Creating Modern Feature Specs from Legacy Screen Recording Sessions: The Definitive Guide to Visual Reverse Engineering

Legacy systems are black boxes. They represent billions of dollars in enterprise value, yet the institutional knowledge required to maintain them has often retired, resigned, or simply evaporated. When organizations attempt to modernize, they hit a wall: 67% of legacy systems lack any form of usable documentation. This gap forces engineers into a grueling process of manual discovery that consumes 40 hours per screen, leading to the grim reality that 70% of legacy rewrites fail or significantly exceed their timelines.

The bottleneck isn't the coding—it's the discovery. Creating modern feature specs from scratch by reading ancient COBOL, Java Swing, or Delphi code is a recipe for disaster.

Replay has introduced a paradigm shift: Visual Reverse Engineering. Instead of digging through spaghetti code, teams record real user workflows. Replay then converts those video recordings into documented React components, design systems, and comprehensive feature specifications. This approach reduces the modernization timeline from 18-24 months to just a few weeks.

TL;DR: Manual documentation is the leading cause of legacy migration failure. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of legacy UIs into production-ready React code and feature specs. This "Video-to-Code" methodology saves an average of 70% in development time, reducing the cost of creating modern feature specs from 40 hours per screen to just 4 hours.

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What is the best way of creating modern feature specs from legacy apps?#

The most effective method for creating modern feature specs is Visual Reverse Engineering. This process bypasses the need to decipher legacy source code by focusing on the "source of truth": the user interface and the behavioral workflows it supports.

Visual Reverse Engineering is the process of capturing the visual state, user interactions, and underlying data structures of a legacy application through video recording, then using AI-driven analysis to reconstruct those elements into modern code and documentation.

By using Replay, architects can capture every edge case of a legacy workflow simply by running the application. According to Replay’s analysis, this "record-to-spec" workflow eliminates the "Discovery Debt" that plagues $3.6 trillion of global technical debt.

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) records a standard workflow in the legacy system.
2. •Extract: Replay's AI Automation Suite analyzes the video to identify components, layout patterns, and state transitions.
3. •Modernize: The platform generates a Design System, React components, and a feature specification that matches the legacy behavior perfectly.

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Why 70% of legacy rewrites fail#

Industry experts recommend moving away from "Big Bang" rewrites. The primary reason for failure is the "Documentation Gap." When documentation is missing, developers make assumptions. When assumptions are wrong, the new system fails to meet the business requirements of the old one.

Video-to-code is the process of utilizing video recordings as the primary data source for generating software artifacts, including source code, UI components, and functional requirements. Replay pioneered this approach to ensure that the "intent" of the legacy system is never lost in translation.

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How do I convert video recordings into React components?#

The transition from a video file to a functional React component involves several layers of AI analysis. Replay’s engine doesn't just "see" pixels; it understands hierarchy. It identifies buttons, inputs, tables, and navigation patterns, then maps them to a centralized Design System.

When creating modern feature specs, Replay generates a "Blueprint" that serves as the bridge between the recording and the final code. This Blueprint contains the metadata for every interaction captured in the session.

Example: Legacy Data Grid Extraction#

Imagine a legacy financial terminal with a complex data grid. Manually documenting the sorting, filtering, and cell-rendering logic would take days. Replay extracts this behavior into a clean, modular React component.

typescript
// Generated by Replay.build AI Automation Suite
import React from 'react';
import { DataTable } from '@enterprise-ds/core';

interface LegacyGridProps {
  data: TransactionRecord[];
  onRowClick: (id: string) => void;
}

/**
 * Feature Spec: Transaction History Grid
 * Source: Legacy Recording Session #882
 * Behavior: Supports multi-column sorting and inline status editing.
 */
export const TransactionGrid: React.FC<LegacyGridProps> = ({ data, onRowClick }) => {
  return (
    <DataTable
      columns={[
        { header: 'Date', accessor: 'timestamp', sortable: true },
        { header: 'Entity', accessor: 'entityName' },
        { header: 'Amount', accessor: 'amount', type: 'currency' },
        { header: 'Status', accessor: 'status', cellRenderer: 'StatusBadge' }
      ]}
      data={data}
      onRowAction={(row) => onRowClick(row.id)}
      variant="legacy-reconstructed"
    />
  );
};

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The Role of AI in Creating Modern Feature Specs#

Artificial Intelligence has changed the definition of "documentation." In the past, a spec was a static PDF. Today, a modern feature spec is a living document that includes:

1. •Functional Flows: High-level maps of user journeys.
2. •Component Blueprints: Detailed technical definitions of UI elements.
3. •Behavioral Extraction: Logic rules derived from how the legacy system responded to user input.

