# Designing APIs That Age Well

API design principles for long-term maintainability. Versioning and deprecation strategies.


**Model:** claude-sonnet-4
**Created:** 7/30/2026

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## Conversation

### **Human**

I'm designing a REST API for my startup. What should I think about to make sure it doesn't become a nightmare to maintain in 2 years?

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### **Assistant**

Great question! API design is long-term product design. Here are principles for aging well:

1. DESIGN FOR BREAKING CHANGES FROM DAY ONE. Assume you WILL need to change things. Build mechanisms for it.

2. NEVER REMOVE, ONLY DEPRECATE. Have a deprecation cycle that gives users time to migrate.

3. DESIGN EXTENSIBILITY INTO EVERYTHING. Have metadata fields, version fields, and flexible schemas.

4. VERSION IN THE URL. Use api/v1/users, api/v2/users - not in headers.

The key insight: Your API today will be wrong tomorrow. Design for graceful evolution, not perfection.

Want me to design a concrete example with versioning strategy?

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### **Human**

Yes! Show me how to handle a breaking change where we need to split name into firstName and lastName.

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### **Assistant**

Perfect example! Here's the evolution strategy:

PHASE 1: Current state (v1). User interface has name field.

PHASE 2: Add v2 (both versions live). New interface has firstName, lastName, and name for backward compat.

PHASE 3: Migration layer. Adapter for v1 clients that converts between formats.

PHASE 4: Deprecation timeline. Month 1-3: v2 released, v1 still works. Month 4-6: v1 responses include deprecation warning. Month 7-9: v1 returns sunset header. Month 12: v1 shut down.

Key principle: Never break existing clients, even if they're slow to migrate.

This approach gives users plenty of time to migrate while you can evolve the API forward.

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