO, SwarmZero

With hands-on help from their founder, we integrated Gmail and Google Drive in just 30 minutes. This level of personal support and commitment is exactly what startups should strive for.

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Co-Founder, Ingram Technologies