Evolving the contract
Two families, and every engine adapter under them, depend on this package's shapes staying stable. That is why it evolves additively and is deliberately the slowest-moving package in the Reality Collective WebXR stack.
Additive by default
New fields on InputSourceSnapshot, such as linearVelocity and angularVelocity, arrive as optional, so an existing provider that does not supply them keeps compiling and keeps passing conformance. A new capability, such as presence, is added to InputCapabilities and to NO_CAPABILITIES at the same time, defaulting to false or "none", so a provider written before the addition still returns a complete, valid capability set. A new provider method, such as setPresenceVisible, is added as optional on InputProvider, and a conformance case then requires it only when the capability it serves is declared true. This is what lets @realitycollective/webxr-input add capabilities across several preview releases without forcing every adapter to update on the same day.
The capability-key test keeps additions honest
Adding a capability in one place only, such as InputCapabilities without the matching entry in INPUT_CAPABILITY_REQUIREMENTS, is caught by a test that checks the requirement list against the capability keys. See Capabilities and negotiation for what that test enforces.
Breaking changes are expensive on purpose
A breaking change, such as renaming a field or removing a capability, requires checking every consuming family first: the interaction core, the UI Extensions core, and every engine adapter under both. That cost is intentional. It keeps this package's shapes the one thing an adapter author can rely on not moving under them, which is the entire point of extracting a contracts package in the first place.
When to retire, not defend
This package does not claim to be a permanent standard. If an engine-free equivalent to this contract emerges upstream, or if spec convergence makes what remains of the problem trivial, the stated plan is to adopt that alternative or retire this package, not to defend its existence for its own sake.