Simchat Demo Interface

The Simchat interface we'll build - a conversational assistant for blockchain data

In this guide, you’ll learn how to build Simchat, an AI chat agent that can provide realtime blockchain insights through natural conversation. Users can ask questions about wallet balances, transaction history, NFT collections, and token information across 60+ EVM chains and Solana, and the agent will fetch and explain the data in a friendly way. By combining OpenAI’s LLMs with the realtime blockchain data provided by Sim APIs, you’ll create a chat agent that makes onchain data accessible to everyone, regardless of their technical expertise.

View Source Code

Access the complete source code for Simchat on GitHub

Try Live Demo

Chat with the finished assistant

​
Prerequisites

Before we begin, ensure you have:

Get your Sim API Key

Learn how to obtain your Sim API key

​
Features

When you complete this guide, your chat agent will have these capabilities:

OpenAI Function Calling

Automatically triggers API requests based on user queries using OpenAI’s function calling feature

Multichain Token Balances

Retrieves native and ERC20/SPL token balances with USD values for any wallet address across EVM chains and Solana

Transaction History

Displays chronological wallet activity including transfers and contract interactions on EVM networks

NFT Collection Data

Shows ERC721 and ERC1155 collectibles owned by wallet addresses across supported EVM chains

Token Metadata

Provides detailed token information, pricing, and holder distributions for EVM and Solana tokens

Chat Interface

Users ask questions in plain English about blockchain data, no technical knowledge required

​
Try the Live Demo

Before diving into building, you can interact with the live chat agent app below. Try these example questions:
  • What tokens does vitalik.eth have?
  • Show me the NFTs in wallet 0xd8da6bf26964af9d7eed9e03e53415d37aa96045
  • What’s the price of USDC?
  • Get token balances for DYw8jCTfwHNRJhhmFcbXvVDTqWMEVFBX6ZKUmG5CNSKK on Solana

​
Project Setup

Let’s start by creating the project structure and installing dependencies.
1

Create Project Directory

Open your terminal and create a new directory:
mkdir simchat
cd simchat
Initialize a new Node.js project:
npm init -y
npm pkg set type="module"
2

Install Dependencies

Install the required packages:
npm install express openai dotenv
These packages provide:
  • express: Web server framework
  • openai: Official OpenAI client library
  • dotenv: Environment variable management
3

Configure Environment Variables

Create a .env file in your project root:
touch .env
Add your API keys:
.env
# Required API keys
OPENAI_API_KEY=your_openai_api_key_here
SIM_API_KEY=your_sim_api_key_here
Never commit your .env file to version control. Add it to .gitignore to keep your API keys secure.
4

Add Starter Code

Create the main app files:
touch server.js
touch chat.html
The server.js file will handle our backend Express server and API logic, while chat.html contains our frontend chat interface.Populate the server.js with this basic Express code:
import express from 'express';
import { OpenAI } from 'openai';
import path from 'path';
import { fileURLToPath } from 'url';
import dotenv from 'dotenv';

dotenv.config();

// Set up __dirname for ES modules
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

// Initialize Express
const app = express();
app.use(express.json());

// Initialize OpenAI client
const openai = new OpenAI({
    apiKey: process.env.OPENAI_API_KEY
});

// Sim API key
const SIM_API_KEY = process.env.SIM_API_KEY;

// Serve the HTML file
app.get('/', (req, res) => {
    res.sendFile(path.join(__dirname, 'chat.html'));
});

// Start server
const PORT = process.env.PORT || 3001;
app.listen(PORT, () => {
    console.log(`Server running on http://localhost:${PORT}`);
});
Add the initial frontend template to chat.html:
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Sim APIs Chat</title>
    <style>
        :root {
            --font-primary: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
            --color-bg-deep: #e0e0e0; /* Light gray page background */
            --color-text-primary: #333333;
            --color-text-secondary: #555555;
            --color-bg-container: #ffffff;
            --color-user-message-bg: #222222; /* Black for user messages */
            --color-user-message-text: #ffffff;
            --color-system-message-bg: #f1f1f1; /* Light gray for system messages */
            --color-system-message-text: #333333;
            --color-input-border: #dddddd;
            --color-send-button-bg: #8a8a8a;
            --color-send-button-icon: #ffffff;
            --color-plus-button-border: #e0e0e0;
            --color-plus-button-text: #555555;
            --border-radius-bubble: 18px;
            --border-radius-container: 20px;
            --border-radius-input: 10px;
        }

