Antigravity × Legacy Code Migration — AI-Driven Code Modernization

Setup and context — Why Legacy Code Migration Matters Now

In modern software development, legacy code is an unavoidable challenge. Code written a few years ago accumulates as technical debt, making new feature development and bug fixes increasingly time-consuming — a reality most development teams face.

Yet a complete rewrite is the riskiest approach possible. What's needed is a way to incrementally and safely modernize systems without taking them offline.

This is where Antigravity shines. AI agents can rapidly analyze massive codebases, map dependencies, draft migration plans, transform code, and generate tests. In this article, we'll walk through a complete strategy for legacy code migration powered by Antigravity.

Three Core Migration Strategies

There are several fundamental strategies for migration, and the right choice depends on your project's circumstances.

The Strangler Fig Pattern

This approach keeps the existing system running while building new features on a modern architecture, gradually replacing the old parts. Originally proposed by Martin Fowler, this pattern minimizes risk while enabling incremental progress.

To implement this pattern with Antigravity, start by defining your migration policy in AGENTS.md.

# Migration Policy
- All new endpoints must be built on the new architecture
- Analyze impact scope before modifying existing code
- Place an Anti-corruption Layer at legacy/modern boundaries

Branch by Abstraction

This technique defines common interfaces that let you swap between legacy and modern implementations. Combined with feature flags, it enables safe rollouts.

// Abstraction layer definition
interface PaymentGateway {
  processPayment(amount: number, currency: string): Promise<PaymentResult>;
  refund(transactionId: string): Promise<RefundResult>;
}
 
// Legacy implementation
class LegacyPaymentGateway implements PaymentGateway {
  async processPayment(amount: number, currency: string) {
    // Calls the existing SOAP API
  }
}
 
// Modern implementation
class ModernPaymentGateway implements PaymentGateway {
  async processPayment(amount: number, currency: string) {
    // Uses the Stripe API
  }
}

Parallel Run

Run both old and new systems simultaneously and compare results. This is especially effective for systems where data integrity is critical.

Dependency Analysis with Antigravity

The first step in any legacy migration is understanding the full dependency graph. Feed your codebase into Antigravity's agents and have them visualize the dependency structure.

Running a Codebase Scan

In Antigravity's terminal, issue a prompt like this:

Analyze the entire project's dependency structure and report:
1. Module-to-module dependency graph
2. Circular dependency detection
3. Modules with the highest coupling
4. External library dependencies and version drift from latest

Antigravity performs static analysis and calculates coupling and cohesion metrics for each module. Use these results to determine migration priority.

The Migration Priority Matrix

Map the analysis results along two axes:

• Business Impact: How much does this module affect revenue or user experience?
• Technical Risk: How broad is the blast radius and how complex is the change?

Start with modules that have high business impact and low technical risk.

Incremental Migration in Practice

Phase 1: Build the Test Harness

Before touching any legacy code, establish tests that guarantee existing behavior. Have Antigravity auto-generate integration tests for every existing endpoint.

Generate characterization tests for all public methods in this module.
Use actual input/output values as assertions to capture the current behavior exactly.

Characterization tests record the "current behavior" of legacy code as-is — bugs and all. This lets you detect regressions during migration.

// Example characterization test generated by Antigravity
describe('LegacyOrderProcessor', () => {
  it('calculates tax-inclusive total (rounds down)', () => {
    const processor = new LegacyOrderProcessor();
    const result = processor.calculateTotal(1000, 0.1);
    // Current implementation uses Math.floor
    expect(result).toBe(1100);
  });
 
  it('returns null on insufficient stock (not an exception)', () => {
    const processor = new LegacyOrderProcessor();
    const result = processor.processOrder('SKU-001', 999);
    // Legacy code returns null by design
    expect(result).toBeNull();
  });
});

Phase 2: Extract Interfaces

With your safety net in place, extract interfaces from the legacy code. Have Antigravity analyze classes and carve out their public APIs as interfaces.

Analyze LegacyOrderProcessor and perform the following:
1. Extract an interface from its public methods
2. Convert the existing class into an implementation of that interface
3. Update all dependent call sites to reference the interface

Phase 3: Develop the Modern Implementation

Build a modern implementation against the extracted interface. This is the phase where you introduce new tech stacks and architecture patterns.