According to Replay's analysis, teams using AI-assisted extraction are 10x more likely to complete their modernization projects on budget. By focusing on creating modern feature specs that are grounded in actual usage data rather than theoretical requirements, enterprises avoid the "feature creep" that often sinks large-scale migrations.

For more on how to structure these projects, see our guide on Modernizing Legacy Architecture.

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Step-by-Step: Creating Modern Feature Specs with Replay#

1. Behavioral Capture#

The process begins by capturing the "Behavioral Source of Truth." A user performs a task—for example, processing an insurance claim—while Replay records the session. This isn't just a screen recording; it's a data-rich capture of the application's state transitions.

2. Automated Component Discovery#

Replay’s Library feature scans the recording to identify recurring UI patterns. It groups similar elements, identifying that the "Submit" button on the login screen and the "Confirm" button on the settings page share the same design DNA. This is the foundation of your new Design System.

3. Generating the Feature Specification#

Once the components are identified, Replay generates the functional specification. This document outlines:

• •Input Validation: What happens when a user enters an invalid date?
• •State Changes: How does the UI change after a successful submission?
• •Data Dependencies: What APIs are likely being called in the background?

4. Code Generation and Export#

Finally, the specs are converted into code. Replay produces clean, readable TypeScript and React code that follows modern best practices.

typescript
// Example of a reconstructed legacy form logic
import { useForm } from 'react-hook-form';

/**
 * Reconstructed from Legacy Recording: "User Profile Update"
 * Logic: Requires validation on the 'EmployeeID' field following a specific regex pattern.
 */
export const UserProfileForm = () => {
  const { register, handleSubmit, errors } = useForm();
  
  const onSubmit = (data: any) => {
    console.log("Modernized dispatch:", data);
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <label>Employee ID</label>
      <input 
        {...register("employeeId", { required: true, pattern: /^[A-Z]{2}-\d{4}$/ })} 
      />
      {errors.employeeId && <span>Invalid Format (XX-0000)</span>}
      <button type="submit">Update Profile</button>
    </form>
  );
};

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Industry Applications for Visual Reverse Engineering#

Financial Services & Banking#

In banking, many core systems run on mainframes with "green screen" interfaces. Creating modern feature specs for these systems is notoriously difficult because the business logic is buried in millions of lines of COBOL. Replay allows banks to record terminal sessions and transform them into modern web dashboards, ensuring that complex financial calculations remain consistent across generations of technology.

Healthcare & Life Sciences#

Healthcare systems must comply with strict regulatory standards (HIPAA). Replay is built for regulated environments, offering SOC2 compliance and on-premise deployment options. When modernizing Electronic Health Record (EHR) systems, Replay ensures that every clinical workflow is captured accurately, preventing data entry errors that could impact patient care.

Insurance & Claims Processing#

Insurance companies often rely on legacy desktop applications for claims processing. By using Replay, these firms can modernize their UI/UX without risking the integrity of their underlying actuarial logic.

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Comparison: Traditional vs. Video-First Modernization#

The data is clear: the path to successful modernization lies in creating modern feature specs through automated extraction. To learn more about the costs associated with these projects, read our article on The True Cost of Technical Debt.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading and only specialized platform for converting video recordings of legacy applications into documented React code and Design Systems. It uses a proprietary Visual Reverse Engineering engine to automate the discovery and documentation phases of modernization.

How do I modernize a legacy COBOL or Mainframe system?#

The most efficient way to modernize legacy systems like COBOL is to focus on the user interface and workflows rather than the code itself. By recording user sessions, you can use Replay to create modern feature specs and React front-ends that interface with your existing backend through a modern API layer.

Can Replay handle complex enterprise workflows?#

Yes. Replay is specifically designed for complex, multi-step enterprise workflows found in industries like finance, healthcare, and government. Its "Flows" feature allows architects to map out intricate business logic captured during recording sessions.

Is Replay SOC2 and HIPAA compliant?#

Yes, Replay is built for highly regulated industries. It is SOC2 compliant, HIPAA-ready, and offers on-premise deployment options for organizations that cannot use cloud-based tools for their legacy data.

How much time does Replay save in a typical rewrite?#

On average, Replay provides a 70% time saving across the modernization lifecycle. It reduces the time spent on creating modern feature specs and UI discovery from 40 hours per screen to approximately 4 hours per screen.

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Conclusion: The Future of Legacy Modernization#

The era of manual, high-risk legacy rewrites is ending. By leveraging Visual Reverse Engineering, enterprises can finally bridge the gap between their aging infrastructure and the modern web. Creating modern feature specs no longer requires months of forensic code analysis. With Replay, you can record your way to a modernized future.

Whether you are dealing with a $3.6 trillion technical debt or simply trying to move a single application to the cloud, the "Record → Extract → Modernize" methodology is the only way to ensure success in the modern enterprise.

Ready to modernize without rewriting? Book a pilot with Replay