        body {
            font-family: var(--font-primary);
            background-color: var(--color-bg-deep);
            color: var(--color-text-secondary);
            margin: 0;
            padding: 0;
            display: flex;
            justify-content: center;
            align-items: center;
            min-height: 100vh;
            padding-top: 0;
            padding-bottom: 0;
            height: 100vh;
            box-sizing: border-box;
        }

        .mobile-container {
            width: 100%;
            max-width: 420px;
            height: 90vh;
            max-height: 800px;
            min-height: 600px;
            background-color: var(--color-bg-container);
            border-radius: var(--border-radius-container);
            display: flex;
            flex-direction: column;
            overflow: hidden;
            align-self: center;
            box-shadow: 0 8px 32px rgba(20, 24, 41, 0.18), 0 1.5px 6px rgba(20, 24, 41, 0.10);
        }

        .chat-header {
            display: flex;
            align-items: center;
            justify-content: space-between;
            padding: 15px 20px;
            border-bottom: 1px solid var(--color-input-border);
            background-color: var(--color-bg-container); /* Ensure it's on top */
            z-index: 10;
        }

        .avatar-info {
            display: flex;
            align-items: center;
        }

        .avatar {
            width: 40px;
            height: 40px;
            border-radius: 50%;
            margin-right: 12px;
            object-fit: cover;
        }

        .user-details {
            display: flex;
            flex-direction: column;
        }

        .user-name {
            font-weight: bold;
            color: var(--color-text-primary);
            font-size: 1rem;
        }

        .user-email {
            font-size: 0.8rem;
            color: var(--color-text-secondary);
        }

        .add-button {
            width: 36px;
            height: 36px;
            border-radius: 50%;
            border: 1px solid var(--color-plus-button-border);
            background-color: var(--color-bg-container);
            color: var(--color-plus-button-text);
            font-size: 1.5rem;
            line-height: 1;
            display: flex;
            justify-content: center;
            align-items: center;
            cursor: pointer;
        }
        .add-button:hover {
            background-color: #f9f9f9;
        }

        .chat-messages {
            flex-grow: 1;
            padding: 20px;
            overflow-y: auto;
            display: flex;
            flex-direction: column;
            gap: 10px;
        }

        .message {
            padding: 10px 15px;
            border-radius: var(--border-radius-bubble);
            max-width: 75%;
            line-height: 1.4;
            word-wrap: break-word;
        }

        .user-message {
            background-color: var(--color-user-message-bg);
            color: var(--color-user-message-text);
            align-self: flex-end;
            border-bottom-right-radius: 5px; /* To match the image's style */
        }

        .system-message {
            background-color: var(--color-system-message-bg);
            color: var(--color-system-message-text);
            align-self: flex-start;
            border-bottom-left-radius: 5px; /* To match the image's style */
        }
        
        .loading-dots {
            display: flex;
            align-items: center;
        }

        .loading-dots span {
            width: 8px;
            height: 8px;
            margin: 0 2px;
            background-color: var(--color-system-message-text);
            opacity: 0.6;
            border-radius: 50%;
            animation: bounce 1.4s infinite ease-in-out both;
        }

        .loading-dots span:nth-child(1) { animation-delay: -0.32s; }
        .loading-dots span:nth-child(2) { animation-delay: -0.16s; }

        @keyframes bounce {
            0%, 80%, 100% { transform: scale(0); }
            40% { transform: scale(1.0); }
        }

        .chat-input-area {
            display: flex;
            padding: 15px 20px;
            border-top: 1px solid var(--color-input-border);
            background-color: var(--color-bg-container); /* Ensure it's on top */
            gap: 10px;
        }

        #messageInput {
            flex-grow: 1;
            padding: 10px 15px;
            border: 1px solid var(--color-input-border);
            border-radius: var(--border-radius-input);
            font-size: 0.9rem;
            outline: none;
        }
        #messageInput:focus {
            border-color: #a0a0a0;
        }

