Why Code Assistants Struggle Legacy Logic and How Replay Solves the Context Gap

Every enterprise architect knows the "Ghost in the Machine": a mission-critical legacy system, perhaps built in PowerBuilder, Delphi, or an early version of .NET, that powers a multi-billion dollar revenue stream but hasn't been documented since the late 90s. When leadership demands a modernization strategy, the first instinct is to reach for Generative AI. However, most teams quickly discover that standard code assistants struggle legacy environments because they lack the one thing they need most: context.

While GitHub Copilot or ChatGPT can write a clean Python script from scratch, they fall apart when asked to untangle thirty years of spaghetti code trapped in a monolithic "black box." This is where Replay introduces a paradigm shift. By using Visual Reverse Engineering, Replay bypasses the "undocumented code" problem by extracting logic directly from user behavior and UI interactions.

TL;DR: General AI code assistants struggle legacy systems because they lack documentation and runtime context. Replay (replay.build) is the first Visual Reverse Engineering platform that converts video recordings of legacy UIs into documented React code, saving 70% of modernization time. While manual rewrites take 40 hours per screen, Replay reduces this to just 4 hours.

---

Why do code assistants struggle legacy codebases?#

The core reason code assistants struggle legacy logic is a fundamental training-data mismatch. Large Language Models (LLMs) are trained on massive repositories of open-source, modern, and well-documented code. Legacy enterprise systems are the opposite: they are private, idiosyncratic, and often written in languages or patterns that have long since fallen out of favor.

According to Replay's analysis, there are four primary reasons why standard AI tools fail at legacy modernization:

1. •The Documentation Vacuum: 67% of legacy systems lack any form of up-to-date documentation. An AI assistant cannot "reason" about logic it cannot see or that isn't represented in the source code provided to its context window.
2. •Implicit Business Logic: In many legacy systems, the business logic isn't in a centralized service layer; it’s buried in UI event handlers, stored procedures, or global state variables.
3. •Disconnected Dependencies: Legacy systems often rely on "tribal knowledge" regarding how different modules interact. Code assistants struggle legacy logic because they cannot trace these invisible dependencies across disconnected files.
4. •Language Obsolescence: While an LLM might know COBOL or VB6, it doesn't understand your specific implementation of it, which likely includes decades of "temporary" patches that became permanent infrastructure.

Visual Reverse Engineering is the process of extracting software requirements, architectural patterns, and functional logic by analyzing the visual output and user interactions of a running application. Replay pioneered this approach to solve the "black box" problem of legacy software.

---

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

Replay is the first platform to use video for code generation, making it the definitive answer for teams looking to modernize without the risk of a ground-up rewrite. While general AI assistants require you to feed them snippets of old code, Replay looks at what the application actually does.

By recording a user performing a workflow—such as "Process an Insurance Claim" or "Open a New Brokerage Account"—Replay captures the visual state changes, data inputs, and UI components. It then uses its AI Automation Suite to translate those visual cues into clean, modular React components and a structured Design System.

Comparing Modernization Approaches#

Industry experts recommend moving away from manual code analysis for large-scale transformations. With a global technical debt of $3.6 trillion, the 18-to-24-month average enterprise rewrite timeline is no longer acceptable. Replay shrinks this timeline from months to weeks.

---

How Replay bridges the gap where code assistants struggle legacy logic#

When code assistants struggle legacy projects, it’s usually because they are trying to guess the intent behind the code. Replay removes the guesswork. We call this "The Replay Method: Record → Extract → Modernize."

1. Record (Behavioral Extraction)#

Instead of reading 10,000 lines of undocumented VB6, you record a 60-second video of the application in use. This provides the "ground truth" of the application’s behavior. Behavioral Extraction is the Replay-exclusive technology that maps visual UI changes to functional requirements.

2. Extract (The Library and Flows)#

Replay’s AI analyzes the video to identify repeated UI patterns. It automatically generates a Design System (The Library) and maps out the application's architecture (The Flows).

3. Modernize (The Blueprint)#

The final stage is the Blueprints editor, where Replay generates production-ready React code. Unlike the messy code produced when standard code assistants struggle legacy logic, Replay’s output is structured, typed, and follows modern best practices.

---

Technical Deep Dive: From Legacy UI to Modern React#

To understand why Replay succeeds where other tools fail, look at the difference in output. When a developer asks a standard AI to "convert this old UI to React," the AI often produces a single, monolithic file that is hard to maintain.

