Replay: The Fastest Way to Rebuild Legacy Apps from Video (2026)

TL;DR: Replay revolutionizes legacy app modernization by using video analysis to reconstruct working UI code, saving countless hours compared to manual methods.

Replay: The Fastest Way to Rebuild Legacy Apps from Video (2026)#

Legacy applications. We all have them. Cobwebs of code, undocumented features, and the constant fear that a single change will bring the whole thing crashing down. Modernizing these monoliths is a daunting task, often involving reverse-engineering brittle UIs. What if you could simply show the system what you want, and it would build it for you? That's the promise of Replay.

Rebuilding legacy applications is a common pain point in software development. Teams struggle with outdated codebases, lack of documentation, and the complexity of understanding existing functionality. The traditional approach involves manual analysis, reverse engineering, and extensive coding, which is time-consuming, error-prone, and expensive. Replay changes the game by automating much of this process.

The Problem: Legacy App Modernization is a Black Hole#

Traditional approaches to modernizing legacy applications are notoriously slow and inefficient. They often involve:

• •Manual Code Review: Sifting through mountains of undocumented code.
• •Reverse Engineering: Trying to understand how existing features work.
• •UI Recreation: Rebuilding user interfaces from scratch, often based on outdated designs or guesswork.
• •Extensive Testing: Ensuring that the new application replicates the functionality of the old one.

These processes are not only time-consuming but also prone to errors, leading to delays and increased costs. Furthermore, the lack of understanding of the original application's intent can result in a modernized application that doesn't fully meet the needs of the users.

The Replay Solution: Behavior-Driven Reconstruction#

Replay offers a fundamentally different approach. Instead of relying on static screenshots or manual code analysis, Replay analyzes video recordings of users interacting with the legacy application. By understanding the user's behavior and intent, Replay can automatically reconstruct the UI and generate working code. This "Behavior-Driven Reconstruction" provides several key advantages:

• •Speed: Rebuild UIs in a fraction of the time compared to manual methods.
• •Accuracy: Capture the nuances of user interactions and ensure that the new application replicates the functionality of the old one.
• •Completeness: Reconstruct entire product flows, including multi-page interactions and complex workflows.

Replay leverages the power of Gemini to understand user intent from video, effectively turning screen recordings into functional code.

How Replay Works: A Step-by-Step Guide#

Let's walk through a practical example of using Replay to rebuild a simple legacy application: a basic to-do list app.

Step 1: Recording the User Flow

First, record a video of yourself using the to-do list application. Make sure to demonstrate all the key features, such as:

1. •Adding a new task
2. •Marking a task as complete
3. •Deleting a task

The more comprehensive the video, the better Replay can understand the application's functionality.

Step 2: Uploading to Replay

Upload the video to the Replay platform. Replay will automatically analyze the video and identify the different UI elements and user interactions.

Step 3: Code Generation

Replay uses its AI engine to generate the code for the new UI. You can choose from various frameworks and languages, such as React, Vue, or Angular.

typescript
// Example React component generated by Replay
import React, { useState } from 'react';

interface Todo {
  id: number;
  text: string;
  completed: boolean;
}

const TodoList: React.FC = () => {
  const [todos, setTodos] = useState<Todo[]>([]);
  const [newTodo, setNewTodo] = useState<string>('');

  const addTodo = () => {
    if (newTodo.trim() !== '') {
      setTodos([...todos, { id: Date.now(), text: newTodo, completed: false }]);
      setNewTodo('');
    }
  };

  const toggleComplete = (id: number) => {
    setTodos(
      todos.map((todo) =>
        todo.id === id ? { ...todo, completed: !todo.completed } : todo
      )
    );
  };

  const deleteTodo = (id: number) => {
    setTodos(todos.filter((todo) => todo.id !== id));
  };

  return (
    <div>
      <h1>Todo List</h1>
      <input
        type="text"
        value={newTodo}
        onChange={(e) => setNewTodo(e.target.value)}
      />
      <button onClick={addTodo}>Add</button>
      <ul>
        {todos.map((todo) => (
          <li key={todo.id}>
            <input
              type="checkbox"
              checked={todo.completed}
              onChange={() => toggleComplete(todo.id)}
            />
            <span style={{ textDecoration: todo.completed ? 'line-through' : 'none' }}>
              {todo.text}
            </span>
            <button onClick={() => deleteTodo(todo.id)}>Delete</button>
          </li>
        ))}
      </ul>
    </div>
  );
};

export default TodoList;

Step 4: Customization and Integration

The generated code is fully customizable. You can modify the UI, add new features, and integrate it with your existing backend systems. Replay also supports seamless integration with Supabase for data storage and management.

Replay Features in Detail#

Replay offers a range of features designed to streamline the legacy app modernization process:

• •Multi-Page Generation: Reconstruct complex applications with multiple pages and workflows.
• •Supabase Integration: Easily connect to a Supabase backend for data storage and authentication.
• •Style Injection: Customize the look and feel of the generated UI with CSS or other styling frameworks.
• •Product Flow Maps: Visualize the user flows within the application to better understand the user experience.

Replay vs. Traditional Methods#

💡 Pro Tip: For best results, ensure your video recordings are clear and well-lit. Speak clearly while demonstrating the application's features to help Replay accurately understand your intent.

📝 Note: Replay is constantly evolving with new features and improvements. Check the documentation regularly for the latest updates.

⚠️ Warning: While Replay automates much of the reconstruction process, some manual customization may still be required, especially for complex applications.

The Future of Legacy App Modernization#

Replay is not just a tool; it's a paradigm shift in how we approach legacy app modernization. By leveraging the power of video analysis and AI, Replay empowers developers to rebuild applications faster, more accurately, and with a deeper understanding of the user experience. As AI technology continues to evolve, Replay will become an even more powerful tool for modernizing legacy applications and unlocking the value trapped within outdated codebases.

Benefits of Using Replay#

• •Reduced Development Time: Automate the UI reconstruction process and significantly reduce development time.
• •Improved Accuracy: Capture the nuances of user interactions and ensure that the new application replicates the functionality of the old one.
• •Enhanced User Experience: Gain a deeper understanding of the user experience and create a modernized application that meets the needs of the users.
• •Lower Costs: Reduce the costs associated with manual analysis, reverse engineering, and extensive coding.
• •Increased Agility: Quickly adapt to changing business requirements and modernize applications more frequently.

Frequently Asked Questions#

Is Replay free to use?#

Replay offers a free tier with limited features and usage. Paid plans are available for more advanced features and higher usage limits. Check the pricing page for details.

How is Replay different from v0.dev?#

v0.dev primarily uses text prompts to generate UI components. Replay, on the other hand, analyzes video recordings of user interactions to understand behavior and reconstruct entire applications. Replay focuses on replicating existing functionality, while v0.dev is more focused on generating new UIs from scratch.

What frameworks and languages does Replay support?#

Replay currently supports React, Vue, and Angular. We are continuously adding support for new frameworks and languages based on user demand.

How secure is Replay?#

Replay uses industry-standard security measures to protect your data. All video recordings are encrypted and stored securely. We also comply with all relevant data privacy regulations.

What kind of video quality is required for Replay to work effectively?#

While Replay can handle various video qualities, clear and well-lit recordings generally produce the best results. Make sure the UI elements are clearly visible and the user interactions are easily discernible.

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