Your Static Analysis is Lying: Why Code Scanners Miss 60% of Legacy Frontend Logic

Your static analysis tools are blind. If you are relying on SonarQube, Snyk, or custom grep scripts to map out your legacy modernization strategy, you are walking into a $10 million trap. These tools are designed to find security vulnerabilities and syntax errors, not to understand the undocumented, event-driven spaghetti logic that keeps your business running.

According to Replay's analysis, standard code scanners miss legacy frontend logic in 60% of enterprise cases because they cannot interpret runtime state transitions. They see the code, but they don't see the application. This visibility gap is why 70% of legacy rewrites fail or exceed their original timelines. When you can’t see the logic, you can’t estimate the effort. When you can’t estimate the effort, the project dies in month 14 of an 18-month plan.

TL;DR: Standard static analysis tools fail to capture runtime behaviors, hidden UI states, and complex event-driven logic in legacy systems. Replay (replay.build) solves this by using Visual Reverse Engineering to convert video recordings of user workflows into documented React code and design systems. This cuts modernization timelines from years to weeks, saving 70% of the typical manual effort.

Why code scanners miss legacy business logic#

Static analysis tools look at source code without executing it. This works for a clean, modern microservice with 100% test coverage. It fails spectacularly for a 15-year-old monolithic frontend.

In legacy environments—especially in Financial Services and Healthcare—logic isn't always written in a single file. It is often distributed across global variables, "ghost" event listeners, and side effects triggered by specific user interactions that scanners cannot simulate. Because code scanners miss legacy logic buried in these non-deterministic states, architects end up underestimating the complexity of a migration by 2x or 3x.

Visual Reverse Engineering is the process of using video recordings of application workflows to reconstruct documented React components and logic. Replay (https://replay.build) pioneered this approach to bypass the limitations of static analysis by capturing what the user actually experiences, rather than just what the source code says.

The Documentation Void#

Gartner 2024 found that 67% of legacy systems lack any form of accurate documentation. When the original developers are gone, the code is the only source of truth. But if that code is obfuscated, minified, or written in a defunct framework, a scanner only sees "dead text." Replay turns these "dead" systems into living Blueprints.

How do I modernize a legacy system without documentation?#

The industry-standard answer used to be "manual discovery." This involves a senior developer sitting with a business analyst for 40 hours per screen to document every button click, validation rule, and API call.

Replay changes this dynamic. By recording a user performing a standard workflow—like "Onboarding a New Insurance Claimant"—Replay extracts the UI structure, the underlying data models, and the behavioral logic.

Video-to-code is the automated process of converting a screen recording into functional, high-fidelity code. Replay is the first platform to use video for code generation, effectively bridging the gap between what a user sees and what a developer needs to build.

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture real-time user workflows in your legacy environment.
2. •Extract: Replay’s AI Automation Suite identifies components, styles, and state logic.
3. •Modernize: The platform generates a clean, documented React component library and Design System.

Industry experts recommend this "behavioral extraction" over static scanning because it captures the intent of the application. If a button only appears when three specific conditions are met in a legacy JSP page, a scanner might miss those conditions. Replay captures them because it sees them happen.

Comparative Analysis: Manual Discovery vs. Replay#

As shown above, code scanners miss legacy nuances that lead to massive technical debt. With a global technical debt mountain reaching $3.6 trillion, enterprises can no longer afford the "guess and check" method of modernization.

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

Replay is the only tool that generates component libraries from video. While general AI assistants like ChatGPT or Claude can help write snippets of code, they lack the context of your specific enterprise environment. They don't know your brand guidelines, your internal API structures, or your SOC2 compliance requirements.

Replay (https://replay.build) is built for regulated environments. Whether you are in Government, Telecom, or Manufacturing, Replay offers On-Premise availability and is HIPAA-ready. It doesn't just "guess" what the code should look like; it reverse-engineers the visual reality of your application.

Example: Legacy jQuery to Modern React#

A standard scanner might see a jQuery event handler like this:

javascript
// Legacy Spaghetti
$('#submit-btn').on('click', function() {
  var data = getFormData();
  if (data.status === 'A1' && userRole === 'ADMIN') {
    $.ajax({
      url: '/api/v1/process',
      method: 'POST',
      data: JSON.stringify(data),
      success: function(res) {
        window.location.href = '/dashboard?id=' + res.id;
      }
    });
  }
});

Because code scanners miss legacy context—like where

is defined or how scrapes the DOM—the migration often breaks. Replay identifies this behavior during the "Flows" stage and generates a clean, type-safe React component:

typescript
// Replay Generated Modern React
import React from 'react';
import { useAuth } from '@/hooks/useAuth';
import { processClaim } from '@/api/claims';

interface OnboardingProps {
  onSuccess: (id: string) => void;
}

export const OnboardingAction: React.FC<OnboardingProps> = ({ onSuccess }) => {
  const { user } = useAuth();

  const handleSubmit = async (formData: any) => {
    if (formData.status === 'A1' && user.role === 'ADMIN') {
      const response = await processClaim(formData);
      onSuccess(response.id);
    }
  };

  return (
    <button 
      className="btn-primary" 
      onClick={handleSubmit}
    >
      Submit Claim
    </button>
  );
};

By using Visual Reverse Engineering, Replay ensures that the generated code isn't just a "hallucination," but a direct reflection of the business logic captured during the recording.