        #sendButton {
            background-color: var(--color-send-button-bg);
            color: var(--color-send-button-icon);
            border: none;
            width: 45px;
            height: 45px;
            border-radius: var(--border-radius-input);
            font-size: 1.5rem; /* Larger for the icon */
            cursor: pointer;
            display: flex;
            justify-content: center;
            align-items: center;
        }
        #sendButton:hover {
            background-color: #757575;
        }

        #sendButton:disabled {
            background-color: #cccccc;
            cursor: not-allowed;
        }

        .error-message {
            background-color: #ffebee;
            color: #c62828;
            border: 1px solid #ffcdd2;
            padding: 10px 15px;
            border-radius: var(--border-radius-bubble);
            align-self: flex-start;
            max-width: 75%;
        }
    </style>
</head>
<body>
    <div class="mobile-container">
        <div class="chat-header">
            <div class="avatar-info">
                <svg xmlns="http://www.w3.org/2000/svg" height="2rem" viewBox="0 0 95 40" fill="none"><path d="M19.9892 39.9686C31.0277 39.9686 39.9762 31.0213 39.9762 19.9843C39.9762 8.94728 31.0277 0 19.9892 0C8.95059 0 0.0020752 8.94728 0.0020752 19.9843C0.0020752 31.0213 8.95059 39.9686 19.9892 39.9686Z" fill="#F4603E"></path><path d="M3.47949 31.257C3.47949 31.257 16.6871 26.9308 39.9651 19.3408C39.9651 19.3408 41.2401 31.7705 28.3541 38.2539C28.3541 38.2539 21.9997 41.2994 15.0284 39.3458C15.0284 39.3458 8.08667 38.0355 3.47949 31.257Z" fill="#1E1870"></path><path d="M58.8644 30.006C57.2803 30.006 55.9338 29.7244 54.825 29.1611C53.7337 28.5979 52.792 27.8587 52 26.9434L54.3497 24.6729C54.9834 25.4121 55.6874 25.9754 56.4619 26.3626C57.2539 26.7498 58.1251 26.9434 59.0756 26.9434C60.1493 26.9434 60.9589 26.7146 61.5045 26.257C62.0502 25.7818 62.323 25.1481 62.323 24.3561C62.323 23.74 62.147 23.2384 61.795 22.8512C61.4429 22.464 60.7829 22.1823 59.8148 22.0063L58.0723 21.7423C54.3937 21.1615 52.5544 19.375 52.5544 16.3828C52.5544 15.5556 52.704 14.8075 53.0033 14.1387C53.3201 13.4699 53.7689 12.8978 54.3497 12.4226C54.9306 11.9474 55.6258 11.5865 56.4355 11.3401C57.2627 11.0761 58.1956 10.9441 59.234 10.9441C60.6245 10.9441 61.839 11.1729 62.8774 11.6305C63.9159 12.0882 64.8047 12.7658 65.544 13.6635L63.1678 15.9076C62.7102 15.3444 62.1558 14.8867 61.5045 14.5347C60.8533 14.1827 60.0349 14.0067 59.0492 14.0067C58.0459 14.0067 57.2891 14.2003 56.7787 14.5875C56.2858 14.9571 56.0394 15.4852 56.0394 16.1716C56.0394 16.8756 56.2418 17.3949 56.6467 17.7293C57.0515 18.0637 57.7027 18.3101 58.6004 18.4685L60.3165 18.7854C62.1822 19.1198 63.5551 19.7182 64.4351 20.5807C65.3328 21.4255 65.7816 22.6136 65.7816 24.1449C65.7816 25.0249 65.6232 25.8258 65.3064 26.5474C65.0071 27.2514 64.5583 27.8675 63.9599 28.3955C63.379 28.9059 62.6574 29.302 61.795 29.5836C60.9501 29.8652 59.9733 30.006 58.8644 30.006Z" fill="var(--color-text-primary)"></path><path d="M70.2257 13.9011C69.5217 13.9011 69.0112 13.7427 68.6944 13.4258C68.3952 13.109 68.2456 12.7042 68.2456 12.2114V11.6833C68.2456 11.1905 68.3952 10.7857 68.6944 10.4689C69.0112 10.1521 69.5217 9.99365 70.2257 9.99365C70.9121 9.99365 71.4138 10.1521 71.7306 10.4689C72.0474 10.7857 72.2058 11.1905 72.2058 11.6833V12.2114C72.2058 12.7042 72.0474 13.109 71.7306 13.4258C71.4138 13.7427 70.9121 13.9011 70.2257 13.9011ZM68.536 15.9076H71.9154V29.6892H68.536V15.9076Z" fill="var(--color-text-primary)"></path><path d="M75.3045 29.6892V15.9076H78.6839V18.2045H78.8159C79.0799 17.4829 79.5023 16.8668 80.0832 16.3564C80.664 15.846 81.4736 15.5908 82.5121 15.5908C83.4625 15.5908 84.281 15.8196 84.9674 16.2772C85.6539 16.7348 86.1643 17.4301 86.4987 18.3629H86.5515C86.7979 17.5885 87.282 16.9372 88.0036 16.4092C88.7428 15.8636 89.6669 15.5908 90.7758 15.5908C92.131 15.5908 93.1695 16.0572 93.8911 16.9901C94.6304 17.9229 95 19.2518 95 20.9767V29.6892H91.6206V21.3199C91.6206 19.3486 90.8814 18.3629 89.4029 18.3629C89.0685 18.3629 88.7428 18.4157 88.426 18.5213C88.1268 18.6093 87.854 18.7502 87.6076 18.9438C87.3788 19.1198 87.194 19.3486 87.0532 19.6302C86.9123 19.8942 86.8419 20.211 86.8419 20.5807V29.6892H83.4625V21.3199C83.4625 19.3486 82.7233 18.3629 81.2448 18.3629C80.928 18.3629 80.6112 18.4157 80.2944 18.5213C79.9951 18.6093 79.7223 18.7502 79.4759 18.9438C79.2471 19.1198 79.0535 19.3486 78.8951 19.6302C78.7543 19.8942 78.6839 20.211 78.6839 20.5807V29.6892H75.3045Z" fill="var(--color-text-primary)"></path></svg>
            </div>
            <button class="add-button" aria-label="Clear chat" onclick="clearChat()">
                <svg fill="#000000" width="32px" height="32px" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg"><path d="M27.1 14.313V5.396L24.158 8.34c-2.33-2.325-5.033-3.503-8.11-3.503C9.902 4.837 4.901 9.847 4.899 16c.001 6.152 5.003 11.158 11.15 11.16 4.276 0 9.369-2.227 10.836-8.478l.028-.122h-3.23l-.022.068c-1.078 3.242-4.138 5.421-7.613 5.421a8 8 0 0 1-5.691-2.359A7.993 7.993 0 0 1 8 16.001c0-4.438 3.611-8.049 8.05-8.049 2.069 0 3.638.58 5.924 2.573l-3.792 3.789H27.1z"></path></svg>
            </button>
        </div>

