/v1 only changes in ways that do not break a caller: new response fields, new optional request fields, new integrations, new capabilities, and new error codes for new situations.
A breaking change removes or renames a field, changes a field’s type, adds, removes, or renames a verdict, changes an existing error code, or changes a default. A breaking change ships as /v2.
From the day /v2 ships, /v1 responses carry Deprecation and Sunset headers. /v1 keeps working for 6 months.
Before launch, nothing is deployed and /v1 may still change. The error body, credits_remaining, stop_reason, and steps[].reason arrived this way.