Postel’s Law
Accept harmless variation when intent is clear, then return consistent and well-defined output.
Be forgiving with people, but precise about meaning, safety, and data.
A phone field rejects spaces and parentheses even though the number is clear. Removing punctuation should not be the person's job. A transfer amount is different. If the value is ambiguous, the system must ask instead of guessing.
Origin
Jon Postel stated the robustness principle in the 1980 TCP specification. RFC 9413 later explained that overly liberal protocol handling can harm interoperability, so tolerance needs explicit limits.
Accept harmless formatting variation such as spaces, punctuation, case, or pasted text when the intended value remains unambiguous.
Normalize the value visibly and let people correct it. Silent transformation is risky when a small change can alter meaning.
Reject dangerous or ambiguous input with a specific explanation. Tolerance must not weaken security, consent, or data integrity.
Produce consistent output and document the contract. Predictable formats make systems easier to integrate, test, and maintain.
Digital systems
A flexible phone field
The field accepts common punctuation and spacing, then displays the number in a consistent format. If the country code is unclear, it asks rather than inventing one.
Physical signals
A reversible connector
A connector that works in either orientation removes a harmless alignment error. Its electrical specification still remains strict about what can safely pass through it.
People must satisfy an arbitrary format even when the system could safely interpret what they mean.
Parse reasonable formats, show the normalized result, and stop for clarification whenever interpretation could change the outcome or create risk.