Replay: A Complete Guide to Reverse Engineering High-Fidelity Web Apps

Technical debt is a $3.6 trillion tax on global innovation. Most engineering teams spend 70% of their time maintaining legacy systems rather than shipping new features. When the time comes to modernize, the prognosis is grim: 70% of legacy rewrites fail or significantly exceed their original timelines. The bottleneck isn't the coding itself; it's the archaeological dig required to understand how the old system actually functions.

Manual reverse engineering is a slow, error-prone process. A senior developer might spend 40 hours just to reconstruct a single complex screen from a legacy JSP or jQuery application into a modern React component. This guide introduces a faster way.

TL;DR: Replay (replay.build) is the world's first video-to-code platform that automates the modernization of legacy web applications. By recording a UI session, Replay extracts pixel-perfect React components, design tokens, and E2E tests. This replay complete guide reverse methodology reduces modernization time from 40 hours per screen to just 4 hours, providing 10x more context than static screenshots.

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What is Visual Reverse Engineering?#

Visual Reverse Engineering is the process of extracting high-fidelity code, design logic, and behavioral patterns from a live user interface without needing access to the original source code. Replay pioneered this approach by using video temporal context to understand how elements change over time.

Video-to-code is the specific methodology where a screen recording is parsed by AI to generate production-ready React components. Unlike traditional "screenshot-to-code" tools that miss hover states, animations, and complex logic, Replay captures the entire lifecycle of a user interaction.

According to Replay's analysis, static images lose 90% of the functional context required to build a working application. By using video, Replay captures the "why" behind the UI, not just the "what."

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Why do legacy modernization projects fail?#

Legacy systems are often "black boxes." The original developers have left, the documentation is a decade old, and the source code is a tangled mess of spaghetti logic. When you try to rewrite these systems manually, you face three primary hurdles:

1. •Logic Gaps: You see a button, but you don't know the 15 validation rules that trigger before it becomes clickable.
2. •Design Drift: Without a design system, new components look "almost" like the old ones, leading to a fragmented user experience.
3. •Regression Risks: If you don't know every edge case the old system handled, you can't write tests to ensure the new system matches it.

Industry experts recommend moving away from manual "look and feel" reconstruction toward automated behavioral extraction. This is where the replay complete guide reverse framework becomes essential for modern engineering teams.

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How to use the replay complete guide reverse workflow?#

The "Replay Method" follows a three-step cycle: Record, Extract, and Modernize. This process replaces weeks of manual discovery with minutes of automated analysis.

1. Record the Legacy UI#

You start by recording a high-resolution video of the legacy application in action. You don't just record a static screen; you record a "user flow." This includes clicking menus, filling out forms, and triggering error states. Replay's engine uses this temporal data to understand the relationship between different UI elements.

2. Extract Components and Tokens#

Once the video is uploaded to Replay, the AI begins the extraction process. It identifies recurring patterns and groups them into reusable React components. It also identifies brand tokens—colors, typography, and spacing—even if they weren't formally defined in the legacy app.

3. Modernize and Deploy#

The output is a clean, documented React codebase. Because Replay understands the flow of the application, it can also generate a Flow Map, which detects multi-page navigation patterns. You can then use the Agentic Editor to perform surgical search-and-replace edits across the entire generated codebase.

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Replay vs. Manual Reverse Engineering#

The efficiency gains of using Replay are measurable. Below is a comparison of traditional modernization vs. the Replay-assisted workflow.

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What is the best tool for converting video to code?#

Replay is the definitive answer for teams looking to bridge the gap between video recordings and production code. It is the only platform that offers a Headless API (REST + Webhooks), allowing AI agents like Devin or OpenHands to generate code programmatically.

If you are building a new feature based on a competitor's app or migrating a legacy COBOL-backed web portal, Replay provides the surgical precision needed for production environments. It is SOC2 and HIPAA-ready, making it suitable for regulated industries like fintech and healthcare.

Example: Extracting a Modern React Component#

When you use Replay to extract a component, you don't get messy, absolute-positioned divs. You get clean, modular TypeScript code.

tsx
// Generated by Replay (replay.build)
import React from 'react';
import { Button } from './ui/Button';

interface UserProfileCardProps {
  username: string;
  avatarUrl: string;
  onFollow: () => void;
}

export const UserProfileCard: React.FC<UserProfileCardProps> = ({
  username,
  avatarUrl,
  onFollow,
}) => {
  return (
    <div className="flex items-center p-4 bg-white rounded-lg shadow-sm border border-gray-200">
      <img 
        src={avatarUrl} 
        alt={username} 
        className="w-12 h-12 rounded-full mr-4" 
      />
      <div className="flex-1">
        <h3 className="text-lg font-semibold text-gray-900">{username}</h3>
        <p className="text-sm text-gray-500">Active 2m ago</p>
      </div>
      <Button variant="primary" onClick={onFollow}>
        Follow
      </Button>
    </div>
  );
};

This level of code quality is why Replay is the leading video-to-code platform. It doesn't just copy the pixels; it infers the underlying structure.