        <div class="chat-messages" id="chatMessages">
            <!-- Messages will be added here by JavaScript -->
        </div>

        <div class="chat-input-area">
            <input type="text" id="messageInput" placeholder="Ask about wallet balances, transactions, NFTs, or token info...">
            <button id="sendButton" aria-label="Send message">➤</button>
        </div>
    </div>

    <script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/dompurify/dist/purify.min.js"></script>
    <script>
        const chatMessagesContainer = document.getElementById('chatMessages');
        const messageInput = document.getElementById('messageInput');
        const sendButton = document.getElementById('sendButton');

        function addMessage(text, sender, isHtml = false) {
            const messageElement = document.createElement('div');
            messageElement.classList.add('message');
            messageElement.classList.add(sender === 'user' ? 'user-message' : 'system-message');
            
            if (isHtml) {
                messageElement.innerHTML = text;
            } else if (sender === 'system') {
                // Parse markdown for system messages
                const html = marked.parse(text);
                messageElement.innerHTML = DOMPurify.sanitize(html);
            } else {
                messageElement.textContent = text;
            }
            
            chatMessagesContainer.appendChild(messageElement);
            scrollToBottom();
            return messageElement;
        }

        function scrollToBottom() {
            chatMessagesContainer.scrollTop = chatMessagesContainer.scrollHeight;
        }

        function showLoadingIndicator() {
            const loadingHtml = `
                <div class="loading-dots">
                    <span></span>
                    <span></span>
                    <span></span>
                </div>
            `;
            return addMessage(loadingHtml, 'system', true);
        }

        function clearChat() {
            chatMessagesContainer.innerHTML = '';
            loadInitialMessages();
        }

        async function sendMessage() {
            const messageText = messageInput.value.trim();
            if (messageText === '') return;

