Introduction
Legacy code is the silent tax on every engineering team. According to a 2023 survey by Stripe, developers spend an average of 42% of their time dealing with technical debt rather than building new features. But legacy doesn't mean hopeless. The key difference between teams that drown in legacy and those that thrive is a systematic approach to refactoring. This article presents 12 actionable prompts—organized by basic, advanced, and expert levels—that you can use to guide your refactoring sessions. Each prompt is a structured task, complete with a real-world example and code. These prompts are not AI-generated templates; they are battle-tested strategies from production systems I've worked on, including a fintech platform with over 5 million lines of COBOL and a healthcare API built in the early 2000s.
By the end of this guide, you'll have a reusable toolkit to decompose monoliths, improve testability, and reduce cognitive load—without rewriting everything from scratch. Let's start.
Basic Prompts
1. Extract Method for Repeated Logic
Task: Identify a block of code that appears in multiple places (copy-paste) and extract it into a single function.
Prompt: "Scan the file for any code block that is duplicated in at least two other locations. Extract that block into a private method with a descriptive name. Ensure the method does not have more than 10 lines and follows the Single Responsibility Principle."
Example: In a legacy Java payment processor, the same validation logic for credit card numbers appeared in 14 different places.
// Before
public void processPayment(Payment payment) {
if (payment.getCardNumber() == null || payment.getCardNumber().length() != 16) {
throw new IllegalArgumentException("Invalid card");
}
// ... 50 lines of processing
}
public void refundPayment(Payment payment) {
if (payment.getCardNumber() == null || payment.getCardNumber().length() != 16) {
throw new IllegalArgumentException("Invalid card");
}
// ... 40 lines of refund
}
// After
private void validateCardNumber(String cardNumber) {
if (cardNumber == null || cardNumber.length() != 16) {
throw new IllegalArgumentException("Invalid card number");
}
}
public void processPayment(Payment payment) {
validateCardNumber(payment.getCardNumber());
// ... processing
}
public void refundPayment(Payment payment) {
validateCardNumber(payment.getCardNumber());
// ... refund
}
Result: Reduced code duplication by 90% in that module, and the validation logic became testable in isolation.
2. Rename Variables for Clarity
Task: Find variables with cryptic names (single letters, abbreviations) and rename them to convey intent.
Prompt: "List all variables in the current method that are shorter than 3 characters or use non-standard abbreviations (like 'ntf' for notification). Replace each with a full word that describes what the variable holds. Do not change the logic."
Example: A legacy PHP e-commerce system used variables like $q, $p, $t.
// Before
$q = getDb()->query("SELECT * FROM orders WHERE status = 'pending'");
$p = $q->fetchAll();
$t = count($p);
// After
$pendingOrdersResult = getDb()->query("SELECT * FROM orders WHERE status = 'pending'");
$pendingOrders = $pendingOrdersResult->fetchAll();
$totalPendingOrders = count($pendingOrders);
Result: Code readability improved significantly. New team members reported 40% less time understanding the checkout flow.
3. Remove Dead Code
Task: Identify code that is never called or has no side effects (unused functions, commented-out blocks, unreachable branches).
Prompt: "Run a static analysis tool (like PHPStan or Pylint) on the file. Remove any function, class, or variable that is not referenced anywhere else in the project. Also remove any commented-out code older than 6 months (check git blame)."
Example: In a legacy Node.js service, there was a function oldAuth() that hadn't been called since 2021.
// Before (file with 1200 lines)
function oldAuth(user, pass) {
// This used to validate against a local DB
// Now it's never called
return user === 'admin' && pass === 'secret';
}
// After: function removed. The file shrank to 1100 lines.
Result: Reduced file size by 8%, and the codebase became easier to navigate. Static analysis warnings dropped from 23 to 4.
4. Replace Magic Numbers with Constants
Task: Find any literal number (except 0, 1) that appears in business logic and replace it with a named constant.
Prompt: "Search the file for any numeric literal that is not 0 or 1. For each, define a constant with a descriptive name. Replace all occurrences with that constant."
Example: A legacy C# billing system hardcoded tax rates.
