How FinCorp Migrated 2M Lines of Java to Go using Agentic Strangler Figs

FinCorp (anonymized) had a 15-year-old Java EE monolith handling $4B in transactions daily. It was brittle, hard to deploy, and impossible to hire for. The migration estimate was 3 years and $12M.

The Agentic Approach

Instead of a manual rewrite, we deployed a custom Agent swarm specialized in the 'Strangler Fig' pattern. The workflow was:

• Analysis: Agent A reads the Java Class, mapping all public methods and dependencies.
• Interface Generation: Agent B creates a Go interface that exactly matches the Java contract.
• Logic Translation: Agent C translates the business logic, method by method, using a specialized prompt tuned on FinCorp's Go style guide.
• Test Verification: Agent D generates a test suite that runs against BOTH the Java and Go versions to ensure parity.

The Results

We achieved a migration velocity of **5,000 lines of code per day**. The entire core transactional engine was ported in 4 months, not 3 years.

Crucially, the human engineers did not write the Go code. They reviewed it. Their job shifted from 'Translator' to 'Auditor'. They caught subtle rounding errors that the Agent missed, but they were spared the months of typing boilerplate.