            // Disable input during processing
            messageInput.disabled = true;
            sendButton.disabled = true;

            // Add user message
            addMessage(messageText, 'user');
            messageInput.value = '';

            // Show loading indicator
            const loadingElement = showLoadingIndicator();

            try {
                // Send to server
                const response = await fetch('/chat', {
                    method: 'POST',
                    headers: {
                        'Content-Type': 'application/json',
                    },
                    body: JSON.stringify({ message: messageText })
                });

                // Remove loading indicator
                chatMessagesContainer.removeChild(loadingElement);

                if (!response.ok) {
                    const errorData = await response.json();
                    throw new Error(errorData.error || 'Server error');
                }

                const data = await response.json();
                
                // Display assistant response
                if (data.message) {
                    addMessage(data.message, 'system');
                }

    } catch (error) {
                if (loadingElement.parentNode) {
                    chatMessagesContainer.removeChild(loadingElement);
                }
                
                console.error('Error:', error);
                addMessage(`Error: ${error.message}`, 'error');
            } finally {
                messageInput.disabled = false;
                sendButton.disabled = false;
                messageInput.focus();
            }
        }

        // Event listeners
        sendButton.addEventListener('click', sendMessage);
        messageInput.addEventListener('keypress', function(event) {
            if (event.key === 'Enter' && !messageInput.disabled) {
                sendMessage();
            }
        });

        // Initial welcome messages
        function loadInitialMessages() {
            addMessage("Hi! I'm your Sim APIs assistant. I can help you explore blockchain data across 60+ EVM chains and Solana.", "system");
            
            setTimeout(() => {
                addMessage("Try asking me about:\n\n- Token balances for any wallet\n- Transaction history\n- NFT collections\n- Token information and pricing\n- Solana token data", "system");
            }, 500);
        }

        // Load initial messages on page load
        document.addEventListener('DOMContentLoaded', loadInitialMessages);
    </script>
</body>
</html>
5

Verify Project Structure

Your project structure should now look like:
simchat/
├── server.js          # Express server with OpenAI integration
├── chat.html          # Chat interface
├── package.json       # Project configuration
├── .env               # API keys (keep private)
└── node_modules/      # Dependencies
Run node server.js in the terminal to start the server. Visit http://localhost:3001 to see the newly scaffolded chat app.

Our newly created chat front-end UI is ready.

If you try to send a message at this point, you’ll get a server error since we haven’t implemented the back-end functionality yet.
If you encounter errors, make sure your .env file contains the correct OPENAI_API_KEY and SIM_API_KEY. Check your terminal for any error messages from server.js.

​
Add OpenAI LLM Chat

Now let’s add the core chat functionality to our Express server using OpenAI’s GPT-4o-mini. We’ll start by defining a system prompt that instructs the LLM on its role and capabilities. Add this SYSTEM_PROMPT variable to your server.js file:
(server.js)
// System prompt that instructs the AI assistant
const SYSTEM_PROMPT = `You are a helpful assistant that can answer questions about blockchain data using Dune's Sim APIs. You have access to various functions that can fetch realtime blockchain data including:

- Token balances for wallets across 60+ EVM chains
- Transaction activity and history
- NFT collections and collectibles
- Token metadata and pricing information
- Token holder distributions
- Supported blockchain networks

When users ask about blockchain data, wallet information, token details, or transaction history, use the appropriate functions to fetch realtime data. Always provide clear, helpful explanations of the data you retrieve.

