← Patterns

Postel's Law (The Robustness Principle)

Core Concept

“Be conservative in what you send, liberal in what you accept.”

Produce strict, well-formed output. Accept messy, varied input gracefully. Don’t break because someone else’s output is slightly different than you expected.

Where It Applies

API Design

Strict output schemas, tolerant input parsing. Don’t fail because a field is missing that you don’t need.

Stage Boundaries in Pipelines

When one stage feeds another, the consuming stage should handle format variation. Parse by meaning (“find the section about milestones”), not by position (“read the third section”). No flag days when formats change.

Team Communication

Be precise in what you commit to (conservative output). Be generous in interpreting what others say (liberal input). Most miscommunication comes from doing the opposite — sloppy commitments, strict interpretations.

Management

When receiving status updates, parse for meaning, not exact wording. When giving direction, be precise.

The Tension

Postel’s Law can enable sloppy producers if taken too far. “Liberal in what you accept” doesn’t mean “accept garbage.” It means handle reasonable variation gracefully. The quality bar on output (conservative sending) is the counterweight.

Cross-References

• Software-Laws — hub for all seven laws and how they interact
• Conway’s Law — Conway says boundaries shape systems, Postel says be tolerant across them. Together: draw boundaries carefully, but don’t couple tightly.