Large Codebase Management with AI
Managing Legacy Code and Technical Debt
5 min read
The Legacy Code Reality
Every long-lived codebase accumulates technical debt:
- Outdated patterns and dependencies
- Missing tests and documentation
- Quick fixes that became permanent
- Knowledge loss as developers leave
AI assistants are particularly valuable here—they can analyze unfamiliar code without the frustration humans experience.
Understanding Legacy Code
Generating Documentation from Code
claude "This file has no documentation. Analyze it and generate:
1. A module overview
2. JSDoc comments for each function
3. A list of potential issues or risks"
AI produces documentation:
/**
* Order Processing Module
*
* Handles the complete order lifecycle from cart to fulfillment.
* Originally written in 2019, uses callback patterns.
*
* Known issues:
* - No error recovery for payment failures
* - Inventory not reserved until payment complete (race condition)
* - Hardcoded tax rates (should use TaxService)
*
* @module services/order
* @deprecated Consider migrating to OrderServiceV2
*/
/**
* Creates a new order from the user's cart
*
* @param {string} userId - The user placing the order
* @param {Cart} cart - Cart object with items
* @param {function} callback - Called with (error, order)
*
* @warning Does not validate cart items against current inventory
* @warning Tax calculation is hardcoded to 8.5%
*/
function createOrder(userId, cart, callback) {
// ... existing code
}
Creating Architectural Overviews
claude "This is a 10-year-old Rails monolith. Create a high-level
architecture document showing:
1. Major components and their responsibilities
2. Data flow between components
3. External integrations
4. Known architectural issues"
Identifying Technical Debt
Automated Debt Detection
claude "Analyze this codebase for technical debt. Look for:
1. Deprecated dependencies
2. Copy-pasted code (DRY violations)
3. Functions over 50 lines
4. Files with no test coverage
5. TODO/FIXME/HACK comments
6. Circular dependencies"
AI produces a debt inventory:
# Technical Debt Inventory
## Critical (Block deployment)
- [ ] lodash@3.10.1 has known vulnerabilities (CVE-2021-23337)
- [ ] moment.js is deprecated, 847 usages found
## High (Fix within sprint)
- [ ] 23 functions exceed 50 lines (see appendix)
- [ ] 12 files have 0% test coverage (core business logic)
- [ ] Circular dependency: payment ↔ order ↔ inventory
## Medium (Plan for next quarter)
- [ ] 156 TODO comments, oldest from 2021
- [ ] 34 HACK comments indicating workarounds
- [ ] 89 instances of copy-pasted validation code
## Low (Track but don't prioritize)
- [ ] Inconsistent naming conventions (camelCase vs snake_case)
- [ ] 12 unused exports
- [ ] Outdated JSDoc comments (45 functions)
## Debt by Directory
- src/services/: 34% of total debt
- src/legacy/: 28% of total debt
- src/utils/: 18% of total debt
Prioritizing Debt Repayment
claude "Given our debt inventory, create a prioritized
refactoring plan. Consider:
- Risk of each item
- Effort required
- Business value
- Dependencies between items"
Safe Refactoring Strategies
The Strangler Fig Pattern
claude "Help me apply the Strangler Fig pattern to replace
our legacy OrderService. Create a plan that:
1. Identifies all consumers of OrderService
2. Creates a new OrderServiceV2 interface
3. Gradually migrates consumers
4. Maintains backward compatibility"
AI creates migration plan:
# OrderService Strangler Fig Migration
## Phase 1: Create Facade (Week 1)
1. Create OrderServiceFacade that wraps OrderService
2. Add feature flag: USE_ORDER_V2
3. Update all 34 consumers to use facade
4. Deploy and verify no behavior change
## Phase 2: Build New Service (Week 2-3)
1. Create OrderServiceV2 with modern patterns
2. Add comprehensive test suite
3. Implement same interface as facade
## Phase 3: Gradual Migration (Week 4-6)
1. Enable V2 for internal users (feature flag)
2. Monitor for errors, compare outputs
3. Enable for 10% → 50% → 100% of users
4. Keep V1 as fallback
## Phase 4: Cleanup (Week 7)
1. Remove feature flag
2. Remove facade
3. Delete legacy OrderService
4. Update documentation
## Rollback Plan
At any phase, disable USE_ORDER_V2 flag to
immediately revert to legacy behavior.
Characterization Testing
claude "This legacy function has no tests. Create characterization
tests that capture its current behavior so we can safely refactor."
// AI generates characterization tests
describe('createOrder (legacy characterization)', () => {
// These tests document CURRENT behavior, not desired behavior
it('returns order with auto-generated ID', () => {
const result = createOrder(user, cart);
expect(result.id).toMatch(/^ORD-\d{8}$/);
});
it('applies hardcoded 8.5% tax rate', () => {
const cart = { items: [{ price: 100 }] };
const result = createOrder(user, cart);
expect(result.tax).toBe(8.5); // Known debt: should use TaxService
});
it('silently ignores invalid items instead of failing', () => {
const cart = { items: [{ price: -50 }] }; // Invalid
const result = createOrder(user, cart);
expect(result.items).toEqual([]); // Quirky behavior we must preserve
});
it('uses callback pattern for async operations', (done) => {
createOrder(user, cart, (err, order) => {
expect(err).toBeNull();
expect(order).toBeDefined();
done();
});
});
});
Modernization Patterns
Gradual TypeScript Migration
claude "Help me add TypeScript to this JavaScript file.
Create types based on:
1. How functions are called
2. What they return
3. External API responses
Keep it gradual—use 'any' for uncertain types."
Dependency Updates
claude "Update our dependencies safely:
1. List all outdated packages
2. Identify breaking changes between versions
3. Create a migration plan with tests
4. Flag packages that need code changes"
# Dependency Update Plan
## Safe Updates (no code changes)
```bash
npm update axios lodash-es date-fns
Minor Code Changes Required
- react-query@3 → @tanstack/react-query@5
- 12 files need import updates
- Hook names unchanged
- AI can auto-fix
Major Migration Required
- redux@4 → zustand@4
- Complete store rewrite
- 45 connected components
- Recommend phased migration
## Creating a Sustainable Path
### Establish Modernization Guidelines
```bash
claude "Create a CONTRIBUTING.md section on technical debt:
1. How to identify debt while working
2. How to document new debt
3. When debt is acceptable
4. How debt gets prioritized for fixing"
Track Progress
claude "Compare our tech debt from 3 months ago
to today. Show:
- Debt items resolved
- New debt introduced
- Net change
- Velocity of debt reduction"
Module Summary
You've learned to:
- Manage context effectively in large codebases
- Navigate and understand unfamiliar code
- Identify and prioritize technical debt
- Safely refactor legacy code with AI assistance
Next, we'll cover advanced debugging and refactoring techniques—using AI to solve complex problems. :::