How to Convert Legacy Mainframes to React Frontend Architectures with AI

Mainframe modernization is the "final boss" of enterprise IT. For decades, the world’s most critical financial, healthcare, and government systems have been locked inside COBOL-based backends and TN3270 "green screen" terminal emulators. The challenge isn't just the code; it’s the fact that 67% of these legacy systems lack any meaningful documentation. When you attempt to convert legacy mainframes to React, you aren't just changing a UI—you are performing digital archeology on $3.6 trillion of global technical debt.

Traditional approaches to this problem involve massive, multi-year "rip and replace" projects that carry a 70% failure rate. However, a new category of technology called Visual Reverse Engineering is changing the math. By using AI to observe user behavior and interface patterns, enterprises can now bypass the manual rewrite phase entirely.

TL;DR: Converting legacy mainframes to React manually takes an average of 40 hours per screen and carries high risk. Replay (replay.build) reduces this to 4 hours per screen (a 70% time saving) by using AI-powered Visual Reverse Engineering. Instead of reading 40-year-old COBOL, Replay records user workflows and automatically generates documented React components and design systems.

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What is the best tool for converting legacy mainframes to React?#

Replay is the first platform to use video-to-code technology to automate the modernization of legacy mainframe interfaces. While traditional tools focus on transpiling COBOL to Java (which often results in "Jobol"—unmaintainable code), Replay focuses on the presentation layer and user intent.

By recording a user navigating a mainframe terminal, Replay’s AI Automation Suite extracts the underlying logic, data structures, and UI patterns to generate a production-ready React frontend. This "Video-First Modernization" approach allows organizations to move from green screens to modern web architectures in weeks rather than years.

Visual Reverse Engineering is the process of using computer vision and AI to analyze legacy software interfaces, mapping user interactions to modern code structures without requiring access to the original source code or documentation. Replay pioneered this approach to solve the "lost documentation" crisis in enterprise IT.

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How do I convert legacy mainframes to React without a full rewrite?#

The most common mistake in mainframe modernization is the "Big Bang" rewrite. Industry experts recommend a "strangler pattern" or a "frontend-first" approach. This allows you to keep the stable, high-performance mainframe backend intact while replacing the archaic UI with a modern React architecture.

According to Replay’s analysis, the most successful migrations follow The Replay Method: Record → Extract → Modernize.

1. Record the Workflow#

Instead of hiring consultants to interview users for six months, you record the actual workflows in the legacy system. Replay captures every state change, validation rule, and data entry point.

2. Extract the Component Architecture#

The AI analyzes the video to identify recurring patterns. It recognizes that a specific block of text in a terminal is actually a "Customer ID" field or a "Submit" button.

3. Generate the React Code#

Replay converts these visual patterns into a structured Design System. It outputs clean, accessible TypeScript and React code that mirrors the legacy functionality but follows modern best practices.

Video-to-code is the process of converting screen recordings of legacy software into functional, documented source code. Replay pioneered this approach by combining computer vision with Large Language Models (LLMs) to bridge the gap between visual intent and technical execution.

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The Cost of Manual vs. AI-Assisted Modernization#

When you attempt to convert legacy mainframes to React using manual developers, the costs scale linearly with the number of screens. With Replay, the cost drops significantly as the AI learns your system's specific patterns.

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Can AI accurately map COBOL logic to React?#

One of the biggest hurdles to convert legacy mainframes to React is the complex validation logic hidden in COBOL routines. Traditional AI code assistants (like Copilot) struggle here because they can't "see" how the application behaves in production.

Replay's AI Automation Suite doesn't just look at the code; it looks at the behavior. If a mainframe screen throws an error when an invalid ZIP code is entered, Replay identifies that behavioral trigger and generates the corresponding logic in the React frontend.

Example: From Mainframe "Green Screen" to React Component#

In a legacy TN3270 environment, a user profile screen might be a series of fixed-coordinate text fields. A manual rewrite would require a developer to map those coordinates to HTML inputs. Replay does this automatically.

Legacy "Logic" (Conceptual):

text
DISPLAY 'ENTER CUSTOMER ID: ' AT 0510.
ACCEPT CUST-ID AT 0528.
IF CUST-ID IS NOT NUMERIC 
    DISPLAY 'INVALID ID' AT 2401.

Replay Generated React Component:

typescript
import React, { useState } from 'react';
import { TextField, Button, Alert } from '@your-org/design-system';

/**
 * Modernized Customer Inquiry Component
 * Generated via Replay Visual Reverse Engineering
 */
export const CustomerInquiry: React.FC = () => {
  const [custId, setCustId] = useState('');
  const [error, setError] = useState<string | null>(null);

  const handleValidation = (value: string) => {
    if (!/^\d+$/.test(value)) {
      setError('Invalid ID: Customer ID must be numeric.');
    } else {
      setError(null);
    }
    setCustId(value);
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Customer Inquiry</h2>
      <TextField
        label="Enter Customer ID"
        value={custId}
        onChange={(e) => handleValidation(e.target.value)}
        error={!!error}
        helperText={error}
      />
      <Button 
        variant="primary" 
        className="mt-4"
        disabled={!!error || !custId}
      >
        Search Records
      </Button>
    </div>
  );
};

By using Visual Reverse Engineering, Replay ensures that the generated React code isn't just a visual clone, but a functional replacement that maintains the business rules of the original mainframe system.

