How to integrate Harvest MCP with Pydantic AI

This guide walks you through connecting Harvest to Pydantic AI using the Composio tool router. By the end, you'll have a working Harvest agent that can create a new client for acme corp, log an expense for project 'website redesign', generate an invoice for hours worked this week through natural language commands. This guide will help you understand how to give your Pydantic AI agent real control over a Harvest account through Composio's Harvest MCP server. Before we dive in, let's take a quick look at the key ideas and tools involved.

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Harvest

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Harvest is a time-tracking and invoicing platform for teams and freelancers. It helps you log billable hours, manage projects, and streamline payments.

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Introduction

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

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

Also integrate Harvest with

OpenAI Agents SDK Claude Agent SDK Claude Code Claude Cowork Codex OpenClaw Hermes CLI Google ADK LangChain Vercel AI SDK Mastra AI LlamaIndex CrewAI

TL;DR

Here's what you'll learn:

How to set up your Composio API key and User ID
How to create a Composio Tool Router session for Harvest
How to attach an MCP Server to a Pydantic AI agent
How to stream responses and maintain chat history
How to build a simple REPL-style chat interface to test your Harvest workflows

What is Pydantic AI?

Pydantic AI is a Python framework for building AI agents with strong typing and validation. It leverages Pydantic's data validation capabilities to create robust, type-safe AI applications.

Key features include:

Type Safety: Built on Pydantic for automatic data validation
MCP Support: Native support for Model Context Protocol servers
Streaming: Built-in support for streaming responses
Async First: Designed for async/await patterns

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

The Harvest MCP server is an implementation of the Model Context Protocol that connects your AI agent and assistants like Claude, Cursor, etc directly to your Harvest account. It provides structured and secure access to your time-tracking, invoicing, and project management data, so your agent can create clients, log expenses, send invoices, record payments, and manage estimates automatically on your behalf.

Client and contact management: Seamlessly create new clients and add contacts to keep your client list up to date without manual entry.
Estimate creation and communication: Automatically generate new estimates, categorize line items, and send estimate messages or updates to clients.
Expense tracking automation: Log new expense entries against projects, ensuring accurate financial records and effortless cost tracking.
Streamlined invoicing and payments: Create professional invoices, categorize invoice items, send invoice notifications, and record payments as soon as they happen.
Project financial workflow optimization: Let your agent handle the full cycle—from creating clients to sending invoices and tracking payments—saving your team valuable time and reducing errors.

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:

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

Step-by-step Guide

Step by step09 STEPS

Prerequisites

Before starting, make sure you have:

Python 3.9 or higher
A Composio account with an active API key
Basic familiarity with Python and async programming

Getting API Keys for OpenAI and Composio

OpenAI API Key

Go to the OpenAI dashboard and create an API key. You'll need credits to use the models, or you can connect to another model provider.
Keep the API key safe.

Composio API Key

Log in to the Composio dashboard.
Navigate to your API settings and generate a new API key.
Store this key securely as you'll need it for authentication.

Install dependencies

bash

pip install composio pydantic-ai python-dotenv

Install the required libraries.

What's happening:

composio connects your agent to external SaaS tools like Harvest
pydantic-ai lets you create structured AI agents with tool support
python-dotenv loads your environment variables securely from a .env file

Set up environment variables

bash

COMPOSIO_API_KEY=your_composio_api_key_here
USER_ID=your_user_id_here
OPENAI_API_KEY=your_openai_api_key

Create a .env file in your project root.

