Best AI Prompts for Legacy Code Migration with Google Antigravity

TL;DR

• Google Antigravity (now part of Vertex AI) provides enterprise-grade code migration capabilities; effective prompts decompose complex migrations into manageable operations.
• The most effective migration prompts specify source and target technologies, provide concrete examples, and request verification of functional equivalence.
• Use Google Antigravity for translating between languages, refactoring patterns, and generating test coverage — not for understanding undocumented business logic.
• The combination of Google Antigravity’s translation capabilities plus human architectural oversight produces safer migrations.
• Always implement phased migrations with validation checkpoints rather than attempting big-bang rewrites.

Introduction

Legacy code migration is a multi-trillion dollar problem globally. Organizations run critical systems written in COBOL, older Java versions, and various proprietary languages that no one maintains expertise in anymore. These systems accumulate security vulnerabilities, cost disproportionate amounts to maintain, and limit the organization’s ability to compete. Yet rewriting them is terrifying — the risk of breaking something critical is real.

The challenge is that legacy migration requires deep understanding of existing code, careful translation to modern equivalents, and comprehensive validation that functionality is preserved. Doing this manually is slow, expensive, and error-prone. Modern AI tools like Google Antigravity can accelerate the translation and refactoring phases significantly.

Google Antigravity provides code generation and transformation capabilities that are particularly useful for migration work. It can understand code in multiple languages, generate equivalent functionality in target languages, and help with the mechanical parts of translation. The key is knowing how to prompt effectively — providing enough context about the source code, the target environment, and the business requirements to get useful migrations.

This guide provides the prompts that make Google Antigravity useful for legacy migration — breaking complex rewrites into manageable operations, generating clean modern code, and ensuring functional equivalence.

Table of Contents

1. The Legacy Migration Landscape
2. Prompt Engineering for Migration
3. COBOL Migration Prompts
4. Java Modernization Prompts
5. Monolith Decomposition Prompts
6. Database Migration Prompts
7. Testing Strategy Prompts
8. Quality Assurance Prompts
9. FAQ
10. Conclusion

1. The Legacy Migration Landscape

Understanding the scope of the problem.

COBOL Systems: Millions of lines of COBOL still run critical infrastructure — banking, government, insurance. COBOL programmers are retiring. New talent is scarce. These systems encode decades of business logic that must be preserved.

Java Legacy: Many organizations run Java applications on Java 7 or earlier. These systems work but accumulate technical debt. Migration to newer Java versions offers performance improvements and security updates.

Monolithic Architectures: Legacy systems are often large monoliths that are hard to change. Microservices migration promises agility but requires careful decomposition of tightly-coupled systems.

Data Migration: Legacy systems use legacy databases — Oracle forms, old SQL Server, even flat files. Data must migrate correctly, with proper transformation and validation.

Testing Gaps: Legacy systems often have minimal test coverage. Without tests, validating migration correctness is challenging. The cost of building tests before migration is often underestimated.

2. Prompt Engineering for Migration

How to prompt effectively for migration tasks.

Context is Everything: Provide the source code, the target language, and the business context. “Translate this” is less effective than “Translate this COBOL batch process for calculating interest to Java. Preserve the rounding behavior described in the comments.”

Concrete Examples: Show input/output examples. For calculation logic, show what outputs should result from specific inputs. This helps verify functional equivalence.

Explicit Requirements: State non-functional requirements explicitly. “Must handle 10,000 transactions per hour” or “Must maintain backward compatibility with existing API” guides the migration approach.

Decompose Complex Problems: Break large migrations into smaller prompts. A 10,000-line program should be migrated piece by piece, with each piece validated before proceeding.

Request Verification: Ask for help building test cases. “What inputs would verify this logic is correct?” generates test ideas you might not have considered.

3. COBOL Migration Prompts

Handle COBOL-specific migration challenges.

COBOL to Java Prompt: “Translate this COBOL program to Java: [paste COBOL]. Translation requirements: Preserve business logic exactly, Maintain numerical precision, Handle the same error conditions, Preserve file handling semantics. Provide the Java equivalent with comments explaining any non-obvious translations.”

COBOL Data Division Prompt: “Convert this COBOL Data Division to Java classes: [paste Data Division]. Generate: Java class definitions with appropriate types, Field mappings from COBOL picture clauses, Validation logic for numeric fields, Serialization methods if needed. Include handling of: COMP-3 packed decimals, Binary fields, Redefines clauses.”

COBOL Perform Logic Prompt: “Translate this COBOL PERFORM structure to modern Java: [paste PERFORM statements]. The business logic being performed: [describe]. Generate equivalent Java method calls that preserve: Loop semantics, GO TO alternatives if any, Section execution order. Handle any fall-through behavior.”