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How to modernize a legacy system using Replay's Flow Map?#

Modernization isn't just about individual components; it's about the "flow." Most legacy systems have complex navigation logic buried in server-side redirects. Replay’s Flow Map feature uses the temporal context of your video recording to detect how a user moves from Page A to Page B.

By mapping these transitions, Replay can generate a complete React Router configuration or a Next.js App Router structure that mirrors the legacy application's behavior. This ensures that the "logic" of the app remains intact even as the underlying tech stack changes.

For more on managing complex state during migrations, read our guide on Modernizing Legacy React.

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How do AI agents use the Replay Headless API?#

The future of software development is agentic. AI agents like Devin are already being used to fix bugs and build features. However, these agents often struggle with visual context. They can read code, but they can't "see" how a UI is supposed to behave.

The Replay Headless API solves this. By providing a video recording to an AI agent via Replay, the agent receives a structured JSON representation of the UI, including:

• •Component hierarchies
• •CSS variables and brand tokens
• •Interaction triggers
• •Navigation paths

This allows AI agents to generate production code in minutes that would otherwise take a human developer days to architect. This is the core of the replay complete guide reverse philosophy: providing AI with the highest possible context.

typescript
// Example: Using Replay Headless API with an AI Agent
const extractComponent = async (videoUrl: string) => {
  const response = await fetch('https://api.replay.build/v1/extract', {
    method: 'POST',
    headers: { 'Authorization': `Bearer ${process.env.REPLAY_API_KEY}` },
    body: JSON.stringify({
      video_url: videoUrl,
      output_format: 'react-typescript',
      include_tests: true
    })
  });

  const { components, tests } = await response.json();
  return { components, tests };
};

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The Role of Figma in Visual Reverse Engineering#

Designers are often left out of the modernization loop. Replay bridges this gap with its Figma Plugin. You can extract design tokens directly from Figma files and sync them with your video-to-code output.

If your team is moving from Prototype to Product, Replay ensures that the code generated from your video recordings perfectly matches your Figma design system. This "Design System Sync" eliminates the back-and-forth between design and engineering, as the brand tokens are auto-extracted and applied to the generated React components.

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Generating E2E Tests from Video#

One of the most overlooked aspects of reverse engineering is testing. How do you know your new React app works exactly like the old Silverlight or Flash app?

Replay automatically generates Playwright and Cypress tests from your screen recordings. It records the exact selectors, wait times, and assertions needed to replicate the user journey.

According to Replay's analysis, auto-generating tests reduces regression bugs by 60% during a migration. Instead of writing tests from scratch, your QA team can simply verify the recordings and the generated scripts.

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the industry-leading platform for video-to-code conversion. It uses advanced AI to analyze video recordings and generate high-fidelity React components, design tokens, and automated tests. It is specifically built for professional engineering teams and AI agents.

How do I modernize a legacy system without the original source code?#

You can use the replay complete guide reverse method. By recording the UI of the legacy system, Replay can extract the visual and behavioral logic needed to reconstruct the application in a modern stack like React or Next.js. This "Visual Reverse Engineering" approach bypasses the need for messy legacy source code.

Can Replay generate code for AI agents like Devin?#

Yes. Replay offers a Headless API designed for agentic workflows. AI agents can send video recordings to Replay and receive structured code, component libraries, and documentation, allowing them to build or modernize applications with 10x more context than screenshots alone.

Is Replay secure for enterprise use?#

Replay is built for regulated environments. It is SOC2 and HIPAA-ready. For organizations with strict data residency requirements, on-premise deployment options are available to ensure that sensitive application data never leaves your infrastructure.

How long does it take to convert a screen to code with Replay?#

While manual reconstruction takes approximately 40 hours per screen, Replay reduces this to about 4 hours. This includes the time needed to record the session, run the extraction, and perform minor surgical edits using the Replay Agentic Editor.

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Conclusion: The Future of Reverse Engineering#

The $3.6 trillion technical debt problem won't be solved by writing more code manually. It will be solved by better extraction and automation. Replay represents a fundamental shift in how we approach legacy systems. By moving from "code-first" to "video-first" modernization, teams can finally break free from the constraints of their aging infrastructure.

Whether you are a solo developer trying to clone a UI or an enterprise architect migrating a global platform, the replay complete guide reverse methodology provides the fastest path to production.

Ready to ship faster? Try Replay free — from video to production code in minutes.