I posted this question in a Khronos forum and was informed that there is no way to "to query the OpenXR runtime library's API version". Since a "1.1.41" version of the SDK was recently posted in GitHub, but many (most?) runtime libraries are still at "1.0", the former advice to create an XrInstance using XR_CURRENT_API_VERSION is liable to fail with XR_ERROR_API_VERSION_UNSUPPORTED. A workaround for that error is to iteratively decrement the patch, minor, and major version fields and try again, until xrCreateInstance either succeeds, or fails for other reasons. The application should either record and adapt to whatever API version the runtime supports, or fail with something like "version unsupported".