Keep your responses concise and focused. When presenting large datasets, summarize the key findings rather than listing every detail.`;
This system prompt sets the context for the LLM, explaining its capabilities and how it should behave when interacting with users. Now let’s implement the basic chat endpoint with Express.js that uses this system prompt. The /chat endpoint will receive POST requests from our frontend chat interface, process them through the LLM, and return responses to display in the chat: 
// Basic chat endpoint
app.post('/chat', async (req, res) => {
    try {
        const { message } = req.body;
        if (!message) return res.status(400).json({ error: 'Message is required' });

        // Create conversation with system prompt
        const messages = [
            { role: "system", content: SYSTEM_PROMPT },
            { role: "user", content: message }
        ];

        // Call OpenAI
        const response = await openai.chat.completions.create({
            model: "gpt-4o-mini",
            messages: messages,
            max_tokens: 2048
        });

        const assistantMessage = response.choices[0].message.content;

        res.json({ 
            message: assistantMessage
        });

    } catch (error) {
        console.error('Chat error:', error);
        res.status(500).json({ 
            error: 'An error occurred while processing your request',
            details: error.message 
        });
    }
});
Run node server.js again and visit http://localhost:3001. You’ll have a working chat interface powered by OpenAI’s gpt-4o-mini model with a custom system prompt, but it won’t be able to fetch realtime blockchain data yet.

The chat is now working with OpenAI responses, but not yet fetching blockchain data

​
Define OpenAI Functions

To make our chatbot fetch realtime blockchain data, we need to use OpenAI’s function calling feature. When the model determines it needs external data, it will call one of these functions with appropriate parameters, and we can then execute the actual API call and provide the results back to the model. Add this functions array to your server.js file: 
// Function definitions for OpenAI function calling
const functions = [
    {
        type: "function",
        function: {
            name: "get_token_balances",
            description: "Get realtime token balances for an EVM wallet address across multiple chains. Returns native and ERC20 token balances with USD values.",
            parameters: {
                type: "object",
                properties: {
                    address: {
                        type: "string",
                        description: "The wallet address to get balances for (e.g., 0xd8da6bf26964af9d7eed9e03e53415d37aa96045)"
                    },
                        description: "Whether to exclude spam tokens from results",
                        default: true
                    }
                },
                required: ["address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_wallet_activity",
            description: "Get chronologically ordered transaction activity for an EVM wallet including transfers, contract interactions, and decoded function calls.",
            parameters: {
                type: "object",
                properties: {
                    address: {
                        type: "string",
                        description: "The wallet address to get activity for"
                    },
                    limit: {
                        type: "number",
                        description: "Maximum number of activities to return (default: 25)",
                        default: 25
                    }
                },
                required: ["address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_nft_collectibles",
            description: "Get NFT collectibles (ERC721 and ERC1155) owned by an EVM wallet address.",
            parameters: {
                type: "object",
                properties: {
                    address: {
                        type: "string",
                        description: "The wallet address to get NFTs for"
                    },
                    limit: {
                        type: "number",
                        description: "Maximum number of collectibles to return (default: 50)",
                        default: 50
                    }
                },
                required: ["address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_token_info",
            description: "Get detailed metadata and pricing information for a specific token on EVM chains.",
            parameters: {
                type: "object",
                properties: {
                    token_address: {
                        type: "string",
                        description: "The token contract address or 'native' for native tokens"
                    },
                    chain_ids: {
                        type: "string",
                        description: "Chain IDs to search on (e.g., '1,137,8453' or 'all')",
                        default: "all"
                    }
                },
                required: ["token_address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_token_holders",
            description: "Get token holders for a specific ERC20 or ERC721 token, ranked by wallet value.",
            parameters: {
                type: "object",
                properties: {
                    chain_id: {
                        type: "number",
                        description: "The chain ID where the token exists (e.g., 1 for Ethereum)"
                    },
                    token_address: {
                        type: "string",
                        description: "The token contract address"
                    },
                    limit: {
                        type: "number",
                        description: "Maximum number of holders to return (default: 100)",
                        default: 100
                    }
                },
                required: ["chain_id", "token_address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_transactions",
            description: "Get detailed transaction information for an EVM wallet address.",
            parameters: {
                type: "object",
                properties: {
                    address: {
                        type: "string",
                        description: "The wallet address to get transactions for"
                    },
                    limit: {
                        type: "number",
                        description: "Maximum number of transactions to return (default: 25)",
                        default: 25
                    }
                },
                required: ["address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_supported_chains",
            description: "Get list of all supported EVM chains and their capabilities.",
            parameters: {
                type: "object",
                properties: {},
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_svm_token_balances",
            description: "Get token balances for a Solana (SVM) address. Returns native and SPL token balances with USD values.",
            parameters: {
                type: "object",
                properties: {
                    address: {
                        type: "string",
                        description: "The Solana wallet address to get balances for (e.g., DYw8jCTfwHNRJhhmFcbXvVDTqWMEVFBX6ZKUmG5CNSKK)"
                    },
                    limit: {
                        type: "number",
                        description: "Maximum number of balances to return (default: 100)",
                        default: 100
                    },
                    chains: {
                        type: "string",
                        description: "Comma-separated list of chains to include",
                        default: "all"
                    }
                },
                required: ["address"],
                additionalProperties: false
            }
        }
    },
    {
        type: "function",
        function: {
            name: "get_svm_token_metadata",
            description: "Get metadata for a Solana token mint address.",
            parameters: {
                type: "object",
                properties: {
                    mint: {
                        type: "string",
                        description: "The Solana token mint address (e.g., So11111111111111111111111111111111111111112)"
                    }
                },
                required: ["mint"],
                additionalProperties: false
            }
        }
    }
];
Each function corresponds to a different Sim API endpoint that we’ll implement next.

