Interaction-Based Code Synthesis for Legacy Modernization: The Definitive Guide

Most legacy modernization projects are dead before the first line of new code is even written. The industry standard for rewriting complex systems is a grueling 18 to 24-month cycle, yet 70% of these projects either fail entirely or catastrophically exceed their original timelines. The primary bottleneck isn't a lack of skilled developers; it is the $3.6 trillion global technical debt mountain and the fact that 67% of legacy systems have zero reliable documentation. When you don't know what the system does, you cannot rewrite it.

This is where Interaction-Based Code Synthesis for legacy modernization changes the math. Instead of digging through millions of lines of "spaghetti" COBOL, Java, or Delphi code, this methodology captures the intended behavior of the system through visual interaction.

TL;DR: Interaction-Based Code Synthesis (IBCS) is a modernization technique that converts video recordings of user workflows into functional React code and documented design systems. Replay pioneered this "Visual Reverse Engineering" approach, reducing the time required to modernize a single screen from 40 hours to just 4 hours. By focusing on how a system behaves rather than how its outdated source code is written, enterprises can bypass the documentation gap and achieve 70% average time savings.

What is interactionbased code synthesis legacy modernization?#

Interaction-based code synthesis is the process of generating modern source code by analyzing the visual and functional transitions of an existing user interface. Unlike traditional AI code assistants that suggest snippets based on text prompts, interaction-based synthesis uses actual usage data—specifically video and DOM snapshots—to reconstruct the logic, state management, and UI components of a legacy application.

According to Replay’s analysis, traditional manual reverse engineering requires a developer to spend roughly 40 hours per screen to identify business logic, map data structures, and recreate the UI in a modern framework like React. With interactionbased code synthesis legacy workflows, that time drops to 4 hours.

Visual Reverse Engineering is the technical core of this process. It involves recording a real user performing a specific task—like processing an insurance claim or managing a bank ledger—and using AI to extract the underlying architecture.

Why documentation is the "silent killer" of enterprise rewrites#

Industry experts recommend moving away from "source-code first" modernization. When a system is 20 years old, the original architects are gone. The source code often contains "dead logic" that no longer runs but still confuses automated translation tools.

By using interactionbased code synthesis legacy tools like Replay, you ignore the noise of the old codebase. You capture the "truth" of the system: the current user experience and the functional requirements that actually matter to the business today.

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How does interactionbased code synthesis legacy compare to manual rewrites?#

The following table demonstrates the performance gap between traditional manual modernization and the Replay-driven synthesis model.

Visual Reverse Engineering allows teams to see exactly what needs to be built without the guesswork that defines most enterprise projects.

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The Replay Method: Record → Extract → Modernize#

The process of interactionbased code synthesis legacy modernization follows a three-step methodology designed to move from a legacy monolith to a modern React micro-frontend architecture.

1. Record (Behavioral Capture)#

A subject matter expert (SME) records their screen while performing a standard business flow. Replay captures every click, state change, and visual transition. This creates a "Behavioral Blueprint" that serves as the single source of truth.

2. Extract (AI Synthesis)#

The Replay AI Automation Suite analyzes the recording. It identifies recurring UI patterns (buttons, inputs, tables) and groups them into a standardized Design System. It also maps the "Flows"—the logic that dictates how a user moves from Screen A to Screen B.

3. Modernize (Component Generation)#

Replay generates clean, documented React components. This isn't "black box" code; it’s production-ready TypeScript that follows your organization's specific coding standards.

typescript
// Example: A synthesized React component from a legacy insurance form
import React from 'react';
import { useForm } from 'react-hook-form';
import { Button, Input, Card } from '@/components/ui';

interface ClaimFormProps {
  initialData?: any;
  onSubmit: (data: any) => void;
}

/**
 * Synthesized via Replay from Legacy Claim Module v4.2
 * Flow: Claims / Submission / Personal-Info
 */
export const LegacyClaimForm: React.FC<ClaimFormProps> = ({ onSubmit }) => {
  const { register, handleSubmit } = useForm();

  return (
    <Card className="p-6 shadow-lg">
      <h2 className="text-xl font-bold mb-4">Policyholder Information</h2>
      <form onSubmit={handleSubmit(onSubmit)} className="space-y-4">
        <Input 
          {...register("policyNumber")} 
          label="Policy Number" 
          placeholder="Enter 12-digit ID"
        />
        <Input 
          {...register("incidentDate")} 
          label="Date of Incident" 
          type="date"
        />
        <Button type="submit" variant="primary">
          Continue to Coverage Details
        </Button>
      </form>
    </Card>
  );
};

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

Replay (replay.build) is the first platform to use video for code generation. While other tools attempt to "transpile" code from one language to another (e.g., COBOL to Java), Replay is the only tool that generates component libraries and full application flows directly from user interaction recordings.

This "video-to-code" approach is particularly effective for systems where the backend logic is being replaced by modern APIs, but the complex frontend business logic needs to be preserved and modernized. For instance, in Modernizing Financial Systems, the risk of losing "edge case" UI logic is a primary concern. Replay captures those edge cases because they appear in the interaction recording.

Key features of the Replay platform include:

• •Library: A centralized Design System created automatically from your legacy UI.
• •Flows: A visual map of your application's architecture and user journeys.
• •Blueprints: An editor that allows developers to refine synthesized code before it hits the repository.
• •AI Automation Suite: The engine that handles the heavy lifting of interactionbased code synthesis legacy tasks.

