Be conservative in what you send, be liberal in what you accept.
Jon Postel's 1980 robustness principle came out of the TCP specification — a rule for letting networks tolerate sloppy peers without becoming sloppy themselves. The same rule maps cleanly onto interfaces: be strict about what you produce, forgiving of what users hand you.
A system should send clean, strict output but accept generous, messy input. On the network this kept incompatible TCP implementations talking; on the interface, it keeps humans from being punished for typing things in their own format.
Accept the user's format, then normalize it yourself. Phone numbers with dots, slashes, or spaces; dates as "tomorrow" or "12/3"; emails with stray whitespace — all reasonable input. Strip, parse, and store the clean version without making the user redo their typing.
The other half of the law: what you produce should be tight, standards-compliant, and unambiguous. Dates in ISO 8601, IDs in a known shape, JSON with a consistent schema. Liberal inputs only work because outputs hold a contract that the rest of the system can rely on.
When input is ambiguous, infer the likely intent and confirm — don't dump the user back to a blank field. "Looks like you meant Dec 3?" beats a red border and a "format invalid" error. Recovery is faster, kinder, and produces more completed flows.
Be liberal with what older clients, older URLs, and older data send you. Don't break links when paths change — redirect. Don't reject saved files in last year's format — upgrade them silently. The user shouldn't pay for your refactor.
Names with hyphens, addresses with apartment numbers, prices with currency symbols — let the parser do the work. The user has a mental shape for what they're entering; meet them at it rather than imposing a stricter shape that only your database cares about.
A field with a smart default is accepting "I don't care" as valid input. Country pre-filled by IP, currency by locale, role by team — every safe assumption is one fewer thing the user has to send. Strict outputs still hold; the system just guessed well.
Postel's law has a counter-case: too-tolerant parsers let bad data calcify into the spec, and the system inherits years of "well, we accept that" cruft. The strict half matters — be liberal at the edge, strict in the core, and don't paper over genuine ambiguity with a guess.
Postel and Tesler reach the same conclusion from opposite directions. Tesler says complexity has to live somewhere; Postel says when it lives at the boundary, put it on the system side. The forgiving interface is the system absorbing the messiness of the real world.
Models are the most liberal input parsers ever built — and the strictest committee about which interpretation they commit to.
"Book me a flight on Friday" used to need a precise form. Now it doesn't. Models accept hand-waving, typos, mixed-language input, voice and text together — the most Postel-compliant front door we've ever shipped. The form fields haven't gone away; they've moved inside the model.
A liberal parser that silently commits to one interpretation is dangerous. The user said "Friday" — the model picked one. Postel's contract still applies: show what you understood, let the user fix it, never collapse genuine ambiguity into a confident answer.