COBOL File Handling Prompt: “Convert this COBOL file handling to Java: [paste file operations]. File format: [sequential, indexed, line sequential]. Record format: [describe]. Generate: Java file I/O or appropriate library usage, Error handling equivalent to COBOL file status, Buffer handling if relevant.”

COBOL Business Rules Prompt: “Extract and translate these business rules from COBOL: [paste relevant code sections]. Business rules identified: [list]. These rules determine: [what they control]. Generate the rules in: Java implementation, Possibly a rules engine format if appropriate. Preserve: Exact calculation logic, Boundary conditions, Error handling.”

4. Java Modernization Prompts

Modernize older Java codebases.

Java Version Upgrade Prompt: “Modernize this Java 7 code to Java 17: [paste code]. Use modern Java features: Stream API, Optional, records where appropriate, Lambda expressions. Preserve: Exact behavior, Error handling, Thread safety if relevant. Show the modernized version with explanations.”

Framework Migration Prompt: “Migrate this from Spring XML config to Java config: [paste XML]. Target: [Spring Boot style Java configuration]. Generate equivalent: @Configuration class, @Bean definitions, ComponentScan if needed. Preserve: Bean names, Dependencies, Scopes.”

JDBC to JPA Prompt: “Convert this JDBC code to JPA/Hibernate: [paste JDBC]. Entity to map: [describe]. Generate: Entity class, Repository interface, JPQL queries where needed. Preserve: Query logic, Transaction handling, Exception semantics.”

Legacy Pattern Update Prompt: “Modernize this legacy Java pattern: [describe pattern, e.g., service locator, static utils]. Current code: [paste]. Target pattern: [what to use instead]. Generate modern equivalent: Dependency injection, Proper OOP design. Include migration path if existing code references the old pattern.”

Multithreading Update Prompt: “Modernize this legacy threading code: [paste]. Current approach: [threads, executors, etc.]. Modern Java concurrency: [java.util.concurrent improvements]. Generate: Updated implementation using modern constructs, Thread safety guarantees, Migration notes for dependent code.”

5. Monolith Decomposition Prompts

Break monoliths into services.

Service Boundary Prompt: “Analyze this monolith module for service extraction: [describe module]. Current dependencies: [list]. Extracted service responsibilities: [what this service would do]. Recommended boundaries: [how to split]. API design: [how it should be called]. This is a candidate for: [microservice, serverless, etc.]. Generate a service specification.”

API Design Prompt: “Design a migration API for extracting this function: [describe function]. Current signature: [method/parameters]. Callers: [what uses this function]. Design: REST endpoint, Request/response format, Error responses. Make it: Easy to call from legacy code, Future-proof for microservices.”

Data Decomposition Prompt: “Analyze data dependencies for service extraction: [describe data usage]. Tables/entities used: [list]. Read patterns: [how data is accessed]. Write patterns: [how data is modified]. Generate: Database design for extracted service, API for data access, Sync strategy for data that remains shared.”

Migration Sequence Prompt: “Plan a migration sequence for this monolith: [describe monolith]. Services to extract: [priority list]. Dependencies between services: [what must migrate first]. Migration order: [sequence]. For each service: Entry criteria, Migration steps, Validation, Rollback trigger.”

Legacy Bridge Prompt: “Design a bridge strategy for gradual migration: [describe migration]. Need to: [keep old and new running simultaneously]. Bridge approach: [API gateway, adapter pattern, etc.]. How to route requests: [between old and new]. Generate the bridge implementation.”

6. Database Migration Prompts

Handle database modernization.

Schema Conversion Prompt: “Convert this legacy schema to modern database: [describe source]. Target: [PostgreSQL, MySQL, etc.]. Source schema: [DDL or description]. Generate: DDL for modern database, Index recommendations, Any data type conversions needed. Preserve: Relationships, Constraints, Stored procedures (migrate to application code if possible).”

Stored Procedure Conversion Prompt: “Migrate these stored procedures to application code: [paste procedures]. Target language: [Java, Python, etc.]. Database: [target DB]. Approach: Move logic out of database into application layer. Generate: Equivalent application code, Updated queries, Transaction handling.”

Data Migration Script Prompt: “Generate a data migration script: Source schema: [describe]. Target schema: [describe]. Data volume: [estimate]. Migration approach: [ETL, dual-write, etc.]. Generate: Extraction queries, Transformation logic, Loading process, Validation checks, Rollback steps.”

Index Strategy Prompt: “Design an index strategy for migrated database: Schema: [describe]. Query patterns: [common queries]. Migration from: [source indexing if any]. Generate: Index recommendations by table, Composite indexes, Any covering indexes for performance. Verify: Primary keys, Foreign keys preserved.”