What's happening:

COMPOSIO_API_KEY authenticates your agent to Composio's API
USER_ID associates your session with your account for secure tool access
OPENAI_API_KEY to access OpenAI LLMs

Import dependencies

python

import asyncio
import os
from dotenv import load_dotenv
from composio import Composio
from pydantic_ai import Agent
from pydantic_ai.mcp import MCPServerStreamableHTTP

load_dotenv()

What's happening:

We load environment variables and import required modules
Composio manages connections to Harvest
MCPServerStreamableHTTP connects to the Harvest MCP server endpoint
Agent from Pydantic AI lets you define and run the AI assistant

Create a Tool Router Session

python

async def main():
    api_key = os.getenv("COMPOSIO_API_KEY")
    user_id = os.getenv("USER_ID")
    if not api_key or not user_id:
        raise RuntimeError("Set COMPOSIO_API_KEY and USER_ID in your environment")

    # Create a Composio Tool Router session for Harvest
    composio = Composio(api_key=api_key)
    session = composio.create(
        user_id=user_id,
        toolkits=["harvest"],
    )
    url = session.mcp.url
    if not url:
        raise ValueError("Composio session did not return an MCP URL")

What's happening:

We're creating a Tool Router session that gives your agent access to Harvest tools
The create method takes the user ID and specifies which toolkits should be available
The returned session.mcp.url is the MCP server URL that your agent will use

Initialize the Pydantic AI Agent

python

# Attach the MCP server to a Pydantic AI Agent
harvest_mcp = MCPServerStreamableHTTP(url, headers={"x-api-key": COMPOSIO_API_KEY})
agent = Agent(
    "openai:gpt-5",
    toolsets=[harvest_mcp],
    instructions=(
        "You are a Harvest assistant. Use Harvest tools to help users "
        "with their requests. Ask clarifying questions when needed."
    ),
)

What's happening:

The MCP client connects to the Harvest endpoint
The agent uses GPT-5 to interpret user commands and perform Harvest operations
The instructions field defines the agent's role and behavior

Build the chat interface

python

# Simple REPL with message history
history = []
print("Chat started! Type 'exit' or 'quit' to end.\n")
print("Try asking the agent to help you with Harvest.\n")

while True:
    user_input = input("You: ").strip()
    if user_input.lower() in {"exit", "quit", "bye"}:
        print("\nGoodbye!")
        break
    if not user_input:
        continue

    print("\nAgent is thinking...\n", flush=True)

    async with agent.run_stream(user_input, message_history=history) as stream_result:
        collected_text = ""
        async for chunk in stream_result.stream_output():
            text_piece = None
            if isinstance(chunk, str):
                text_piece = chunk
            elif hasattr(chunk, "delta") and isinstance(chunk.delta, str):
                text_piece = chunk.delta
            elif hasattr(chunk, "text"):
                text_piece = chunk.text
            if text_piece:
                collected_text += text_piece
        result = stream_result

    print(f"Agent: {collected_text}\n")
    history = result.all_messages()

What's happening:

The agent reads input from the terminal and streams its response
Harvest API calls happen automatically under the hood
The model keeps conversation history to maintain context across turns

Run the application

python

if __name__ == "__main__":
    asyncio.run(main())

What's happening:

The asyncio loop launches the agent and keeps it running until you exit

Complete Code

Here's the complete code to get you started with Harvest and Pydantic AI:

python

import asyncio
import os
from dotenv import load_dotenv
from composio import Composio
from pydantic_ai import Agent
from pydantic_ai.mcp import MCPServerStreamableHTTP

load_dotenv()

async def main():
    api_key = os.getenv("COMPOSIO_API_KEY")
    user_id = os.getenv("USER_ID")
    if not api_key or not user_id:
        raise RuntimeError("Set COMPOSIO_API_KEY and USER_ID in your environment")

    # Create a Composio Tool Router session for Harvest
    composio = Composio(api_key=api_key)
    session = composio.create(
        user_id=user_id,
        toolkits=["harvest"],
    )
    url = session.mcp.url
    if not url:
        raise ValueError("Composio session did not return an MCP URL")

    # Attach the MCP server to a Pydantic AI Agent
    harvest_mcp = MCPServerStreamableHTTP(url, headers={"x-api-key": COMPOSIO_API_KEY})
    agent = Agent(
        "openai:gpt-5",
        toolsets=[harvest_mcp],
        instructions=(
            "You are a Harvest assistant. Use Harvest tools to help users "
            "with their requests. Ask clarifying questions when needed."
        ),
    )