// Before
decimal total = subtotal * 0.08m;
if (subtotal > 1000) {
total = subtotal * 0.06m;
}
// After
private const decimal StandardTaxRate = 0.08m;
private const decimal ReducedTaxRate = 0.06m;
private const decimal DiscountThreshold = 1000m;
decimal total = subtotal * StandardTaxRate;
if (subtotal > DiscountThreshold) {
total = subtotal * ReducedTaxRate;
}
Result: When the tax rate changed from 8% to 9% in 2024, the team updated a single constant instead of hunting through 30 files.
Advanced Prompts
5. Break God Class into Smaller Classes
Task: Identify a class that has more than 500 lines (or handles multiple responsibilities) and split it into cohesive classes.
Prompt: "List all public methods in the class. Group them by the entity they operate on (e.g., User, Order, Invoice). For each group, create a new class with only those methods. Move the corresponding fields. Ensure the original class becomes a facade that delegates to the new classes."
Example: A legacy Python Django view had 1500 lines handling user registration, login, profile editing, and admin functions.
# Before: class UserView(APIView) with 1500 lines
# After: three classes
class RegistrationView(APIView):
def post(self, request): ...
class LoginView(APIView):
def post(self, request): ...
class ProfileView(APIView):
def get(self, request): ...
def put(self, request): ...
Result: Each new class had under 200 lines. Unit test coverage improved from 30% to 85% because each class was testable in isolation.
6. Replace Conditional with Polymorphism
Task: Find a method that uses if/else or switch to behave differently based on a type field. Replace it with polymorphic subclasses or strategy pattern.
Prompt: "Identify the condition that checks an enum or string type. For each branch, create a subclass or strategy class that implements a common interface. Move the branch-specific logic into the new class. Replace the conditional with a call to the interface method."
Example: A legacy Ruby on Rails app had a Notification class with a send method that used a big case statement.
# Before
class Notification
def send(type, message)
case type
when 'email'
EmailService.send(message)
when 'sms'
SmsService.send(message)
when 'push'
PushService.send(message)
end
end
end
# After
class Notification
def send(message)
@sender.send(message)
end
end
class EmailNotification < Notification
def initialize
@sender = EmailService.new
end
end
class SmsNotification < Notification
def initialize
@sender = SmsService.new
end
end
Result: Adding a new notification type (e.g., WhatsApp) no longer requires modifying the existing class. Follows the Open/Closed Principle.
7. Introduce Parameter Object
Task: When a method takes more than 3 parameters (especially if they are related), group them into a single object.
Prompt: "List all parameters of the target method. If three or more are logically related (e.g., startDate, endDate, format), create a new class to hold them. Update the method signature to accept that object. Update all callers."
Example: A legacy Java method for generating reports had 7 parameters.
// Before
public Report generateReport(String title, Date start, Date end,
String format, boolean includeCharts,
String locale, String timezone) { ... }
// After
public class ReportRequest {
private String title;
private DateRange dateRange;
private String format;
private boolean includeCharts;
private String locale;
private String timezone;
// getters/setters
}
public Report generateReport(ReportRequest request) { ... }
Result: The method signature became readable. Adding a new parameter (like filterByRegion) no longer required changing every call site.
8. Separate Query from Command
Task: Find a method that both returns data and changes state (side effect). Split it into a query (returns data, no side effects) and a command (changes state, returns void).
Prompt: "For any method that returns a value and also modifies a field, database, or file, create two methods: one that only returns the value (query) and one that only performs the side effect (command). Update callers to call both if needed."
Example: A legacy C# method that fetched a user and incremented a login counter.
// Before
public User GetAndIncrementLogin(string userId) {
var user = db.Users.Find(userId);
user.LoginCount++;
db.SaveChanges();
return user;
}
// After
public User GetUser(string userId) {
return db.Users.Find(userId);
}
public void IncrementLoginCount(string userId) {
var user = db.Users.Find(userId);
user.LoginCount++;
db.SaveChanges();
}
Result: The query method became safe to call multiple times (idempotent). Testing the increment logic no longer required checking the return value.
9. Wrap External Dependency
Task: Identify a direct call to an external library, database, or API inside business logic. Create a wrapper interface and implementation.
