postels-lawlisted

Apply Postel's Law when designing APIs, data interchange formats, network protocols, input validation, or any interface that will receive data from other systems or users. Trigger on phrases like "how strict should our API be?", "should we accept slightly malformed input?", "what should we send vs. accept?", "how do we handle unexpected fields?", or any question about the balance between strictness and permissiveness in an interface design. Postel's Law is a foundational principle of robust interface design, with important modern caveats.
The-Artificer-of-Ciphers-LLC/skills-from-the-artificer · ★ 2 · AI & Automation · score 73

Install: claude install-skill The-Artificer-of-Ciphers-LLC/skills-from-the-artificer

# Postel's Law (Robustness Principle) > "Be conservative in what you send, liberal in what you accept." > — Jon Postel, RFC 760, 1980 ## The core idea When building systems that communicate with other systems, Postel recommended: - **Send only well-formed, minimal, correct data.** Don't rely on the other side tolerating your quirks. - **Accept data that is slightly off.** Be forgiving of minor variations, extra fields, or slightly non-standard inputs from senders. This principle shaped the early internet. HTML parsers that accepted malformed tags, TCP stacks that handled variations in packet formatting — the early web grew partly because implementors were lenient about what they accepted. ## The case for it **Real-world senders make mistakes.** Other systems, other developers, other versions of software — they will send you data that is slightly wrong. If you reject anything imperfect, you become brittle and hard to integrate with. **Interoperability requires tolerance.** When you have many producers of data and many consumers, allowing slightly varied input enables the ecosystem to grow. Strict rejection creates integration friction. **Graceful degradation.** A system that accepts imperfect input can provide partial value; one that rejects it provides none. ## The modern critique — and important caveats Postel's Law has critics, and their arguments are strong enough to warrant caution: **Permissive acceptance hides bugs.** When you silently accept malformed input