How to Build Your Own Replit Alternative with AI Agents

Cloud development environments are everywhere now. Replit, GitHub Codespaces, Gitpod, CodeSandbox - they all let you write, run, and deploy code from a browser without installing anything locally.

What if you could build your own? Not as a weekend project, but a real product with instant environments, AI coding assistance, and per-user isolation.

This guide walks through the full architecture.

Why Build Your Own

Three reasons teams build their own cloud IDE:

1. Educational platforms - coding bootcamps and schools need controlled environments where students can't break things or access each other's work
2. Internal tools - companies want developers to code in managed environments with pre-configured access to internal services
3. Product features - SaaS products embed code editing for users (think: automation builders, data pipelines, webhook editors)

You don't need to build VS Code from scratch. The pieces already exist - you're assembling them with the right infrastructure.

The Core Components

┌─────────────────────────────────────┐
│          Your Web App                │
│  ┌──────────┐  ┌──────────────────┐ │
│  │  Monaco   │  │  Terminal        │ │
│  │  Editor   │  │  (xterm.js)      │ │
│  └──────┬───┘  └────────┬─────────┘ │
│         │               │            │
│         └───────┬───────┘            │
│                 │ WebSocket          │
└─────────────────┼────────────────────┘
                  │
      ┌───────────┴───────────┐
      │   User's Workspace     │
      │   (Firecracker microVM)│
      │                        │
      │   - Full Linux env     │
      │   - Language runtimes  │
      │   - User's files       │
      │   - AI agent (optional)│
      └────────────────────────┘

Frontend: Monaco editor (the engine behind VS Code) + xterm.js terminal emulator, both running in the browser.

Backend: Each user gets their own isolated workspace - a full Linux VM with their files, installed packages, and running processes.

Connection: WebSocket bridges the browser to the workspace for terminal I/O, file operations, and real-time sync.

Step 1: The Code Editor

Use Monaco Editor - it's the same engine that powers VS Code, and it's open source. It gives you:

• Syntax highlighting for every major language
• IntelliSense (autocomplete)
• Multi-cursor editing
• Find and replace
• Minimap
• Themes (including VS Code themes)

Embed it in your React/Next.js app. Configure it to read and write files through your backend API.

When the user edits a file, your frontend sends the change to the workspace's filesystem. When a file changes on disk (e.g., npm install modifies package-lock.json), you push the update to the editor.

Step 2: The Terminal

xterm.js gives you a terminal emulator in the browser. Connect it to a PTY (pseudo-terminal) running in the user's workspace over WebSocket.

The user types npm run dev, xterm.js sends keystrokes over WebSocket, the workspace's shell processes them, and output streams back in real-time.

PTY support is built into Oblien workspaces - you create a terminal session via the SDK, and it returns a WebSocket URL. Attach xterm.js to that WebSocket, and you have a fully interactive terminal with:

• Color support (ANSI 256-color)
• Mouse events
• Window resize handling
• Copy/paste
• Unicode/emoji support

Step 3: Per-User Workspaces

This is where it gets interesting. Each user needs their own environment - their own filesystem, installed packages, running processes.

When a user starts a new project:

1. Create a workspace - select the base image (Node.js, Python, Go, etc.)
2. Clone their repo (if applicable) - git clone inside the workspace
3. Install dependencies - npm install, pip install -r requirements.txt, etc.
4. Start the project - npm run dev, python app.py, etc.
5. Expose a preview URL - the user's dev server gets a public URL

Each workspace is a full Linux VM. The user can install anything, run anything, modify any system configuration - none of it affects other users.

Step 4: Preview URLs

When the user runs a dev server (e.g., Next.js on port 3000), expose that port as a public URL.

The user sees https://a1b2c3-3000.preview.oblien.com - their running app, accessible from any browser. No port forwarding. No ngrok. No DNS configuration.

Update the preview URL in real-time as code changes - hot reload works through the preview URL because it's a direct connection to the workspace's dev server.

Step 5: Adding AI Assistance

This is what makes your cloud IDE better than Replit. Run an AI coding agent inside each workspace.

The AI agent has:

• Full filesystem access (reads and writes files)
• Terminal access (runs commands, sees output)
• Context awareness (understands the project structure)
• Conversation memory (remembers previous interactions per session)

When the user asks "add a login page with Google OAuth," the agent:

1. Reads the project structure
2. Installs necessary packages (next-auth, Google OAuth adapter)
3. Creates the auth configuration file
4. Builds the login page component
5. Updates the relevant routes
6. Tests the build

The user watches the agent work in real-time - files appearing in the editor, commands running in the terminal.

File Sync Architecture

The trickiest engineering challenge: keeping the browser editor in sync with the workspace filesystem.

Option A: Poll-based

Every 2 seconds, check for file changes. Simple but laggy.

Option B: File watchers (recommended)

Run inotify inside the workspace. When a file changes on disk:

1. The file watcher detects the change
2. It sends a notification over WebSocket
3. Your frontend updates the editor and file tree

When the user edits in the browser:

1. The editor sends the change over WebSocket
2. Your backend writes the change to the file in the workspace
3. Debounce to avoid writing on every keystroke (save after 300ms of no typing)

Option C: CRDT-based (for collaboration)

If multiple users can edit the same workspace, use Yjs or Automerge for conflict-free real-time editing. This is what most collaborative editors use.

Making It Fast: Workspace Lifecycle

Cold start (first time)

User creates a project → new workspace boots in ~130ms → dependencies install (10-60 seconds depending on project) → ready.

Warm resume (returning user)

User's workspace was paused → resume in ~2 seconds → everything is exactly where they left it. Running processes, installed packages, terminal history - all preserved because the VM state was frozen, not destroyed.

Hot start (active user)

User switches tabs and comes back → instant, the workspace never stopped.

Lifecycle policy

• Active: workspace runs normally
• Idle (no activity for 30 min): workspace pauses (no charge)
• User returns: workspace resumes in seconds
• Inactive (no activity for 7 days): workspace snapshots and deletes
• User returns: workspace restores from snapshot in ~5 seconds

The Cost Model

Per-user workspaces sound expensive, but they're not:

• Active coding: ~$0.001/minute for a 2 CPU, 2 GB workspace
• Paused: near-zero cost (just disk storage)
• Deleted with snapshot: minimal storage cost

For a user who codes 4 hours/day:

• Active: 240 min × $0.001 = $0.24/day
• Paused: 20 hours × near-zero = ~$0.00
• Total: ~$7/month per user

Compare that to a $20-40/month Codespaces or Gitpod plan. You have headroom to offer a compelling price while maintaining margins.

What You End Up With

A cloud development environment where:

The Build Order

Start simple, expand later:

1. Week 1 - Monaco editor + xterm.js terminal, connected to a single workspace
2. Week 2 - Per-user workspace creation and lifecycle management
3. Week 3 - File tree, file sync, preview URLs
4. Week 4 - AI agent integration
5. Week 5 - Pause/resume, workspace templates, polished UX
6. Week 6 - Billing, user management, team features

The infrastructure layer (workspace creation, terminal, file operations, preview URLs) is handled by Oblien's SDK. Your focus is the frontend UX and the product features that differentiate you.

Related reading → Cloud Dev Environments Guide | Oblien Documentation