Simon Willison’s Weblog

With enough users, every observable behavior becomes a dependency - regardless of what you promised. Someone is scraping your API, automating your quirks, caching your bugs.

This creates a career-level insight: you can’t treat compatibility work as “maintenance” and new features as “real work.” Compatibility is product.

Design your deprecations as migrations with time, tooling, and empathy. Most “API design” is actually “API retirement.”

— Addy Osmani, 21 lessons from 14 years at Google