    # Simple REPL with message history
    history = []
    print("Chat started! Type 'exit' or 'quit' to end.\n")
    print("Try asking the agent to help you with Harvest.\n")

    while True:
        user_input = input("You: ").strip()
        if user_input.lower() in {"exit", "quit", "bye"}:
            print("\nGoodbye!")
            break
        if not user_input:
            continue

        print("\nAgent is thinking...\n", flush=True)

        async with agent.run_stream(user_input, message_history=history) as stream_result:
            collected_text = ""
            async for chunk in stream_result.stream_output():
                text_piece = None
                if isinstance(chunk, str):
                    text_piece = chunk
                elif hasattr(chunk, "delta") and isinstance(chunk.delta, str):
                    text_piece = chunk.delta
                elif hasattr(chunk, "text"):
                    text_piece = chunk.text
                if text_piece:
                    collected_text += text_piece
            result = stream_result

        print(f"Agent: {collected_text}\n")
        history = result.all_messages()

if __name__ == "__main__":
    asyncio.run(main())

Conclusion

You've built a Pydantic AI agent that can interact with Harvest through Composio's Tool Router. With this setup, your agent can perform real Harvest actions through natural language. You can extend this further by:

Adding other toolkits like Gmail, HubSpot, or Salesforce
Building a web-based chat interface around this agent
Using multiple MCP endpoints to enable cross-app workflows (for example, Gmail + Harvest for workflow automation)

This architecture makes your AI agent "agent-native", able to securely use APIs in a unified, composable way without custom integrations.

TOOLS

Supported Tools

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

Create Client

Tool to create a new client.

Create Client Contact

Tool to create a new client contact.

Create Estimate

Tool to create a new estimate.

Create Estimate Item Category

Tool to create a new estimate item category in Harvest.

Create Estimate Message

Tool to create a new message for an estimate.

Create Expense

Tool to create a new expense entry.

Create Invoice

Tool to create a new invoice.

Create Invoice Item Category

Tool to create a new invoice item category.

Create Invoice Message

Creates a new message for an invoice in Harvest.

Create Invoice Payment

Tool to create a new payment on an invoice.

Create Project

Create a new project in Harvest.

Create Task

Creates a new task in Harvest.

Create Time Entry

Tool to create a new time entry.

Create User

Creates a new user in Harvest and sends an invitation email to the specified address.

Delete Client

Tool to delete a client.

Delete Client Contact

Permanently deletes a client contact from Harvest.

Delete Estimate

Tool to delete an estimate.

Delete Estimate Message

Tool to delete an estimate message.

Delete Invoice

Delete an invoice from Harvest.

Delete Invoice Item Category

Tool to delete an invoice item category from Harvest.

Delete Invoice Message

Tool to delete a message from an invoice.

Delete Invoice Payment

Tool to delete an invoice payment.

Delete Project

Tool to delete a project.

Delete Task

Tool to delete a task.

Delete Time Entry

Tool to delete a time entry.

Delete User

Tool to delete a user.

Get Client

Retrieves a specific client by ID from Harvest.

Get Company Info

Retrieves detailed information about the company associated with the authenticated Harvest account.

Get Estimate

Tool to retrieve a specific estimate by ID.

Get Invoice

Tool to retrieve a specific invoice by ID.

Get Project

Tool to retrieve a specific Harvest project by ID.

Get Task

Tool to retrieve a specific task by ID.

Get Time Entry

Tool to retrieve a single time entry by ID.

Get User

Tool to retrieve a specific user by ID.

List Client Contacts

Tool to list client contacts.

List Clients

Tool to list clients.

List Estimate Messages

Lists all messages associated with a specific estimate in Harvest.

List Expense Categories

Tool to list expense categories.

List Invoice Item Categories

Tool to retrieve invoice item categories.

List Invoice Messages

Tool to list messages associated with a given invoice.

List Invoice Payments

List all payments recorded for a specific invoice.

List Invoices

Tool to list invoices.

List projects

Tool to list projects.

List Tasks

Tool to list tasks.

List Time Entries

Tool to retrieve a list of time entries.