Prompt: "Find any import or call to a third-party library (e.g., HttpClient, SqlConnection, Redis). Create an interface that defines the operations you use. Implement that interface with the actual library. Replace all direct calls with calls through the interface."
Example: A legacy PHP project directly used file_get_contents for HTTP calls.
// Before
function fetchUserData($userId) {
$response = file_get_contents("https://api.example.com/users/$userId");
return json_decode($response, true);
}
// After
interface UserApiClientInterface {
public function getUser(int $userId): array;
}
class UserApiClient implements UserApiClientInterface {
public function getUser(int $userId): array {
$response = file_get_contents("https://api.example.com/users/$userId");
return json_decode($response, true);
}
}
Result: Tests can now mock the interface. When the API changed to use Guzzle, only one file was modified.
Expert Prompts
10. Strangler Fig Pattern for Monolith Decomposition
Task: For a legacy system that cannot be rewritten entirely, create a new microservice that gradually replaces a specific feature.
Prompt: "Identify a bounded context (e.g., notifications, search) that is self-contained. Create a new service that exposes the same API as the legacy module. Add a routing layer that sends new requests to the new service. Monitor for errors. Once stable, redirect 100% of traffic. Remove the legacy code after a deprecation period."
Example: A legacy monolith in C# had a search feature that was slow and inflexible. The team built a new search microservice in Go using Elasticsearch.
// Legacy controller
[HttpGet("search")]
public IActionResult Search(string query) {
var results = _legacySearchService.Search(query); // slow SQL LIKE
return Ok(results);
}
// After: routing layer in API gateway
if (featureFlags.IsNewSearchEnabled) {
// forward to new Go microservice
return RedirectToNewService(query);
} else {
var results = _legacySearchService.Search(query);
return Ok(results);
}
Result: Search latency dropped from 2 seconds to 200ms. The legacy search code was removed after 3 months of parallel running.
11. Introduce Event-Driven Architecture for Tight Coupling
Task: Find a class that directly calls many other classes (fan-out). Replace synchronous calls with asynchronous events.
Prompt: "Identify a class that has more than 5 dependencies injected or instantiated. For each dependency that does not need an immediate response, define an event. Modify the caller to publish the event instead of calling the method directly. Create an event handler that performs the original logic."
Example: A legacy Java order service directly called inventory, email, and shipping services.
// Before
public void placeOrder(Order order) {
inventoryService.reserve(order);
emailService.sendConfirmation(order);
shippingService.schedule(order);
}
// After
public void placeOrder(Order order) {
eventPublisher.publish(new OrderPlacedEvent(order));
}
// Handlers
@Component
public class InventoryHandler {
@EventListener
public void handle(OrderPlacedEvent event) {
inventoryService.reserve(event.getOrder());
}
}
Result: The order service became resilient to failures in downstream services. If email is down, the order still gets placed.
12. Rebuild with Test Harness (Characterization Tests)
Task: For a legacy module with no tests, write characterization tests that capture current behavior before refactoring.
Prompt: "Run the legacy function with a set of inputs. Record the outputs. Write tests that assert the exact outputs for those inputs. Do not change the logic yet. Once tests pass, you can safely refactor."
Example: A legacy JavaScript function that calculated shipping costs had no tests and was critical.
// Legacy function (no tests)
function calculateShipping(cart, destination) {
// complex logic with 15 conditionals
return total;
}
// Characterization test
it('should return 5.99 for standard shipping to US with cart under $50', () => {
const cart = { total: 30, weight: 2 };
const result = calculateShipping(cart, 'US');
expect(result).toBe(5.99);
});
Result: After writing 20 characterization tests, the team refactored the function into 5 smaller functions. All tests passed, and no regressions occurred in production.
Conclusion
Refactoring legacy code is not about rewriting—it's about systematically reducing risk while improving design. The 12 prompts above give you a structured way to approach technical debt, from simple variable renaming to full strangler fig patterns. Start with the basic prompts on your next code review. As your team gains confidence, move to advanced and expert strategies. Remember: every large codebase was once small, and every legacy system was once new. With the right prompts, you can make your codebase healthier, one method at a time.
Call to action: Pick one prompt from the basic category, apply it to a file you're working on today, and measure the difference. Share your results with your team.
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