Why LLMs Alone Can’t Fix Your Legacy Debt: The Case for Visual Context

Throwing a Large Language Model (LLM) at a 20-year-old Java monolith or a tangled COBOL backend is like asking a translator to rewrite a book they’ve never read, based only on a blurry table of contents and a few torn pages. You might get something that looks like code, but it won't behave like your business.

The global technical debt crisis has reached a staggering $3.6 trillion. For the Enterprise Architect, the pressure to modernize is no longer about "innovation"—it’s about survival. Yet, 70% of legacy rewrites fail or exceed their timelines, usually because the "source of truth" is buried under layers of undocumented patches and "tribal knowledge" from engineers who retired a decade ago.

While the industry is currently obsessed with AI-driven code generation, the reality is that LLMs alone cannot solve legacy modernization. They lack the one thing necessary for a successful migration: runtime context.

TL;DR: LLMs lack the runtime context and state-awareness to accurately reconstruct legacy business logic; Replay bridges this gap by using video as the source of truth to automate reverse engineering with 70% time savings.

Why LLMs Alone Fail at Legacy Logic Extraction#

The primary reason legacy modernization projects stall is not a lack of coding talent—it’s a lack of understanding. 67% of legacy systems lack any meaningful documentation. When you feed an LLM a snippet of legacy code, you are asking it to perform "static analysis on steroids." It can tell you what the syntax does, but it cannot tell you why the code exists or how it interacts with a complex, stateful environment at 2:00 PM on a Friday during peak load.

1. The Hallucination of Business Logic#

LLMs are probabilistic, not deterministic. If a legacy function handles a complex insurance claim calculation with 15 edge cases, an LLM might identify 12 of them and "hallucinate" the remaining three based on common patterns it saw in its training data. In a regulated environment—like Financial Services or Healthcare—a 5% error rate in business logic is a catastrophic failure.

2. The "Black Box" Problem#

Legacy systems are often "black boxes" where the inputs and outputs are known, but the internal transformations are a mystery. Static analysis cannot see the side effects. It doesn't see the hidden database trigger, the obscure message queue, or the specific way the UI handles a 404 error from a 1998-era API.

3. Missing Runtime State#

Modernizing a screen isn't just about copying HTML/CSS. It’s about understanding the state machine. What happens to the "Submit" button when the "User Role" is "Supervisor" but the "Account Balance" is negative? An LLM reading a flat file cannot reconstruct this dynamic behavior without seeing it in action.

The Context Gap: Why LLMs Alone Can't See the Runtime#

To truly modernize, you need to move from "Code Archaeology" to "Observed Reality." This is where Replay changes the paradigm. Instead of guessing what the code does by reading it, Replay records what the system actually does during a real user workflow.

By capturing the telemetry of a legacy session—network calls, DOM changes, state transitions, and user inputs—Replay creates a "Visual Source of Truth." When you combine this runtime data with AI, the results shift from "educated guesses" to "documented facts."

💰 ROI Insight: Manual reverse engineering typically takes 40 hours per screen for an Enterprise Architect. With Replay’s visual extraction, that time is reduced to 4 hours. That is a 90% reduction in manual labor.

From Black Box to Documented Codebase: The Replay Workflow#

Replay doesn't just "generate code"; it reverse-engineers the intent of the legacy system. Here is how a Senior Architect utilizes the platform to bypass the "Archaeology" phase.

Step 1: Recording the Workflow#

A subject matter expert (SME) performs a standard task in the legacy application—for example, "Onboarding a New Patient" in a healthcare portal. Replay records every interaction, every API call, and every state change.

Step 2: Visual Extraction and Blueprinting#

Replay’s engine analyzes the recording. It identifies the underlying data structures and the component hierarchy. It doesn't just see a "table"; it sees a

with specific sorting logic and API dependencies.

Step 3: Generating Modern React Components#

Instead of a mess of spaghetti code, Replay generates clean, documented React components that mirror the legacy behavior but use modern patterns.

typescript
// Example: Replay-generated component from a legacy healthcare portal recording
// Logic preserved: Conditional rendering based on 'insurance_verified' flag observed in runtime.

import React, { useState, useEffect } from 'react';
import { Button, Alert, Spinner } from '@acme-org/design-system'; 
import { usePatientData } from '../hooks/usePatientData';

interface PatientOnboardingProps {
  patientId: string;
  onComplete: () => void;
}

export const PatientOnboarding: React.FC<PatientOnboardingProps> = ({ patientId, onComplete }) => {
  const { data, loading, error } = usePatientData(patientId);
  const [isSubmitting, setIsSubmitting] = useState(false);

  // Replay identified this specific business rule from the legacy network trace:
  // If insurance is not verified, the 'Complete' button must remain disabled.
  const canComplete = data?.insurance_verified && data?.forms_signed;

  if (loading) return <Spinner />;
  