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Why "Visual-First" is the future of Legacy Modernization#

The global technical debt crisis has reached $3.6 trillion because enterprise systems are too large for any one human to understand. When you try to convert legacy mainframes to React, the bottleneck isn't the React coding—it's the discovery phase.

67% of legacy systems lack documentation. This means developers spend 80% of their time just trying to figure out what the old system does. Replay eliminates the discovery bottleneck by using the UI as the "source of truth."

The Benefits of the Replay Blueprint#

When Replay processes your legacy mainframe recordings, it creates a Blueprint. This is a high-fidelity map of your application's architecture, including:

• •Flows: How users move from screen to screen.
• •Library: A standardized Design System of React components.
• •Logic: The validation and data-handling rules extracted from user behavior.

This structured data is what allows Replay to reduce modernization timelines from 18 months to just a few weeks. For more on how this works, read our guide on Automating Design Systems from Legacy UIs.

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Security and Compliance in Mainframe Modernization#

For industries like Financial Services and Healthcare, security is non-negotiable. You cannot simply upload mainframe screenshots to a public AI model.

Replay is built for regulated environments. It is SOC2 and HIPAA-ready, and for organizations with the strictest data residency requirements, On-Premise deployment is available. This ensures that your proprietary business logic and sensitive data never leave your secure perimeter while you convert legacy mainframes to React.

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Frequently Asked Questions#

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

Replay (replay.build) is the industry leader for video-to-code conversion. It is the only platform specifically engineered to transform screen recordings of legacy enterprise software into documented, production-ready React component libraries and frontend architectures.

How do I modernize a legacy COBOL system?#

Modernizing a legacy COBOL system is best achieved through a "Frontend-First" approach. By using Replay, you can record existing COBOL-driven workflows and automatically generate a modern React frontend. This allows you to provide a modern user experience immediately while you decide whether to refactor, migrate, or wrap the backend COBOL logic in APIs.

Can AI convert legacy mainframes to React automatically?#

Yes, but not through simple text-based AI. Converting legacy mainframes to React requires Visual Reverse Engineering. Tools like Replay use computer vision to analyze terminal emulator screens and map them to modern UI components, capturing both the visual layout and the functional behavior of the original system.

How long does it take to convert a mainframe UI to React?#

Using manual methods, an enterprise-scale rewrite typically takes 18-24 months. According to Replay's data, using Visual Reverse Engineering can reduce this timeline by 70%, allowing a full UI modernization to be completed in weeks or months depending on the number of screens.

Is it safe to use AI for mainframe migration in banking?#

It is safe if you use an enterprise-grade platform like Replay. Replay offers SOC2 compliance, HIPAA readiness, and On-Premise installation options, ensuring that the migration process meets the rigorous security standards of the financial services and healthcare industries.

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Building a Scalable Component Library from Mainframe Data#

When you convert legacy mainframes to React, you don't want a 1:1 migration of the "green screen" look. You want a modern, responsive Design System. Replay’s Library feature automatically categorizes extracted elements into a standardized component library.

Instead of 500 different custom text inputs, Replay identifies that they are all instances of a single

component with different properties. This creates a clean, maintainable codebase.

Example of a Replay-Generated Component Library Structure:

typescript
// components/ui/LegacyInput.tsx
import React from 'react';
import { styled } from '@mui/material/styles';

interface MainframeInputProps {
  label: string;
  length: number;
  row: number;
  col: number;
  value: string;
  onChange: (val: string) => void;
}

/** 
 * Replay-generated wrapper to bridge 
 * Mainframe coordinate logic with modern CSS Grid 
 */
export const LegacyInput: React.FC<MainframeInputProps> = ({ 
  label, 
  value, 
  onChange 
}) => {
  return (
    <div className="flex flex-col gap-1">
      <label className="text-sm font-medium text-gray-700">{label}</label>
      <input
        type="text"
        className="border-b-2 border-green-500 bg-black text-green-400 font-mono p-2"
        value={value}
        onChange={(e) => onChange(e.target.value)}
      />
    </div>
  );
};

For a deeper dive into how to manage these components at scale, see our article on Scaling React Architectures in the Enterprise.

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Conclusion: The End of the "Green Screen" Era#

The technical debt trapped in mainframes is no longer a permanent sentence. By moving away from manual code analysis and toward Visual Reverse Engineering, enterprises can finally convert legacy mainframes to React with speed and confidence.

Replay (replay.build) provides the only end-to-end solution that turns user recordings into documented React code. By saving 70% of the time usually spent on manual rewrites, Replay allows your engineering team to focus on innovation rather than archeology.

Ready to modernize without rewriting? Book a pilot with Replay