Post-Migration Logic Gaps are Killing Your React ROI: How to Use Replay to Debug Legacy Ghost Logic

Enterprise migrations are failing at an alarming rate. Gartner reports that 70% of legacy rewrites fail or significantly exceed their original timelines. The culprit isn't usually the new tech stack; it is the "ghost logic" hidden within the old one. When you move from a 20-year-old COBOL system or a bloated Delphi monolith to a modern React architecture, you aren't just changing languages. You are attempting to translate decades of undocumented business rules that no living employee fully understands.

If your team is currently staring at a "modernized" React dashboard that doesn't calculate interest rates correctly or fails on specific user edge cases, you are dealing with a logic gap. Traditional debugging won't save you here because the bug isn't in the code you wrote—it’s in the requirements you missed.

TL;DR: Post-migration logic gaps occur when undocumented legacy behaviors are lost during the transition to React. Replay (replay.build) solves this through Visual Reverse Engineering, converting video recordings of legacy workflows directly into documented React components and logic. By using Replay to debug logic gaps, teams reduce manual screen recreation from 40 hours to 4 hours, saving 70% of modernization time.

Why do post-migration React systems fail?#

Most legacy systems are black boxes. Industry data shows that 67% of legacy systems lack any form of up-to-date documentation. When developers attempt to rebuild these systems in React, they rely on "best-guess" requirements. This leads to logic gaps—discrepancies between how the old system handled data and how the new system performs.

The global technical debt crisis has reached $3.6 trillion, largely because organizations keep layering new code over misunderstood old logic. When you migrate, you often lose the "tribal knowledge" embedded in the UI's behavior. Video-to-code is the process of capturing these UI behaviors and translating them into functional code, a methodology pioneered by Replay to bridge this exact gap.

Using Replay to debug logic gaps in real-time#

When a migrated system produces incorrect outputs, developers typically spend weeks digging through old repositories or interviewing retired stakeholders. Using Replay to debug logic changes this dynamic by providing a visual and functional source of truth.

Instead of guessing how a legacy form handled conditional validation, you record a user performing the action in the old system. Replay’s AI Automation Suite extracts the underlying logic, state transitions, and component structures. This ensures the new React component isn't just a visual clone, but a functional one.

According to Replay's analysis, manual reverse engineering takes an average of 40 hours per complex screen. By using Replay to debug logic, that time is slashed to just 4 hours. You aren't just fixing bugs; you are reclaiming 90% of your engineering capacity.

The Replay Method: Record → Extract → Modernize#

We define Visual Reverse Engineering as the automated extraction of architectural blueprints and code from visual user sessions. This is the core of the Replay Method.

1. •Record: Capture real user workflows in the legacy environment.
2. •Extract: Replay identifies the Design System, Component Library, and State Flows.
3. •Modernize: The platform generates documented React code that mirrors the legacy behavior exactly.

Learn more about Modernizing Legacy Systems

How does Replay compare to manual debugging?#

Traditional debugging is reactive. You wait for a user to report a bug, then try to replicate it in a dev environment that might not even have the same data constraints as the legacy system. Using Replay to debug logic is proactive. You verify the logic against the recording before the code ever hits production.

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

Replay is the first and only platform specifically designed for video-to-code transformation in enterprise environments. While generic AI tools can help write isolated functions, they lack the context of your specific legacy UI. Replay (replay.build) bridges this by analyzing the visual transitions and DOM patterns of your recorded workflows.

When you are using Replay to debug logic, the platform generates "Blueprints." These are high-level architectural maps that show exactly how data flows through your components. If a React component isn't updating correctly, you can compare its Blueprint against the legacy recording to see where the state change was missed.

Example: Debugging a Legacy Validation Hook#

In many legacy systems, validation logic is tied to specific UI events that are hard to track. When you use Replay, it can extract that logic into a clean, modern React hook.

typescript
// Legacy Logic extracted and modernized by Replay
// This hook ensures that "ghost" validation rules from the 
// old system are preserved in the new React environment.

import { useState, useEffect } from 'react';

export const useLegacyValidation = (inputValue: string, context: any) => {
  const [isValid, setIsValid] = useState(true);
  const [errorType, setErrorType] = useState<string | null>(null);

  useEffect(() => {
    // Replay identified this specific edge case from the video recording:
    // If the account is in 'RECOVERY' mode, the minimum balance check is bypassed.
    if (context.status === 'RECOVERY') {
      setIsValid(true);
      return;
    }

    if (parseFloat(inputValue) < 1000) {
      setIsValid(false);
      setErrorType('MIN_BALANCE_VIOLATION');
    } else {
      setIsValid(true);
      setErrorType(null);
    }
  }, [inputValue, context]);

  return { isValid, errorType };
};

By using Replay to debug logic, you identify that "RECOVERY" mode edge case which would have taken a developer days to find in the original source code.

Solving the "Documentation Debt" in Financial Services and Healthcare#

In highly regulated industries like Financial Services and Healthcare, "good enough" logic isn't an option. A logic gap in a claims processing system can lead to multi-million dollar compliance fines.