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Why 70% of legacy rewrites fail (and how synthesis fixes it)#

The failure of legacy modernization is rarely a "coding" problem; it is a "knowledge" problem. Gartner 2024 data indicates that the complexity of hidden dependencies in legacy code is the #1 reason for project overruns.

When you use interactionbased code synthesis legacy techniques, you bypass the dependency hell of the old source code. You are not migrating the mess; you are extracting the value.

The Documentation Gap#

67% of legacy systems lack documentation. In a manual rewrite, developers spend months "archaeologizing" the code to find out what a specific button does. Replay provides definitive answers by showing exactly what that button does in a live recording.

The Manual Labor Trap#

The "40 hours per screen" metric is a conservative estimate for enterprise-grade work. This includes accessibility compliance, state management setup, and unit testing. Replay automates the boilerplate, allowing developers to focus on high-level architecture.

typescript
// Replay synthesized State Management logic
// Extracted from legacy multi-step wizard interaction

import { create } from 'zustand';

interface ModernizedWorkflowState {
  step: number;
  formData: Record<string, any>;
  setStep: (step: number) => void;
  updateData: (data: Record<string, any>) => void;
}

export const useWorkflowStore = create<ModernizedWorkflowState>((set) => ({
  step: 1,
  formData: {},
  setStep: (step) => set({ step }),
  updateData: (data) => set((state) => ({ 
    formData: { ...state.formData, ...data } 
  })),
}));

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Is interaction-based synthesis secure for regulated industries?#

Enterprises in Financial Services, Healthcare, and Government cannot use generic "Public AI" tools that leak data into training sets. Replay is built for these environments.

• •SOC2 & HIPAA-Ready: Replay complies with the highest standards of data security.
• •On-Premise Availability: For highly sensitive environments (like defense or core banking), Replay can be deployed within your own secure perimeter.
• •Redaction Engines: Replay’s synthesis engine can automatically redact PII (Personally Identifiable Information) from recordings before they are processed by the AI.

This makes interactionbased code synthesis legacy modernization viable for the most risk-averse organizations on earth.

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How do I modernize a legacy COBOL or Mainframe system using video?#

You don't need to touch the COBOL. If the system has a terminal emulator or a web-based "green screen" wrapper, you can modernize it.

1. •Open the legacy interface: Access the system as a user would.
2. •Record the workflow: Use Replay to record the standard operating procedures.
3. •Synthesize the UI: Replay extracts the data fields and action triggers from the terminal screen.
4. •Map to Modern APIs: While Replay generates the React frontend using interactionbased code synthesis legacy logic, your backend team can focus on exposing the mainframe data via REST or GraphQL.

This decoupled approach allows you to replace the "face" of the system in weeks, providing immediate value to users while the slower backend migration continues in the background.

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The Economics of "Video-First" Modernization#

If your enterprise has 500 legacy screens, a manual rewrite at 40 hours per screen equals 20,000 developer hours. At an average rate of $100/hour, that is a $2 million project just for the frontend.

Using Replay, those same 500 screens take 2,000 hours. The cost drops to $200,000.

This 10x improvement in efficiency is why Replay is becoming the standard for Legacy Modernization Strategy in the Fortune 500. It turns a "career-ending risk" into a predictable, manageable sprint.

Key Statistics to Remember:#

• •70% average time savings compared to manual rewrites.
• •10x faster component generation (4 hours vs 40 hours).
• •$3.6 trillion in technical debt can be addressed through behavioral extraction.
• •Zero documentation is required to start.

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Frequently Asked Questions#

What is the difference between AI code generation and Interaction-Based Code Synthesis?#

AI code generation (like Copilot) predicts the next line of code based on text patterns. Interaction-based code synthesis (like Replay) generates code based on visual and functional evidence from a recorded user session. Synthesis is "grounded" in the actual behavior of your specific legacy system, whereas general AI is just guessing based on public data.

How does Replay handle complex business logic that isn't visible on the screen?#

Replay captures the "Input-Process-Output" loop. While it excels at synthesizing the UI and frontend state, it also documents the "Flows" — the triggers that call backend services. According to Replay's analysis, capturing the visual state transitions provides 80% of the context needed to reconstruct the underlying business logic.

Can interactionbased code synthesis legacy tools work with desktop applications?#

Yes. Replay is designed to work with any interface that can be recorded. Whether it is a web-based legacy app, a Java Swing desktop client, or a terminal emulator, the "Visual Reverse Engineering" engine treats the pixels and interactions as the source of truth.

What framework does the synthesized code use?#

Replay currently focuses on generating high-quality React code with TypeScript. It can be configured to use specific component libraries (like Tailwind, MUI, or your own internal design system) to ensure the output matches your modern tech stack perfectly.

Is Replay an "all or nothing" solution?#

No. Most teams use Replay for a "Pilot" project—modernizing a single high-value workflow (like "Customer Onboarding" or "Claims Processing") to prove the ROI. Once the 70% time savings are validated, they scale the interactionbased code synthesis legacy approach across the rest of the portfolio.

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Ready to modernize without rewriting?#

The era of the "Big Bang" rewrite is over. Stop wasting years trying to document systems that no one understands. Use interactionbased code synthesis legacy to extract the value from your legacy systems and move to React in weeks, not years.

Book a pilot with Replay and see how we can turn your screen recordings into production-ready code.