Automating Contract Discovery from Legacy Monoliths: A Visual Reverse Engineering Approach

The most expensive "dark matter" in your enterprise isn't the server rack gathering dust—it’s the undocumented API contracts buried within your legacy monolith. When documentation is missing (which, according to Replay’s analysis, is the case for 67% of legacy systems), developers are forced into a state of "Software Archaeology." They spend weeks clicking through ancient UIs with Chrome DevTools open, manually scribbling down JSON payloads and trying to guess which fields are mandatory. This manual process is the primary reason why the average enterprise rewrite takes 18 months and why 70% of these projects eventually fail or exceed their timelines.

We are currently facing a $3.6 trillion global technical debt crisis. The bottleneck isn't writing the new code; it's understanding what the old code actually does. Automating contract discovery from these monolithic UI interactions is no longer a luxury—it is a prerequisite for survival in regulated industries like Financial Services and Healthcare.

TL;DR: Manual reverse engineering of legacy APIs takes roughly 40 hours per screen. By using Replay for automating contract discovery from UI recordings, enterprises reduce this to just 4 hours. Replay captures real user workflows, maps network traffic to visual components, and generates documented React code and OpenAPI specs automatically, saving 70% of the typical modernization timeline.

---

The Hidden Cost of Manual API Mapping#

Industry experts recommend that before a single line of a new microservice is written, the existing "contract" between the frontend and backend must be fully understood. In a legacy Java or .NET monolith, these contracts are often implicit. There is no Swagger UI. There is no Postman collection. The "documentation" is the source code itself, often written by people who left the company during the Obama administration.

When you attempt to modernize without automating contract discovery from the source, you fall into the "Parity Trap." You spend months building a new system that fails because you missed a single

or a deeply nested XML field that the backend requires for validation.

Visual Reverse Engineering is the process of capturing live application behavior—both the visual state and the underlying network telemetry—to reconstruct the technical architecture without manual code review.

Replay changes this dynamic by treating the UI as the "source of truth." By recording a user performing a standard workflow (like "Onboard New Customer"), Replay captures every API call, every state change, and every validation rule.

---

The Mechanics of Automating Contract Discovery from UI Interactions#

How do we move from a video recording to a production-ready API contract? The process involves three distinct layers of extraction: Network Interception, Schema Inference, and Contextual Mapping.

1. Network Interception and Traffic Normalization#

Legacy systems often communicate using a "mush" of protocols. You might see REST-ish endpoints sitting alongside SOAP envelopes and binary blobs. Automating contract discovery from these interactions requires a tool that can normalize this traffic.

Replay’s AI Automation Suite intercepts these calls during the recording phase. It doesn't just look at the

responses; it analyzes the headers, cookies, and payload structures.

2. Schema Inference#

Once the traffic is captured, the system must infer the data types. Is

always a UUID, or does it occasionally revert to an integer in older records? Replay uses a probabilistic model to determine the schema, identifying optional vs. required fields based on multiple session recordings.

3. Contextual Mapping (The "Flows" Feature)#

This is where Replay excels. It links the network call to the specific UI component that triggered it. If a user clicks "Submit" and an

call is fired, Replay’s Flows engine maps the form fields in the UI to the keys in the JSON payload.

Learn more about mapping complex user flows

---

Comparison: Manual Discovery vs. Replay Automation#

The following table represents data collected from enterprise modernization projects in the Insurance and Banking sectors over a 12-month period.

---

Implementing Discovered Contracts in Modern React#

Once you have succeeded in automating contract discovery from your legacy system, the next step is implementation. Replay doesn't just give you a PDF of the API; it gives you the code.

According to Replay's analysis, the most successful modernization teams use the discovered contracts to generate TypeScript interfaces immediately. This ensures type safety between the legacy backend (which may stay in place for years) and the new React frontend.

Code Example 1: Inferred TypeScript Interface#

Below is an example of a TypeScript interface generated by Replay after observing a legacy "Loan Application" screen. Note how it handles the "Dark Debt"—the inconsistent naming conventions typical of legacy systems.

typescript
/**
 * Generated by Replay AI Automation Suite
 * Source: Legacy "Loan_App_v4" Screen
 * Discovery Method: Visual Reverse Engineering
 */

export interface LegacyLoanContract {
  // Inferred from 'application_id' input field
  appId: string;
  
  // Legacy systems often use inconsistent casing
  USER_DETAILS: {
    firstName: string;
    last_name: string; // Captured as-is from legacy JSON
    dob: string;      // ISO format inferred from 150+ samples
  };

  // Optional fields identified through multiple session recordings
  coSignerInfo?: {
    taxId: string;
    relationship: 'SPOUSE' | 'BUSINESS_PARTNER' | 'OTHER';
  };

  // Metadata required by the legacy SOAP wrapper
  _metadata: {
    transactionId: string;
    timestamp: number;
  };
}

