Unfortunately it's part of http. One thing you can do is to send both the standard Retry-After header for tools that rely on it, and a nonstandard but unambiguo

Unfortunately it's part of http. One thing you can do is to send both the standard Retry-After header for tools that rely on it, and a nonstandard but unambiguous Retry-After-Seconds. That makes responses self-documenting. (One downside is that you now have two numbers, and if you introduce a bug that makes them different, it will be more confusing.)