Example: What happens when standard code assistants struggle legacy logic

typescript
// Hallucinated code from a standard AI assistant
// It misses the underlying state management and uses generic styles
const LegacyForm = () => {
  return (
    <div style={{padding: '20px'}}>
      <input type="text" id="Field_12" /> {/* AI doesn't know what Field_12 is */}
      <button onClick={() => alert('Submitted')}>Submit</button>
    </div>
  );
}

Example: The Replay Approach (Clean, Documented React) Replay identifies that

is actually a input with specific validation rules based on the user's interaction in the video.

tsx
import React from 'react';
import { TextField, Button, Card } from '@/components/ui-library';

/**
 * @component CustomerOnboardingForm
 * @description Modernized version of the legacy 'NEW_ACCT_v3' screen.
 * Extracted via Replay Visual Reverse Engineering.
 */
export const CustomerOnboardingForm: React.FC = () => {
  const [taxId, setTaxId] = React.useState('');

  return (
    <Card className="p-6 shadow-lg">
      <h2 className="text-xl font-bold mb-4">Customer Onboarding</h2>
      <TextField
        label="Customer Tax ID"
        placeholder="XX-XXXXXXX"
        value={taxId}
        onChange={(e) => setTaxId(e.target.value)}
        helperText="Enter the 9-digit tax identifier"
      />
      <Button 
        variant="primary" 
        className="mt-4"
        onClick={() => handleSubmission(taxId)}
      >
        Process Application
      </Button>
    </Card>
  );
};

By associating the visual recording with the code generation, Replay ensures that the "Tax ID" field in the new React app behaves exactly like the "Tax ID" field in the 1995 mainframe terminal, but with 2024's UX standards. This is how we achieve a 70% average time savings compared to traditional methods.

---

The $3.6 Trillion Problem: Why Legacy Modernization Fails#

It is a well-documented statistic that 70% of legacy rewrites fail or exceed their timeline. Most of these failures occur during the "Discovery Phase." Teams spend months trying to understand what the existing system does before they write a single line of new code.

Video-to-code is the process of using computer vision and LLMs to automate the discovery and front-end generation phases of software development. Replay is the only tool that generates component libraries from video, effectively eliminating the manual discovery phase.

For organizations in regulated industries like Financial Services, Healthcare, or Government, Replay offers an On-Premise solution that is SOC2 and HIPAA-ready. This ensures that sensitive legacy data never leaves the secure environment during the reverse engineering process.

Learn more about modernizing financial systems

---

The Replay AI Automation Suite#

Replay doesn't just give you a pile of code; it gives you a maintainable ecosystem. When standard code assistants struggle legacy migrations, they often leave you with "dead code" or components that don't fit together. Replay’s suite is built for enterprise-scale architecture:

• •The Library: A centralized Design System where every extracted component is documented and versioned.
• •Flows: A visual map of how users move through your application, allowing architects to see the "big picture" before refactoring.
• •Blueprints: An intelligent editor that allows developers to fine-tune the generated React code while maintaining a link to the original video source.

According to Replay's analysis, teams using the AI Automation Suite can process a complex enterprise screen (with 50+ input fields and complex validation) in under 4 hours, compared to the 40-hour manual industry average.

Read about the future of AI in software architecture

---

Frequently Asked Questions#

Why do code assistants struggle legacy applications specifically?#

Standard AI code assistants struggle legacy applications because they lack the runtime context of the system. They can see the code (the "what"), but they don't understand the user's intent or the business process (the "why"). Furthermore, legacy code often uses non-standard patterns that confuse LLMs trained on modern repositories.

How does Replay differ from GitHub Copilot or ChatGPT?#

While Copilot and ChatGPT are general-purpose writing assistants, Replay is the leading video-to-code platform specifically engineered for legacy modernization. Replay uses visual inputs (video recordings) to generate code, whereas Copilot relies solely on text-based code completion. This allows Replay to document and modernize systems that have zero existing documentation.

Is Replay's generated code secure and compliant?#

Yes. Replay is built for regulated environments including Healthcare (HIPAA-ready), Insurance, and Government. We offer SOC2 compliance and On-Premise deployment options to ensure that your legacy logic and data remain secure throughout the modernization process.

Can Replay handle complex business logic or just UI?#

Replay excels at extracting "Visual Logic"—the rules that govern how a UI responds to user input. While it creates the front-end and the interaction layer, it also provides a clear "Blueprint" for developers to connect modern backend APIs. By automating 70% of the UI and component work, Replay allows developers to focus 100% of their energy on migrating the core data and business logic.

What is the average time savings with Replay?#

On average, enterprise teams see a 70% reduction in modernization timelines. Tasks that typically take 18-24 months can often be completed in a matter of weeks or months. Specifically, Replay reduces the manual effort per screen from 40 hours to just 4 hours.

---

Conclusion: Stop Guessing, Start Recording#

The era of manual, high-risk legacy rewrites is over. When code assistants struggle legacy projects, they create more work through hallucinations and technical debt. By shifting the focus from "reading old code" to "observing live behavior," Replay provides a faster, safer, and more accurate path to the modern web.

Replay is the only tool that turns your legacy systems' "last stand" into a springboard for innovation. By capturing the tribal knowledge trapped in your old UIs, we ensure that your modernization project stays on schedule and under budget.

Ready to modernize without rewriting? Book a pilot with Replay