  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Onboarding: {data?.name}</h2>
      
      {!data?.insurance_verified && (
        <Alert type="warning" message="Insurance verification pending in legacy system." />
      )}

      <div className="mt-8 flex justify-end gap-4">
        <Button variant="secondary" onClick={() => window.history.back()}>
          Cancel
        </Button>
        <Button 
          variant="primary" 
          disabled={!canComplete || isSubmitting}
          onClick={onComplete}
        >
          {isSubmitting ? 'Processing...' : 'Complete Onboarding'}
        </Button>
      </div>
    </div>
  );
};

Step 4: Automating API Contracts#

One of the biggest pain points in modernization is the "API Guessing Game." Replay monitors the network traffic during the recording and automatically generates OpenAPI (Swagger) specifications for the legacy endpoints.

yaml
# Generated by Replay AI Automation Suite
openapi: 3.0.0
info:
  title: Legacy Patient API (Extracted)
  version: 1.0.0
paths:
  /api/v1/patients/{id}/verify:
    post:
      summary: Extracted from "Patient Onboarding" workflow
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/VerificationResult'

The Future Isn't Rewriting—It's Understanding#

The "Big Bang Rewrite" is a relic of the past. It’s too risky, too expensive, and usually results in a new system that is missing 20% of the critical features of the old one. The future of Enterprise Architecture is Continuous Modernization.

By using Replay, teams can adopt a "Strangler Fig" approach with 10x the velocity. You don't have to guess what the legacy system does; you see it. You don't have to manually document technical debt; Replay audits it for you.

⚠️ Warning: Relying on LLMs alone to interpret legacy code without runtime validation often leads to "Silent Regressions"—bugs that don't crash the system but result in incorrect data processing that may go unnoticed for months.

Built for Regulated Environments#

Unlike generic AI tools that require sending your proprietary code to a public cloud, Replay is built for the Enterprise.

• •SOC2 & HIPAA Ready: Essential for Healthcare and FinServ.
• •On-Premise Available: Keep your sensitive legacy logic within your own firewall.
• •Technical Debt Audit: Automatically identify which parts of your legacy system are candidates for immediate replacement vs. those that can be wrapped in a modern UI.

Implementation: From 18 Months to 18 Days#

How does this look in practice? Let's look at a typical modernization roadmap using Replay vs. the traditional manual approach.

The Manual Path (The "Archaeology" Method)#

1. •Discovery (3 months): Interviewing stakeholders, trying to find documentation.
2. •Analysis (4 months): Engineers reading 100k lines of code to map dependencies.
3. •Prototyping (3 months): Building a basic version that inevitably misses edge cases.
4. •Development (8 months): Constant back-and-forth as "new" legacy requirements are discovered.

The Replay Path (The "Visual" Method)#

1. •Recording (3 days): SMEs record the top 50 critical workflows.
2. •Extraction (5 days): Replay generates the Library (Design System) and Flows (Architecture).
3. •Refinement (10 days): Engineers use Replay Blueprints to tweak the generated React components and connect them to new APIs.

📝 Note: Replay doesn't just give you code; it gives you an Architecture. The "Flows" feature maps how different screens and services interact, providing a visual map of your legacy system that never existed before.

Frequently Asked Questions#

How does Replay handle "Spaghetti Code" in the backend?#

Replay focuses on the observable behavior of the system. While it doesn't "fix" your backend code, it documents exactly how the frontend interacts with it. This allows you to create clean API contracts, making it much easier to replace the backend services one by one (the Strangler Fig pattern) without breaking the user experience.

Does this replace my developers?#

No. Replay is a "force multiplier" for Senior Architects and Developers. It eliminates the "grunt work" of manual reverse engineering (the 40 hours per screen). It allows your senior talent to focus on high-level architecture and new feature development rather than playing "code detective" in a 20-year-old codebase.

What about business logic preservation?#

This is Replay’s core strength. Because Replay captures the runtime state, it sees exactly how logic branches based on data. An LLM might guess a business rule; Replay proves it by showing the rule in action across multiple recorded sessions.

Is it compatible with my legacy stack?#

If it runs in a browser or can be captured via a terminal/UI session, Replay can analyze it. We have helped companies modernize everything from Silverlight and Flex to JSP, ASP.NET, and even green-screen terminal emulators.

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The $3.6 Trillion Question#

Every day you wait to modernize, your technical debt accrues interest. The risk of a "Big Bang" failure is too high, and the promise of "LLMs alone" is a dangerous shortcut.

The future of the Enterprise isn't about writing more code—it's about understanding the code you already have. Replay provides the visual context that turns "legacy" from a liability into a documented asset.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.