// Modern implementation
class ModernOrderProcessor implements OrderProcessor {
  constructor(
    private readonly taxService: TaxService,
    private readonly inventoryService: InventoryService,
    private readonly eventBus: EventBus
  ) {}
 
  async calculateTotal(
    baseAmount: number,
    taxRate: number
  ): Promise<number> {
    // New: uses an external tax rate service
    const actualRate = await this.taxService.getRate(taxRate);
    return Math.round(baseAmount * (1 + actualRate));
  }
 
  async processOrder(
    sku: string,
    quantity: number
  ): Promise<OrderResult> {
    const available = await this.inventoryService.check(sku, quantity);
    if (!available) {
      throw new InsufficientStockError(sku, quantity);
    }
    // Event-driven: downstream processing runs asynchronously
    await this.eventBus.publish(new OrderCreatedEvent(sku, quantity));
    return { status: 'accepted', sku, quantity };
  }
}

Phase 4: Feature Flag-Driven Switching

Use feature flags to safely toggle between implementations. Route a small percentage of traffic to the new implementation first, confirm everything works, then flip the switch entirely.

// Feature flag routing
function getOrderProcessor(userId: string): OrderProcessor {
  if (featureFlags.isEnabled('modern-order-processor', userId)) {
    return container.resolve(ModernOrderProcessor);
  }
  return container.resolve(LegacyOrderProcessor);
}

Database Schema Migration

Schema migration must happen carefully alongside code migration.

The Expand-Contract Pattern

Split schema changes into two phases: Expand and Contract.

1. Expand: Add new columns or tables without breaking existing ones
2. Migrate: Copy and transform data into the new format
3. Contract: Drop old columns and tables

When asking Antigravity to generate migration scripts, always request rollback procedures as well.

Generate migration scripts for the following schema changes.
Include:
- Up migration (apply changes)
- Down migration (rollback)
- Data migration script
- Pre/post validation queries

Testing Strategy: Guaranteeing Migration Safety

Building the Test Pyramid

Migration projects require a testing pyramid that goes beyond the usual layers — contract tests and parallel-run tests become critical.

• Unit tests: Verify individual methods of the new implementation
• Integration tests: Validate data flow across legacy/modern boundaries
• Contract tests: Guarantee interface compatibility
• Parallel-run tests: Compare old and new outputs, detect discrepancies

// Parallel-run test example
describe('ParallelRun: OrderProcessor', () => {
  it('produces matching results between old and new', async () => {
    const legacy = new LegacyOrderProcessor();
    const modern = new ModernOrderProcessor(taxService, inventoryService, eventBus);
 
    const testCases = generateTestCases(1000); // 1000 random test cases
 
    for (const tc of testCases) {
      const legacyResult = legacy.calculateTotal(tc.amount, tc.taxRate);
      const modernResult = await modern.calculateTotal(tc.amount, tc.taxRate);
 
      expect(modernResult).toBe(legacyResult);
    }
  });
});

Risk Management and Rollback Planning

Automating Regression Detection

Use Antigravity to embed migration-specific checks into your CI pipeline.

# .github/workflows/migration-check.yml
name: Migration Regression Check
on: [push, pull_request]
 
jobs:
  parallel-run:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run parallel comparison tests
        run: npm run test:parallel-run
      - name: Check migration coverage
        run: npm run test:migration-coverage -- --threshold 95

Documenting Rollback Procedures

Prepare for the worst by documenting rollback procedures in both AGENTS.md and your operations documentation. Ensuring Antigravity can immediately report the current migration state and rollback steps on demand is essential.

Practical Example: Migrating from Express.js to Hono

Let's walk through a concrete migration — moving an Express.js API server to Hono.

Step 1: Extract the Routing Layer

// Abstract router interface
interface AppRouter {
  get(path: string, handler: RequestHandler): void;
  post(path: string, handler: RequestHandler): void;
  use(middleware: MiddlewareHandler): void;
  listen(port: number): Promise<void>;
}

Step 2: Adapter Implementation

Have Antigravity generate adapters for both Express and Hono, switching between them with feature flags. Migrate endpoint by endpoint, verifying performance and correctness at each step.

Step 3: Middleware Migration

Rewrite middleware — authentication, logging, error handling — in a framework-agnostic form. This is the most labor-intensive part, but having Antigravity analyze existing middleware and decompose it into pure functions significantly speeds up the process.

Conclusion — Make AI Your Code Modernization Partner

Legacy code migration is both a technical and an organizational challenge. Antigravity dramatically accelerates the technical work — dependency analysis, test generation, code transformation — but the strategic decisions around migration approach and risk management still require human judgment.

The strategies covered here — the Strangler Fig Pattern, Branch by Abstraction, and feature flag-driven incremental switching — can be combined to safely modernize systems without taking them offline.

Migration is a marathon. Rather than trying to change everything at once, accumulate small wins one step at a time. With Antigravity as a powerful partner at your side, each step forward brings you closer to a modern, maintainable codebase.