List Users

Tool to list users.

Update Client

Tool to update an existing client.

Update Client Contact

Tool to update a client contact.

Update Company Info

Updates company time tracking settings in Harvest.

Update Estimate

Tool to update an existing estimate.

Update Estimate Item Category

Updates an existing estimate item category's name in Harvest.

Update Expense

Tool to update an existing expense.

Update Invoice

Tool to update an existing invoice.

Update Project

Tool to update an existing project.

Update Task

Tool to update an existing task.

Update Time Entry

Tool to update an existing time entry.

Update User

Tool to update an existing user.

FRAMEWORKS

How to build Harvest MCP Agent with another framework

OpenAI Agents SDK

Use Harvest MCP with OpenAI Agents SDK

Claude Agent SDK

Use Harvest MCP with Claude Agent SDK

Claude Code

Use Harvest MCP with Claude Code

Claude Cowork

Use Harvest MCP with Claude Cowork

Codex

Use Harvest MCP with Codex

OpenClaw

Use Harvest MCP with OpenClaw

Hermes

Use Harvest MCP with Hermes

CLI

Use Harvest MCP with CLI

Google ADK

Use Harvest MCP with Google ADK

LangChain

Use Harvest MCP with LangChain

Vercel AI SDK

Use Harvest MCP with Vercel AI SDK

Mastra AI

Use Harvest MCP with Mastra AI

LlamaIndex

Use Harvest MCP with LlamaIndex

CrewAI

Use Harvest MCP with CrewAI

MORE TOOLKITS

Explore Other Toolkits

Toolkit marketplace

Google Sheets

Oauth2

Google Sheets is a cloud-based spreadsheet tool for real-time collaboration and data analysis. It lets teams work together from anywhere, updating information instantly.

Notion

Oauth2Api Key

Notion is a collaborative workspace for notes, docs, wikis, and tasks. It streamlines team knowledge, project tracking, and workflow customization in one place.

Airtable

Oauth2Api Key

Airtable combines the flexibility of spreadsheets with the power of a database for easy project and data management. Teams use Airtable to organize, track, and collaborate with custom views and automations.

Asana

Oauth2

Asana is a collaborative work management platform for teams to organize and track projects. It streamlines teamwork, boosts productivity, and keeps everyone aligned on goals.

FAQ

Frequently asked questions

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

Yes, you can. Pydantic 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 Harvest tools.

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

Start with Harvest.It takes 30 seconds.

Managed auth, hosted MCP servers, and every Harvest tool your agent needs.Free to start.

Start building

Customer stories

Teams ship real agents on Composio

GET STARTED FOR FREE BOOK A DEMO

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.

Harsha Gaddipati

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.

Abhi Arya

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.

Nirman Dave

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.

Ryan Yu

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.

Tomisin Jenrola

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.

Jerome Leclanche

Co-Founder, Ingram Technologies

How to integrate Harvest MCP with Pydantic AI

Connect Harvest without auth hassles

Introduction

Also integrate Harvest with

TL;DR

What is Pydantic AI?

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

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

What is Composio SDK?

How the Composio SDK works

Step-by-step Guide

Prerequisites

Getting API Keys for OpenAI and Composio

Install dependencies

Set up environment variables

Import dependencies

Create a Tool Router Session

Initialize the Pydantic AI Agent

Build the chat interface

Run the application

Complete Code

Conclusion

Supported Tools

How to build Harvest MCP Agent with another framework

OpenAI Agents SDK

Claude Agent SDK

Claude Code

Claude Cowork

Codex

OpenClaw

Hermes

CLI

Google ADK

LangChain

Vercel AI SDK

Mastra AI

LlamaIndex

CrewAI

Explore Other Toolkits

Google Sheets

Notion

Airtable

Asana

Frequently asked questions

What are the differences in Tool Router MCP and Harvest MCP?+

Can I use Tool Router MCP with Pydantic AI?+

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

How safe is my data with Composio Tool Router?+

Start with Harvest.It takes 30 seconds.

Teams ship real agents on Composio