Modernization of Legacy COBOL Codebase With Claude Code

 Modernizing Legacy COBOL Codebases with Claude Code: A Step-by-Step Demo

In today's fast-paced digital world, many organizations still rely on legacy mainframe systems written in COBOL. These systems handle critical operations—like credit card management—but maintaining them is increasingly challenging due to a lack of documentation, retiring experts, and difficulty hiring skilled developers. Enter Claude Code, Anthropic's powerful agentic coding tool that empowers developers to modernize these codebases with unprecedented speed and confidence.

In this post, we'll walk through a real demo using Claude Code to modernize a sample COBOL codebase. The example draws from AWS's mainframe modernization demo repository: a medium-sized credit card management system with around 100 files, including COBOL programs, copybooks, and JCL scripts.

 Phase 1: Discovery and Documentation

Legacy codebases often come with little to no documentation, embedding vital business logic and regulatory requirements in obscure code. The original developers may have moved on, and COBOL expertise is rare and expensive.

Claude Code tackles this head-on. We start by creating a specialized sub-agent—a COBOL documentation expert and translator—using Claude Code's agent commands. Sub-agents run in parallel with isolated context windows, keeping the main process clean and efficient.

We enable "thinking mode" and instruct Claude Code to analyze the entire codebase architecture. It systematically creates a to-do list for all 94 files, tracking progress to ensure complete coverage without duplicates.

The results are impressive: far beyond basic comments. For instance, take the interest calculation program (CBAC04C):

- It extracts the full business workflow.

- Details how the program reads transaction category balances.

- Looks up interest rates by account group.

- Applies fallback rules for edge cases.

- Updates account records accordingly.

Claude Code does this for every file. It also generates two key index files in plain text:

- **catalog.txt**: Translates cryptic names (e.g., CBAC04C becomes "Interest Calculator Batch Program").

- **relationships.txt**: Maps dependencies in a simple pipe-delimited format.

Using these, Claude Code produces Mermaid diagrams visualizing the daily batch processing workflow—from transaction inputs through interest posting to customer statements.

In the demo, Claude Code ran autonomously for about an hour, producing over 100 pages of detailed documentation. But it's capable of much more: running for over 30 hours on larger projects. These techniques scale seamlessly to enterprise-sized codebases.

 Phase 2: Migration and Verification

With thorough documentation in hand, we move to migration. We tasked Claude Code with converting a core feature—the interest calculation program—to Java.

Switching to "planning mode," Claude Code first thinks through the strategy without jumping into edits. It identifies tricky COBOL patterns like line-break processing and multi-file coordination, then outlines a five-phase plan:

1. Create the project structure.

2. Translate data models from copybooks to Java classes.

3. Build an I/O layer compatible with original file formats.

4. Convert business logic, preserving COBOL-specific behaviors.

5. Develop a dual test harness: one using GNU COBOL 3.2.0 for the original, and one in Java 17.

The output isn't just a line-by-line translation. Claude Code generates idiomatic Java: proper classes, design patterns, robust error handling, and logging—code that modern teams can easily maintain.

Verification is crucial for confidence in mission-critical systems. Claude Code creates multiple test data files and runs them against both versions. It compares not only final outputs but intermediate calculations, file writes, and data transformations.

The result? Perfect bit-for-bit fidelity. Every calculation, rule, and edge case is preserved exactly.

 Why This Matters for Your Organization

This demo uses a relatively small application, but the techniques are fully scalable. Claude Code empowers developers to modernize legacy COBOL codebases with efficiency and confidence that was impossible just a year ago.

Whether you're dealing with undocumented mainframes or hard-to-maintain systems, tools like Claude Code bridge the gap—preserving business logic while unlocking modern architectures, cloud deployment, and easier maintenance.

If you're facing mainframe modernization challenges, explore Claude Code today. It could transform years of painstaking work into weeks of guided, AI-accelerated progress.

*Watch the original demo video on YouTube: [Claude Code modernizes a legacy COBOL codebase](https://www.youtube.com/watch?v=OwMu0pyYZBc)*