Use Claude to Incrementally Replace Legacy Code with the Strangler Fig Pattern

Rewriting legacy code all at once is how projects fail. The Strangler Fig pattern — replacing old code incrementally while both versions run side by side — is safer but tedious to set up. Claude Code handles the wiring so you focus on the new implementation.

I want to replace the old OrderProcessor with a new one, but both need to 
run simultaneously during the migration. Set up the strangler fig pattern: 
route to the new implementation when a feature flag is on, fall back to 
the old one when it's off, and log which path each request takes.

Claude reads the old code, creates the new interface, and wires up the routing:

// Claude creates a facade that routes between old and new
class OrderProcessorFacade {
  async process(order) {
    if (featureFlags.isEnabled('new-order-processor')) {
      logger.info('routing to new processor', { orderId: order.id });
      return this.newProcessor.process(order);
    }
    logger.info('routing to legacy processor', { orderId: order.id });
    return this.legacyProcessor.process(order);
  }
}

Use this for different migration scenarios:

# Replace one method at a time
Migrate the OrderService to the new pattern, starting with just the 
createOrder method. Keep all other methods on the legacy implementation 
until we're ready to migrate them.

# Run both and compare
Route all requests through both the old and new implementations. 
Compare the results and log any differences — but return the old 
result to users until we've verified the new one matches.

# Gradual percentage rollout
Route 5% of traffic to the new implementation. If the error rate 
stays flat for a week, increase to 25%, then 50%, then 100%.

# Clean up after migration
The new OrderProcessor has been handling 100% of traffic for 2 weeks 
with no issues. Remove the legacy implementation, the feature flag, 
the facade, and the comparison logging.

Big rewrites fail. Incremental replacements succeed — let Claude set up the dual-running infrastructure so you can migrate one piece at a time.

via Claude Code