# Public API Extending Rules
Public API URI must be versioned by prefix /v*(e.g. /v1).
Any modification to an existing public API MUST avoid breaking changes and MUST maintain backward compatibility.
The fundamental principle is that you can’t break existing clients, because you don’t know what they implement, and you don’t control them. In doing so, you need to turn a backwards-incompatible change into a compatible one.
– Mark Nottingham (opens new window)
In particular, any change to an public API MUST follow the following Rules for Extending (applied for everything and API expose to the public including URIs, query params, request body, response body, headers...):
- You MUST NOT take anything away (related: minimal-api-surface.md, robustness-principle.md)
- You MUST NOT change processing rules
- You MUST NOT make optional things required
- Anything you add MUST be optional (related robustness-principle.md)
The goal is to keep published API stable over the time. Clients already using those APIs must be able to continue using the APIs through new releases, updates, deployments without being forced to change anything from their side.