The catalog that named the patterns most developers use without knowing it. Gateway, Repository, Unit of Work, Data Mapper — these aren't enterprise-only concepts.

In agentic systems, the session-state patterns reappear as agent memory strategies. The Gateway pattern is exactly what a tool adapter is.

The Gang of Four book. Still the most precise vocabulary for structural decisions in software.

Tool routing is Chain of Responsibility. Model selection is Strategy. Event-driven agent communication is Observer. The book is 30 years old and describes your architecture better than your architecture docs do.

Bounded contexts, ubiquitous language, aggregates, domain events. Evans gave us the language for decomposing complex domains into manageable, autonomous pieces.

In multi-agent systems, bounded contexts are the natural unit of agent specialization. The ubiquitous language problem is just harder when one of the speakers hallucinates.

The dependency rule: source code dependencies must point inward, toward higher-level policies, never toward lower-level mechanisms.

Swapping Claude for GPT should touch exactly one adapter at the outer ring.

Message channels, routers, transformers, dead-letter queues, saga patterns.

Multi-agent orchestration is messaging. Every "agent framework" released since 2023 is a partial reimplementation of this book.

Feedback latency determines quality. The faster you learn whether a change works, the better your software gets.

Eval suites are your test stage. Canary deployments are your production gate.

Legacy code is code without tests. Feathers' contribution is the concept of seams — points where you can alter behavior without editing the code.

LLM calls are the ultimate legacy code: opaque, non-deterministic, untestable without seams. If you're writing AI code without reading Feathers, your test suite is a lie.