​
Integrate Sim APIs

Now we need to connect OpenAI’s function calls to actual Sim API requests. When the model calls a function like get_token_balances, we need to:
  1. Map that OpenAI function call to the correct Sim API endpoint
  2. Make the HTTP request with proper authentication
  3. Return the data back to the model
We’ll implement this with three components: a generic API caller, endpoint configurations, and an execution function that ties them together.

​
Build the Generic API Caller

First, let’s create a reusable function that handles all HTTP requests to Sim APIs: 
// Generic API call function for Sim APIs
async function apiCall(endpoint, params = {}) {
    try {
        const queryString = Object.keys(params).length 
            ? '?' + new URLSearchParams(params).toString() 
            : '';
        
        const response = await fetch(`https://api.sim.dune.com${endpoint}${queryString}`, {
            headers: {
                'X-Sim-Api-Key': SIM_API_KEY,
                'Content-Type': 'application/json'
            }
        });
        
        if (!response.ok) throw new Error(`API request failed: ${response.statusText}`);
        return await response.json();
    } catch (error) {
        return { error: error.message };
    }
}
This function handles all the common functionality needed for Sim API requests:
  • URL construction with query parameters
  • Authentication headers
  • Error handling
  • JSON parsing
By centralizing this logic, we avoid code duplication and ensure consistent error handling across all API calls.

​
Configure API Endpoints

Next, we’ll create a more comprehensive configuration object that handles all the different parameter patterns used by Sim APIs: 
// API endpoint configurations
const API_CONFIGS = {
    get_token_balances: (address) => {
        const queryParams = new URLSearchParams({ metadata: 'url,logo' });
        return [`/v1/evm/balances/${address}`, queryParams];
    },
    
    get_wallet_activity: (address, limit = 25) => 
        [`/v1/evm/activity/${address}`, { limit: Math.min(limit, 10) }],
    
    get_nft_collectibles: (address, limit = 50) => 
        [`/v1/evm/collectibles/${address}`, { limit: Math.min(limit, 10) }],
    
    get_token_info: (token_address, chain_ids = 'all') => 
        [`/v1/evm/token-info/${token_address}`, { chain_ids }],
    
    get_token_holders: (chain_id, token_address, limit = 100) => 
        [`/v1/evm/token-holders/${chain_id}/${token_address}`, { limit: Math.min(limit, 10) }],
    
    get_transactions: (address, limit = 25) => 
        [`/v1/evm/transactions/${address}`, { limit: Math.min(limit, 10) }],
    
    get_supported_chains: () => 
        ['/v1/evm/supported-chains', {}],
    
    get_svm_token_balances: (address, limit = 100, chains = 'all') => {
        const queryParams = new URLSearchParams();
        if (chains) queryParams.append('chains', chains);
        if (limit) queryParams.append('limit', Math.min(limit, 20));
        return [`/beta/svm/balances/${address}`, queryParams];
    },
    
    get_svm_token_metadata: (mint) => 
        [`/beta/svm/token-metadata/${mint}`, {}]
};
This configuration handles all the different patterns of Sim APIs: simple objects for basic query parameters, URLSearchParams for complex query strings, multiple path parameters, and endpoints with no parameters.

