X_LOCALE_INVALID
Means
not a well-formed BCP 47 tag
Typical cause
en_US, '', or a raw Accept-Language value reaching any @ultimat3/time entry point that takes a locale — describeCron, the date/time formatters, formatRelative, formatDuration and zoneAbbrev. Every one of them raised a bare RangeError out of Intl until 2026-08-23; a RangeError carries no code, no fix: and no --json, so a header value a user controls took a request down with a message nothing could act on. assertLocale is the single gate now — grep -rn 'assertLocale(' packages/time/src is the list
Fix
pass en, en-GB, de-DE — screen header input with Intl.DateTimeFormat.supportedLocalesOf([tag])