Basic Hono Server

With Devcaster's managed authentication and tool calling, it's easy to build AI agents that interact with the real world while reducing boilerplate for setup and authentication management. This cookbook will guide you through building and serving agents using Devcaster, OpenAI, and Hono.js.

Prerequisites

Node.js 18.x or higher
npm or yarn package manager
Devcaster API key
OpenAI API key
Basic knowledge of OAuth
Understanding of building HTTP services (preferably using Hono.js)

Building an AI agent that can interact with `gmail` service

First, let's start with building a simple AI agent embedded with tools from Devcaster that lets the agent interact with the gmail service.

import { OpenAI } from 'openai';
import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';

export async function runGmailAgent(
    devcasterClient: Devcaster<OpenAIProvider>,
    openaiClient: OpenAI,
    userId: string,  // Devcaster uses the User ID to store and access user-level authentication tokens.
    prompt: string,
): Promise<any[]> {
    // Step 1: Fetch the necessary Gmail tools list with Devcaster
    const tools = await devcasterClient.tools.get(
        userId,
        {
            tools: [
                "GMAIL_FETCH_EMAILS",
                "GMAIL_SEND_EMAIL", 
                "GMAIL_CREATE_EMAIL_DRAFT"
            ]
        }
    );

    // Step 2: Use OpenAI to generate a response based on the prompt and available tools
    const response = await openaiClient.chat.completions.create({
        model: "gpt-4.1",
        tools,
        messages: [{ role: "user", content: prompt }],
    });

    // Step 3: Handle tool calls with Devcaster and return the result
    const result = await devcasterClient.provider.handleToolCalls(
        userId,
        response
    );
    return result;
}

This is a simple agent without state management and agentic loop implementation, so the agent can't perform complicated tasks. If you want to understand how devcaster can be used with agentic loops, check other cookbooks with more agentic frameworks.

To invoke this agent, authenticate your users with Devcaster's managed authentication service.

Authenticating users

To authenticate your users with Devcaster you need an authentication config for the given app. In this case you need one for gmail.

To create an authentication config for gmail you need client_id and client_secret from your Google OAuth Console. Once you have the credentials, use the following piece of code to set up authentication for gmail.

import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';

export async function createAuthConfig(devcasterClient: Devcaster<OpenAIProvider>) {
    /**
     * Create a auth config for the gmail toolkit.
     */
    const clientId = process.env.GMAIL_CLIENT_ID;
    const clientSecret = process.env.GMAIL_CLIENT_SECRET;
    if (!clientId || !clientSecret) {
        throw new Error("GMAIL_CLIENT_ID and GMAIL_CLIENT_SECRET must be set");
    }

    return devcasterClient.authConfigs.create(
        "GMAIL",
        {
            "name": "default_gmail_auth_config",
            "type": "use_custom_auth",
            "authScheme": "OAUTH2",
            "credentials": {
                "clientId": clientId,
                "clientSecret": clientSecret,
            },
        },
    );
}

This will create a Gmail authentication config to authenticate your app's users. Ideally, create one authentication object per project, so check for an existing auth config before creating a new one.

import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';
// ---cut---
export async function fetchAuthConfig(devcasterClient: Devcaster<OpenAIProvider>) {
    /**
     * Fetch the auth config for a given user id.
     */
    const authConfigs = await devcasterClient.authConfigs.list();
    for (const authConfig of authConfigs.items) {
        if (authConfig.toolkit.slug === "gmail") {
            return authConfig;
        }
    }

    return null;
}

Devcaster platform provides devcaster managed authentication for some apps to fast-track your development, gmail being one of them. You can use these default auth configs for development, but for production, always use your own oauth app configuration.

Once you have authentication management in place, we can start with connecting your users to your gmail app. Let's implement a function to connect users to your gmail app via devcaster.

import { Hono } from 'hono';
import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';

declare function fetchAuthConfig(client: Devcaster<OpenAIProvider>): Promise<{ id: string } | null>;
declare function createAuthConfig(client: Devcaster<OpenAIProvider>): Promise<{ id: string }>;
declare const devcasterClient: Devcaster<OpenAIProvider>;
// ---cut---
// Function to initiate a connected account
export async function createConnection(devcasterClient: Devcaster<OpenAIProvider>, userId: string) {
    /**
     * Create a connection for a given user id and auth config id.
     */
    // Fetch or create the auth config for the gmail toolkit
    let authConfig = await fetchAuthConfig(devcasterClient);
    if (!authConfig) {
        authConfig = await createAuthConfig(devcasterClient);
    }

    // Create a connection for the user
    return devcasterClient.connectedAccounts.initiate(
        userId,
        authConfig.id,
    );
}

// Setup Hono
const app = new Hono();

// Connection initiation endpoint
app.post("/connection/create", async (c) => {
    /**
     * Create a connection for a given user id.
     */
    // For demonstration, using a default user_id. Replace with real user logic in production.
    const userId = "default";

    // Create a new connection for the user
    const connectionRequest = await createConnection(devcasterClient, userId);
    return c.json({
        "connection_id": connectionRequest.id,
        "redirect_url": connectionRequest.redirectUrl,
    });
});

Now, you can make a request to this endpoint on your client app, and your user will get a URL which they can use to authenticate.

Set Up Hono service

We will use Hono.js to build an HTTP service that authenticates your users and lets them interact with your agent. This guide will provide best practices for using devcaster client in production environments.

