The plain-English version
Changing an API’s data shape or rules while accounting for clients using earlier behavior.
Why it matters
Interfaces and servers are not always updated at exactly the same time.
Let’s make it click
A server adds an optional field before a later client begins depending on it.
Vibe Bro’s heads-up
Renaming a required field in one deployment can break an older mobile app that remains installed.
The docs behind it
Documentation links checked .
Editorial review · October 28, 2026
Supported: Google's compatibility guidance supports old-client constraints and breaking rename/removal examples; the text avoids promising every optional addition is safe.