Postel's Law

ELI5 — The Vibe Check

Postel's Law: 'Be conservative in what you send, be liberal in what you accept.' Your API should accept slightly malformed input gracefully but always produce perfectly formatted output. It's the golden rule of protocol design — be strict with yourself, forgiving with others.

Real Talk

Postel's Law (the Robustness Principle) suggests implementations should be strict in generating protocol output and tolerant in parsing input. While it improved early internet interoperability, modern criticism notes it can mask bugs, encourage non-compliance, and create security vulnerabilities through lenient parsing.

When You'll Hear This

"Postel's Law says accept dates in multiple formats but always return ISO 8601." / "Being too liberal in what we accept led to security bugs — Postel's Law has limits."