Query Optimization Prompt: “Analyze and optimize these migrated queries: [paste queries]. Table sizes: [describe]. Performance issues: [symptoms]. Target: [acceptable performance]. Generate: Optimized queries, Index recommendations, Rewrite suggestions if queries are fundamentally inefficient.”

7. Testing Strategy Prompts

Build comprehensive test coverage.

Test Case Generation Prompt: “Generate test cases for migrated code: Source behavior: [how original worked]. Migrated code: [new implementation]. Test approach: [black box testing of business logic]. Generate: Input/output test cases, Edge cases from original code, Boundary conditions, Error conditions that should be handled.”

Equivalence Verification Prompt: “Design equivalence tests for migration: Original code logic: [describe]. Migrated implementation: [describe]. How to verify equivalence: [methodology]. Test cases that prove: Same outputs for same inputs, Same error handling, Same edge cases. Generate a comprehensive test suite.”

Performance Benchmark Prompt: “Design performance benchmarks for migration: Original system performance: [baseline metrics]. Requirements: [performance targets]. Migrated system: [describe]. Generate: Benchmark scenarios, Load testing approach, Comparison methodology, Acceptance criteria.”

Integration Test Prompt: “Design integration tests for migrated components: Components: [list]. Dependencies: [external systems]. Migration scope: [what changed]. Generate: Integration test cases, Test data needed, Mock/stub requirements, Failure scenarios to test.”

Regression Test Prompt: “Design regression tests for migration: Areas migrated: [list]. Risk of regression: [high/medium/low]. Existing test coverage: [what exists]. Additional tests needed: [list]. Generate tests that would catch: Functional regressions, Performance regressions, Security regressions.”

8. Quality Assurance Prompts

Ensure migration quality.

Code Review Prompt: “Review this migrated code for quality: Original requirement: [what it should do]. Migrated code: [paste]. Quality concerns: [specific areas to check]. Check: Functional equivalence to original, Modern best practices, Error handling, Performance, Security. Provide a quality assessment and any issues found.”

Security Scan Prompt: “Scan this migrated code for security issues: Code: [paste]. Common vulnerabilities: [OWASP Top 10, etc.]. Original security posture: [what protections existed]. Check for: Injection vulnerabilities, Authentication issues, Data exposure, API security. Generate: Security assessment, Recommended fixes.”

Code Quality Prompt: “Assess code quality of this migration: Code: [paste]. Standards target: [your code standards]. Generate quality metrics: [complexity, maintainability, etc.]. Issues to address: [list]. Recommendations: [improvements]. Target a grade: [A, B, etc. for code quality].”

Documentation Review Prompt: “Review documentation completeness for migration: What exists: [list docs]. Migration scope: [what was migrated]. Missing documentation: [what’s needed]. Generate: Required documentation checklist, Draft documentation for any missing pieces, Update recommendations.”

Migration Handoff Prompt: “Create a migration handoff document: Source system: [describe]. Target system: [describe]. Migration date: [when]. Outstanding issues: [list]. Operational considerations: [what operations needs to know]. Generate: Complete handoff document, Outstanding items list, Sign-off checklist.”

FAQ

Can Google Antigravity handle complete COBOL to Java migration? No single tool handles complete migration. Google Antigravity helps with code translation, but understanding business logic, managing migration risks, and validating results require human expertise throughout. Use it to accelerate mechanical translation, not to replace judgment.

How do I ensure functional equivalence? Build comprehensive tests against the original system first. Then verify migrated code produces identical results. Use comparison testing, A/B testing in production if possible, and thorough user acceptance testing. Never assume migration is correct without validation.

What is the safest approach for large migrations? Strangler fig migration — gradually replacing pieces of the legacy system while keeping it running. This allows validation at each step, minimizes risk, and enables rollback if issues occur. Big-bang migrations are high-risk for critical systems.

How do I handle undocumented business logic? This is the hardest part. Interview domain experts. Trace data through the system. Look for comments, naming conventions, and code structure that reveal intent. Use AI to explain code, but validate explanations with business stakeholders before proceeding.

When should I migrate vs. rehost vs. replace? Rehost (lift-and-shift) is fastest but does not address technical debt. Migrate (move with modernization) is balanced. Replace (rewrite) is highest risk but produces best long-term results. Choose based on: system criticality, available expertise, timeline, and budget.

Conclusion

Legacy migration is high-risk but often necessary. Google Antigravity accelerates the mechanical parts of translation while human judgment handles architectural decisions, business logic interpretation, and validation. The key is decomposing complex migrations into manageable pieces and validating each piece before proceeding.

Your next step is to assess your legacy portfolio and identify one component that is a good migration candidate. Use the appropriate prompts to plan the migration, generate the new code, and build comprehensive tests. One successful migration builds confidence for increasingly ambitious modernization efforts.