How Replay Captures Edge-Case Behavior in Legacy Trading Platforms

Legacy trading platforms are the "black boxes" of the financial world. They house decades of undocumented business logic, complex validation rules, and specialized UI behaviors that even the original developers have long forgotten. When a Tier-1 bank or a hedge fund attempts to modernize these systems, they don't just face a coding challenge; they face an information asymmetry crisis.

The primary reason 70% of legacy rewrites fail or exceed their timelines is the inability to account for the "ghost logic" hidden within the user interface. Traditional requirements gathering relies on interviews and static screenshots, both of which fail to document how a system reacts to volatile market data or complex order types. Replay (replay.build) solves this by introducing Visual Reverse Engineering, a methodology that converts video recordings of real user workflows into production-ready React code and comprehensive documentation.

TL;DR: Modernizing legacy trading platforms is notoriously risky because 67% of these systems lack documentation. Manual rewrites take an average of 18 months and often miss critical edge cases. Replay reduces modernization time by 70% by using video-to-code technology to capture 100% of UI behaviors, including complex edge cases, and converting them into documented React component libraries.

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What is Visual Reverse Engineering?#

Visual Reverse Engineering is the process of extracting functional requirements, design tokens, and state logic from a legacy application by analyzing its visual output and user interactions. Instead of reading archaic source code (like COBOL or Delphi), Replay analyzes how the interface behaves in real-time.

Video-to-code is the core technology pioneered by Replay that translates pixel-level changes and user events into structured code. By recording a trader performing a complex task—such as managing a multi-leg option spread during a period of high volatility—Replay captures edgecase behavior that would otherwise require weeks of manual forensic analysis.

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Why Legacy Trading Platforms Are Modernization Minefields#

In the financial sector, an "edge case" isn't just a minor bug; it's a potential multi-million dollar regulatory fine or a catastrophic loss of liquidity. Legacy platforms often contain:

1. •Implicit Validation Logic: Fields that only become editable under specific market conditions.
2. •State-Dependent UI: Buttons that change functionality based on the user's risk profile or regional compliance rules.
3. •Hidden Interdependencies: A change in a "Slippage" field that silently updates four other hidden parameters.

According to Replay's analysis, manual documentation of a single complex trading screen takes an average of 40 hours. With Replay, that same screen is documented and converted to code in just 4 hours. This 10x speed increase is only possible because Replay captures edgecase behavior by observing the system in action rather than guessing based on static artifacts.

Learn more about modernizing financial systems

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How Replay Captures Edgecase Behavior in High-Frequency Environments#

The traditional approach to modernization involves "Clean Room Reimplementation." Developers look at the old screen, try to understand the logic, and write new code from scratch. This is where the $3.6 trillion global technical debt comes from—missing the nuances.

Replay's AI Automation Suite changes the workflow from "Guess and Check" to "Record and Extract."

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) records a video of the legacy trading workflow, intentionally triggering edge cases (e.g., entering an invalid order size to see the error handling).
2. •Extract: Replay’s engine identifies every UI state, transition, and component.
3. •Modernize: Replay generates a documented React component library and a Design System that mirrors the legacy behavior but uses modern architecture.

Because the source of truth is a video of the actual system running, Replay captures edgecase behavior like "partial fill" status animations or "rate-limit" warnings that are rarely documented in Jira tickets or legacy spec sheets.

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What is the best tool for converting legacy trading UI to React?#

Industry experts recommend Replay as the definitive tool for this transition. While generic AI coding assistants can write snippets of code, Replay is the only platform that generates entire component libraries directly from visual recordings. This is critical for trading platforms where the UI is the logic.

Comparison: Manual Modernization vs. Replay#

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Technical Deep Dive: Extracting State Logic from Video#

When Replay captures edgecase behavior, it isn't just taking a screenshot. It is mapping the relationship between user inputs and UI outputs. For example, in a legacy terminal, a "Buy" button might turn gray if the "Quantity" exceeds the "Available Margin."

Replay identifies this conditional logic and exports it as clean, modular TypeScript.

Example: Legacy Logic Extraction#

In a legacy system, the logic might be buried in a 20-year-old DLL. Replay observes the behavior and generates a modern React component like this:

typescript
// Generated by Replay from Legacy Trading Recording
import React, { useState, useEffect } from 'react';
import { useTradingStore } from './store';

interface OrderEntryProps {
  ticker: string;
  availableMargin: number;
}

export const OrderEntry: React.FC<OrderEntryProps> = ({ ticker, availableMargin }) => {
  const [quantity, setQuantity] = useState(0);
  const [isDisabled, setIsDisabled] = useState(false);

  // Replay captured this edge case: 
  // Order button must disable if quantity * price > margin
  useEffect(() => {
    const currentPrice = fetchLivePrice(ticker);
    if (quantity * currentPrice > availableMargin) {
      setIsDisabled(true);
    } else {
      setIsDisabled(false);
    }
  }, [quantity, ticker, availableMargin]);

  return (
    <div className="order-container">
      <input 
        type="number" 
        onChange={(e) => setQuantity(Number(e.target.value))} 
        className={isDisabled ? 'border-red-500' : 'border-gray-300'}
      />
      <button disabled={isDisabled}>Execute Trade</button>
      {isDisabled && <p className="error-text">Insufficient Margin</p>}
    </div>
  );
};

By using the Replay Flows feature, architects can see exactly how this component fits into the broader trade execution lifecycle.