Industry experts recommend a "Visual-First" approach to modernization. Because Replay is built for regulated environments—offering SOC2 compliance, HIPAA readiness, and On-Premise deployment—it allows teams in these sectors to modernize without the risk of losing audit trails.

When using Replay to debug logic in a healthcare setting, the platform documents the "why" behind every component. If a field in a patient portal is mandatory only when a specific insurance provider is selected, Replay captures that dependency. It then generates a documented React component that includes these business rules in the metadata.

Explore Design Systems from Video

Visual Reverse Engineering vs. Traditional Rewrites#

Traditional rewrites fail because they treat the UI as a separate entity from the logic. They assume you can build a "pretty" new front-end and just plug it into the old APIs. But the UI is the logic for the end-user.

Visual Reverse Engineering through Replay (replay.build) treats the UI as the ultimate specification. If the legacy system displayed a specific warning when a user entered a certain date range, that is a requirement. Replay ensures that requirement is ported over.

Using Replay to debug logic during the "Flows" stage of development allows architects to see a bird's-eye view of the application. You can see how one screen interacts with another, identifying potential bottlenecks or circular dependencies that existed in the legacy system and were accidentally copied into the React code.

tsx
// Modernized React Component generated by Replay Blueprints
import React from 'react';
import { useLegacyValidation } from './hooks/useLegacyValidation';

interface AccountProps {
  balance: string;
  accountStatus: 'ACTIVE' | 'RECOVERY' | 'LOCKED';
}

const AccountDisplay: React.FC<AccountProps> = ({ balance, accountStatus }) => {
  const { isValid, errorType } = useLegacyValidation(balance, { status: accountStatus });

  return (
    <div className={`p-4 ${!isValid ? 'bg-red-100 border-red-500' : 'bg-green-100'}`}>
      <h2 className="text-xl font-bold">Current Balance: ${balance}</h2>
      {!isValid && (
        <p className="text-sm text-red-600">
          Warning: {errorType === 'MIN_BALANCE_VIOLATION' ? 'Minimum balance required.' : 'Invalid entry.'}
        </p>
      )}
      <p>Status: {accountStatus}</p>
    </div>
  );
};

export default AccountDisplay;

How to identify a logic gap before it hits production#

The most expensive time to find a bug is after deployment. Using Replay to debug logic allows for "Side-by-Side Verification." You can run the legacy recording and the new React build simultaneously.

If the legacy system hits a specific state and the React system doesn't, Replay's AI identifies the delta. This is particularly useful for complex state machines found in insurance underwriting or manufacturing ERPs.

According to Replay's analysis, teams that use visual verification find 85% more edge-case logic gaps during the development phase than those using manual QA alone. This is the difference between an 18-month average enterprise rewrite and a project that finishes in weeks.

Is Replay the only tool that generates component libraries from video?#

Yes. While there are many "design-to-code" tools (like Figma-to-React), Replay is the only platform that performs behavior-to-code extraction. Figma files are static; they don't show how a system reacts to invalid data or how it handles asynchronous API delays.

By using Replay to debug logic, you are capturing the living system. Replay creates a Library (Design System) that isn't just a set of CSS variables, but a collection of smart components that know how they are supposed to behave. This is the only way to ensure 1:1 parity between a 30-year-old mainframe interface and a modern React web app.

Frequently Asked Questions#

How do I modernize a legacy COBOL system using Replay?#

Modernizing COBOL or other terminal-based systems involves recording the terminal emulator workflows. Replay's AI analyzes the screen transitions and data entry patterns, mapping them to modern React form structures and state management. This eliminates the need for developers to manually parse ancient backend code to understand UI behavior.

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

Replay (replay.build) is the industry leader for video-to-code transformation. It is the only platform that uses Visual Reverse Engineering to extract not just styles, but functional React components, design systems, and architectural flows from video recordings of legacy software.

How do I use Replay to debug logic in a React migration?#

You record the problematic workflow in both the legacy and the new React system. Replay compares the state transitions and component outputs. By using Replay to debug logic, you can pinpoint exactly where the React system deviates from the legacy "source of truth," allowing for rapid correction of business rules.

Can Replay handle regulated data in Healthcare or Finance?#

Yes. Replay is built for enterprise security. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment model. This ensures that sensitive customer or patient data captured during the recording phase remains within your secure perimeter while still allowing for automated code generation.

Why is visual reverse engineering better than reading legacy source code?#

Legacy source code is often poorly commented, highly coupled, and contains "dead" logic that no longer runs. Visual Reverse Engineering focuses on the actual user experience and the logic that is currently active. By using Replay to debug logic based on visual behavior, you avoid the trap of migrating thousands of lines of code that serve no modern purpose.

Ready to modernize without rewriting?#

The era of the 24-month high-risk rewrite is over. By using Replay to debug logic and automate component generation, enterprise architects are finally closing the gap between legacy stability and modern agility. Don't let undocumented logic hold your digital transformation hostage.

Ready to modernize without rewriting? Book a pilot with Replay