​
Execute Function Calls

Now we need an enhanced callFunction that can handle both regular objects and URLSearchParams: 
// Function to execute API calls based on function name
async function callFunction(name, args) {
    if (!API_CONFIGS[name]) return JSON.stringify({ error: `Unknown function: ${name}` });
    
    const [endpoint, params] = API_CONFIGS[name](...Object.values(args));
    const result = await apiCall(endpoint, params);
    return JSON.stringify(result);
}
This approach maintains the streamlined API_CONFIGS pattern while properly handling all the different parameter types and patterns used by the various Sim API endpoints. The apiCall function can handle both URLSearchParams objects (for complex queries) and regular objects (for simple query parameters).

​
Update the Chat Endpoint

Finally, we need to update our chat endpoint to handle function calls. Replace your existing /chat endpoint with this version that includes function calling support: 
// Enhanced chat endpoint with function calling
app.post('/chat', async (req, res) => {
    try {
        const { message } = req.body;
        if (!message) return res.status(400).json({ error: 'Message is required' });

        // Create conversation with system prompt
        const messages = [
            { role: "system", content: SYSTEM_PROMPT },
            { role: "user", content: message }
        ];

        // Call OpenAI with function definitions
        const response = await openai.chat.completions.create({
            model: "gpt-4o-mini",
            messages: messages,
            tools: functions,
            tool_choice: "auto",
            max_tokens: 2048
        });

        let assistantMessage = response.choices[0].message;
        
        // Handle function calls if present
        if (assistantMessage.tool_calls) {
            messages.push(assistantMessage);
            
            // Execute each function call
            for (const toolCall of assistantMessage.tool_calls) {
                const functionResult = await callFunction(
                    toolCall.function.name, 
                    JSON.parse(toolCall.function.arguments)
                );
                
                messages.push({
                    role: "tool",
                    tool_call_id: toolCall.id,
                    content: functionResult
                });
            }
            
            // Get final response with function results
            const finalResponse = await openai.chat.completions.create({
                model: "gpt-4o-mini",
                messages: messages,
                tools: functions,
                tool_choice: "auto",
                max_tokens: 2048
            });
            
            assistantMessage = finalResponse.choices[0].message;
        }

        res.json({ 
            message: assistantMessage.content,
            function_calls: assistantMessage.tool_calls || []
        });

    } catch (error) {
        console.error('Chat error:', error);
        res.status(500).json({ 
            error: 'An error occurred while processing your request',
            details: error.message 
        });
    }
});
This enhanced endpoint now supports the full function calling workflow: it sends the user’s message to OpenAI with the available functions, executes any function calls that the model makes, and then sends the function results back to get the final conversational response. Restart your server and test the function calling functionality. Try asking questions like What tokens does vitalik.eth have? and watch as your chat agent fetches realtime data from Sim APIs to provide accurate, up-to-date responses.

​
Conclusion

You’ve successfully built a realtime chat agent that makes blockchain data accessible through natural conversation. By combining OpenAI’s LLMs with Sim APIs’ comprehensive blockchain data, you’ve created a tool that can instantly fetch and explain complex onchain information across 60+ EVM chains and Solana. This foundation provides everything you need to build your own specialized blockchain chat assistants. Consider extending it for specific use cases like:
  • NFT Discovery Bot: Integrate marketplace data, rarity rankings, and collection insights
  • Portfolio Manager: Include transaction categorization, P&L tracking, and tax reporting features
  • Trading Assistant: Add price alerts, technical indicators, and market sentiment analysis
The complete source code on GitHub includes additional features like full session management and enhanced error handling that weren’t covered in this guide Explore the repository to see the additional features in action.