Code Example 2: Connecting the Modern UI to the Discovered Contract#

With the contract discovered, Replay's Library feature allows you to generate a React component that is pre-wired to this data structure. This eliminates the "hand-off" friction between design and engineering.

tsx
import React from 'react';
import { useQuery } from '@tanstack/react-query';
import { LegacyLoanContract } from './contracts/loan-contract';

// Replay-generated component based on visual recording
export const LoanSummaryCard: React.FC<{ appId: string }> = ({ appId }) => {
  const { data, isLoading, error } = useQuery<LegacyLoanContract>({
    queryKey: ['loan', appId],
    queryFn: () => fetch(`/api/legacy/loans/${appId}`).then(res => res.json()),
  });

  if (isLoading) return <div className="skeleton-loader" />;
  if (error) return <div>Error loading legacy data.</div>;

  return (
    <div className="p-6 bg-white shadow-lg rounded-xl">
      <h2 className="text-xl font-bold">Application: {data?.appId}</h2>
      <div className="mt-4 grid grid-cols-2 gap-4">
        <div>
          <label className="text-sm text-gray-500">Applicant</label>
          <p>{data?.USER_DETAILS.firstName} {data?.USER_DETAILS.last_name}</p>
        </div>
        <div>
          <label className="text-sm text-gray-500">Date of Birth</label>
          <p>{data?.USER_DETAILS.dob}</p>
        </div>
      </div>
    </div>
  );
};

---

Why "Video-to-Code" is the Future of Enterprise Architecture#

Video-to-code is the process of converting screen recordings into functional, documented source code and design tokens. This is the core engine behind Replay.

By automating contract discovery from visual interactions, we solve the "Context Gap." In traditional development, a business analyst writes a requirement, a designer creates a Figma file, and a developer looks at the old code to figure out the API. Information is lost at every step.

With Replay, the recording is the requirement. The AI analyzes the recording to understand:

1. •The Visuals: What colors, fonts, and spacing are used? (Design System generation)
2. •The Logic: What happens when this button is clicked? (Flow generation)
3. •The Data: What information is sent to the server? (Contract discovery)

For organizations in highly regulated sectors, Replay offers an On-Premise solution and is HIPAA-ready, ensuring that sensitive PII (Personally Identifiable Information) captured during the recording process is handled with SOC2-compliant rigor.

Read about our SOC2 and HIPAA compliance

---

Overcoming the Challenges of Automating Contract Discovery from SOAP and XML#

Many legacy systems, particularly in Government and Telecom, don't use clean JSON. They use massive XML SOAP envelopes. Manual discovery here is a nightmare; a single request can be several thousand lines long.

Automating contract discovery from these systems requires deep packet inspection. Replay’s engine flattens these complex XML structures into usable JSON schemas that modern React applications can consume. This "Adapter Pattern" approach allows you to build a modern frontend today while the backend team works on a multi-year migration to microservices.

The Role of Blueprints in Architecture#

Within the Replay platform, Blueprints act as the bridge. Once a contract is discovered, the Blueprint editor allows architects to refine the inferred types, add documentation, and export them as OpenAPI/Swagger files. This creates a "living documentation" that stays in sync with the actual behavior of the legacy system.

---

Frequently Asked Questions#

How does Replay handle dynamic data during contract discovery?#

Replay uses a multi-session analysis approach. By recording the same workflow with different data inputs, Replay’s AI identifies which fields are static, which are dynamic, and which are conditional. This ensures that the automating contract discovery from UI interactions captures the full range of possible API states, not just a single "happy path" snapshot.

Can Replay discover hidden or background API calls?#

Yes. Replay captures all network activity initiated by the browser or client application during the recording session. This includes background polling, analytics pings, and secondary data fetches that are often missed during manual documentation efforts. This comprehensive visibility is crucial for ensuring the new system maintains full feature parity.

Is it possible to use Replay with Citrix or virtualized environments?#

Replay is designed to work with web-based legacy interfaces. For environments where the UI is delivered via virtualization (like Citrix), Replay can still be used if the underlying web application is accessible via a standard browser. For thick-client legacy apps, Replay offers specialized integration paths to capture telemetry.

What happens to the discovered contracts after the migration is over?#

The contracts discovered by Replay serve as the permanent documentation for your new architecture. Because they are exported in standard formats like TypeScript and OpenAPI, they become part of your new CI/CD pipeline, preventing future technical debt from accumulating.

---

The Path Forward: From 18 Months to 18 Weeks#

The goal of modernization isn't just to have "new code." The goal is to regain agility. When you spend 18 months on a rewrite, you aren't just losing money—you are losing market share.

By automating contract discovery from your existing systems, you bypass the most painful, error-prone phase of the project. You move directly from "What does this do?" to "Let's build something better." Replay provides the map, the compass, and the vehicle for this journey.

The $3.6 trillion technical debt mountain is tall, but with Visual Reverse Engineering, it is no longer insurmountable.

Ready to modernize without rewriting? Book a pilot with Replay