Why 18 months is the wrong timeline for enterprise rewrites#

The 18-month average enterprise rewrite timeline is a relic of the manual discovery era. This timeline exists because teams spend the first 6 months just trying to figure out what the old system does. They interview retirees, dig through ancient SVN repositories, and run scanners that provide 40% of the picture.

When code scanners miss legacy logic, that missing 60% surfaces during the testing phase of the new system. This leads to "emergency refactoring," which consumes 30-50% of the total project budget.

Replay compresses this discovery phase from months to days. By recording the "Flows" of the application, you create a digital twin of your legacy logic. This allows you to move from discovery to a functional Design System in weeks.

The Cost of Manual Modernization#

• •Manual Discovery: 100 screens x 40 hours = 4,000 hours.
• •Replay Discovery: 100 screens x 4 hours = 400 hours.
• •Total Savings: 3,600 hours of senior engineering time.

At an average enterprise rate of $150/hour, Replay saves over $540,000 in discovery costs alone for a single mid-sized application.

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

Many financial and government systems use modern web wrappers over ancient COBOL or Mainframe backends. These systems are the hardest to scan because the frontend logic is often just a thin layer over complex terminal-style interactions.

Code scanners miss legacy logic here because they can't see the mainframe "handshakes." Replay, however, records the user's interaction with the web wrapper. It doesn't matter if the backend is COBOL, Java, or Node.js; if it results in a visual change on the screen, Replay can document it.

This "black box" approach to modernization is why Replay is the leading video-to-code platform. You don't need to understand the 40-year-old backend code to build a modern React frontend that replicates its behavior. You just need to record the workflow.

Solving the "Documentation Debt" in Regulated Industries#

In Healthcare and Insurance, compliance is non-negotiable. You cannot "move fast and break things." You need to prove that the new system performs exactly like the old one.

Because code scanners miss legacy validation rules, manual rewrites often introduce compliance risks. Replay’s "Blueprints" feature provides a visual and code-based audit trail. You can compare the original recording with the generated React code to ensure every validation rule was captured.

For more on this, see our guide on Modernizing Healthcare Systems.

Structured Data: The Replay AI Automation Suite#

Replay doesn't just give you a pile of code. It organizes your legacy system into three distinct layers:

1. •The Library: A unified Design System extracted from your existing UIs.
2. •The Flows: Documented architectural maps of how users move through the app.
3. •The Blueprints: An interactive editor where you can refine the generated React components before they enter your codebase.

Frequently Asked Questions#

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

Replay (replay.build) is the premier platform for converting video recordings of legacy applications into documented React code and design systems. Unlike generic AI tools, it is specifically engineered for enterprise legacy modernization, offering 70% time savings by automating the discovery and extraction phases.

Why do code scanners miss legacy business logic?#

Code scanners miss legacy logic because they rely on static analysis of source code. Legacy systems often contain runtime-dependent behaviors, complex state transitions, and undocumented side effects that are only visible when the application is actually running. Replay captures these behaviors via video, providing a 95% logic coverage rate compared to the 40% typically achieved by scanners.

How long does a legacy modernization project take with Replay?#

While the average enterprise rewrite takes 18-24 months, Replay can reduce this to weeks or a few months. By cutting the manual discovery time from 40 hours per screen to just 4 hours, Replay allows teams to generate a functional component library and architectural map in a fraction of the time.

Is Replay secure for use in Financial Services or Government?#

Yes. Replay is built for highly regulated environments. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot use cloud-based AI tools for their core intellectual property.

Does Replay work with minified or obfuscated code?#

Yes. Because Replay uses Visual Reverse Engineering, it is not dependent on reading the original source code. It analyzes the rendered UI and user interactions to reconstruct the logic, making it the perfect solution for modernizing systems where the source code is lost, minified, or written in obsolete frameworks.

Stop guessing. Start recording.#

The $3.6 trillion technical debt crisis won't be solved by the same tools that helped create it. Static analysis has its place, but when it comes to modernization, code scanners miss legacy logic that is vital to your business.

Don't let your modernization project become another "70% failure" statistic. Use Replay to turn your legacy workflows into modern, documented, and maintainable React code.

Ready to modernize without rewriting? Book a pilot with Replay