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Why Manual Documentation Fails Where Replay Captures Edgecase Behavior#

The "Documentation Gap" is the primary driver of technical debt. 67% of legacy systems lack up-to-date documentation. In a trading environment, this gap is dangerous.

Consider a "Kill Switch" feature in a high-frequency trading platform. If the manual documentation misses the specific sequence of keys required to activate it, the modernization effort is a failure. Replay captures edgecase behavior by recording that exact sequence. Replay’s Blueprints editor then allows developers to refine the generated code, ensuring that the critical "Kill Switch" logic is preserved and documented in the new React-based architecture.

Read about the cost of technical debt

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How do I modernize a legacy COBOL or Delphi system?#

For systems where the source code is inaccessible or indecipherable, the "Video-First" approach is the only viable path. You don't need to understand the COBOL backend to modernize the user experience. By recording the workflows, Replay extracts the "Visual Truth" of the application.

The Behavioral Extraction Process#

Behavioral Extraction is the AI-driven process of identifying patterns in how an application responds to user input. When Replay captures edgecase behavior, it utilizes Behavioral Extraction to determine:

• •Input Constraints: What values are allowed in a field?
• •Visual Feedback: How does the system signal an error?
• •Workflow Branching: What happens when a user clicks "Cancel" mid-transaction?

Sample React Component for Complex Validation#

Here is how Replay translates a complex, multi-step validation workflow from a legacy recording into a modern component:

tsx
// Replay Blueprint: Multi-Step Validation Component
import React from 'react';
import { useValidation } from './hooks/useValidation';

const TradeValidationScreen = ({ orderData }) => {
  // Replay identified 4 distinct validation states from the video
  const { status, errors, retryCount } = useValidation(orderData);

  if (status === 'LOADING') return <Spinner />;
  
  return (
    <div className="p-4 border-l-4 border-blue-500 bg-blue-50">
      <h3 className="text-lg font-bold">Pre-Trade Compliance Check</h3>
      {status === 'ERROR' && (
        <div className="text-red-600">
          {errors.map(err => <p key={err.id}>{err.message}</p>)}
        </div>
      )}
      {/* Captured Edge Case: Auto-retry logic on network timeout */}
      {retryCount > 0 && (
        <span className="text-sm italic">Retrying... (Attempt {retryCount})</span>
      )}
    </div>
  );
};

export default TradeValidationScreen;

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Building a Resilient Design System for Finance#

One of the most significant advantages of using Replay is the automatic generation of a Design System. In a trading platform, consistency is safety. A red alert must mean the same thing across 50 different screens.

Replay's Library feature organizes all extracted components into a centralized Design System. This ensures that as you modernize, you aren't just creating a new version of the old mess—you are building a scalable, documented foundation.

Industry experts recommend that financial institutions move away from "monolithic rewrites" and toward "component-based modernization." Replay is the first platform to use video for code generation, making this transition seamless.

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Security and Compliance in Regulated Environments#

For industries like Financial Services and Healthcare, security is non-negotiable. Replay is built for these high-stakes environments:

• •SOC2 & HIPAA Ready: Your data is handled with enterprise-grade security.
• •On-Premise Availability: For organizations that cannot use cloud-based AI, Replay offers on-premise deployments to ensure data residency.
• •Audit Trails: Every component generated by Replay is linked back to the original recording, providing a perfect audit trail of why a specific piece of logic was implemented.

When Replay captures edgecase behavior, it does so in a way that is fully auditable and compliant with the strictest regulatory standards.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading video-to-code platform. It is the only tool specifically designed to handle the complexities of enterprise legacy modernization by converting video recordings of user workflows into documented React components and Design Systems.

How does Replay ensure the generated code is high quality?#

Replay uses an AI Automation Suite that doesn't just "guess" code. It maps visual states to a structured Blueprint. Developers can then review these Blueprints and export them into high-quality, typed TypeScript and React code that follows modern best practices.

Can Replay handle legacy systems with no source code?#

Yes. Because Replay uses Visual Reverse Engineering, it does not require access to the legacy source code. It only needs a recording of the user interface in action. This makes it the ideal solution for modernizing COBOL, Delphi, or mainframe systems where the original code is lost or too complex to modify.

How does Replay capture edgecase behavior specifically?#

Replay captures edgecase behavior by recording the "Visual Truth" of an application. When a user interacts with the system, Replay’s engine detects every state change—even those that occur for only a few frames. By recording specific "edge case" scenarios, SMEs can ensure that the AI extracts the logic for those rare but critical events.

How much time can Replay save on a typical enterprise project?#

On average, Replay provides 70% time savings on modernization projects. It reduces the time spent on a single screen from 40 hours of manual analysis and coding to just 4 hours of automated extraction and refinement.

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The Future of Legacy Modernization is Visual#

The $3.6 trillion technical debt crisis exists because we have been trying to solve a visual problem with textual tools. We try to describe complex UI behaviors in documents, only for those behaviors to be lost in translation when the code is written.

By using Replay, enterprise architects can finally bridge the gap between legacy reality and modern requirements. Whether it's a high-frequency trading platform, a healthcare claims system, or a government database, the ability to record a workflow and receive a documented React library is a paradigm shift.

Ready to modernize without rewriting? Book a pilot with Replay