Setup dependencies

Hono allows dependency injection patterns to simplify the usage of SDK clients that must be singletons. We recommend using devcaster SDK client as singleton.

import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';
import { OpenAI } from 'openai';

let _devcasterClient: Devcaster<OpenAIProvider> | null = null;

export function provideDevcasterClient(): Devcaster<OpenAIProvider> {
    /**
     * Provide a Devcaster client.
     */
    if (_devcasterClient === null) {
        _devcasterClient = new Devcaster({ 
            provider: new OpenAIProvider() 
        });
    }
    return _devcasterClient;
}

// A Devcaster client dependency.
export type DevcasterClient = Devcaster<OpenAIProvider>;

Invoke agent via Hono

When invoking an agent, make sure you validate the user_id.

import { Devcaster } from '@devcaster/core';
import { OpenAIProvider } from '@devcaster/openai';
import { OpenAI } from 'openai';
import { Hono } from 'hono';

const app = new Hono();
declare const devcasterClient: Devcaster<OpenAIProvider>;
declare const openaiClient: OpenAI;
declare function runGmailAgent(c: Devcaster<OpenAIProvider>, o: OpenAI, u: string, p: string): Promise<any[]>;
// ---cut---
export function checkConnectedAccountExists(
    devcasterClient: Devcaster<OpenAIProvider>,
    userId: string,
): Promise<boolean> {
    /**
     * Check if a connected account exists for a given user id.
     */
    // Fetch all connected accounts for the user
    return devcasterClient.connectedAccounts.list({ userIds: [userId], toolkitSlugs: ["GMAIL"] }).then(connectedAccounts => {

        // Check if there's an active connected account
        for (const account of connectedAccounts.items) {
            if (account.status === "ACTIVE") {
                return true;
            }

            // Ideally you should not have inactive accounts, but if you do, delete them.
            console.log(`[warning] inactive account ${account.id} found for user id: ${userId}`);
        }
        return false;
    });
}

export async function validateUserId(userId: string, devcasterClient: Devcaster<OpenAIProvider>): Promise<string> {
    /**
     * Validate the user id, if no connected account is found, create a new connection.
     */
    if (await checkConnectedAccountExists(devcasterClient, userId)) {
        return userId;
    }

    throw new Error("No connected account found for the user id");
}

// Endpoint: Run the Gmail agent for a given user id and prompt
app.post("/agent", async (c) => {
    /**
     * Run the Gmail agent for a given user id and prompt.
     */
    const request = await c.req.json();
    
    // For demonstration, using a default user_id. Replace with real user logic in production.
    const userId = "default";

    // Validate the user id before proceeding
    await validateUserId(userId, devcasterClient);

    // Run the Gmail agent using Devcaster and OpenAI
    const result = await runGmailAgent(
        devcasterClient,
        openaiClient,
        userId,
        request.prompt,
    );
    return c.json(result);
});

Putting everything together

So far, we have created an agent with ability to interact with gmail using the devcaster SDK, functions to manage connected accounts for users and a Hono service. Now let's run the service.

Clone the repository

git clone git@github.com:devcasterhq/devcaster-hono
cd devcaster-hono/

Setup environment

cp .env.example .env

Fill the api keys

DEVCASTER_API_KEY=
OPENAI_API_KEY=

Install dependencies

npm install

Run the HTTP server
```
npm run dev
```

Testing the API with curl

Assuming the server is running locally on http://localhost:8000.

Check if a connection exists

curl -X POST http://localhost:8000/connection/exists

Create a connection

Note: The body fields are required by the API schema, but are ignored internally in this example service.

curl -X POST http://localhost:8000/connection/create \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "auth_config_id": "AUTH_CONFIG_ID_FOR_GMAIL_FROM_THE_DEVCASTER_DASHBOARD"
  }'

Response includes connection_id and redirect_url. Complete the OAuth flow at the redirect_url.

Check connection status

Use the connection_id returned from the create step.

curl -X POST http://localhost:8000/connection/status \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "connection_id": "CONNECTION_ID_FROM_CREATE_RESPONSE"
  }'

Run the Gmail agent

Requires an active connected account for the default user.

curl -X POST http://localhost:8000/agent \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "prompt": "Summarize my latest unread emails from the last 24 hours."
  }'

Fetch emails (direct action)

curl -X POST http://localhost:8000/actions/fetch_emails \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "default",
    "limit": 5
  }'

These examples are intended solely for testing purposes.

Using Devcaster for managed auth and tools

Devcaster reduces boilerplate for building AI agents that access and use various apps. In this cookbook, to build Gmail integration without Devcaster, you would have to write code to

manage Gmail OAuth app
manage user connections
tools for your agents to interact with Gmail

Using Devcaster simplifies all of the above to a few lines of code as shown in the cookbook.

Best practices

🎯 Effective Prompts:

Be specific: "Send email to john@company.com about tomorrow's 2pm meeting" works better than "send email"
Include context: "Reply to Sarah's email about the budget with our approval"
Use natural language: The agent understands conversational requests

🔑 User Management:

Use unique, consistent user_id values for each person
Each user maintains their own Gmail connection
User IDs can be email addresses, usernames, or any unique identifier

Troubleshooting

Connection Issues:

Ensure your .env file has valid DEVCASTER_API_KEY and OPENAI_API_KEY
Check if the user has completed Gmail authorization.
Verify the user_id matches exactly between requests

API Errors:

Check the server logs for detailed error messages
Ensure request payloads match the expected format
Visit /docs endpoint for API schema validation

Gmail API Limits:

Gmail has rate limits; the agent will handle these gracefully
For high-volume usage, consider implementing request queuing

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