I get the feeling you may not have gone 0-1 on an API before. In general, you have 1 consumer when you're starting off, and if you're lucky your API gathers more consumers over time.
In that initial implementation period, it's more time-consuming to have to update a spec nobody uses. Maintaining specs separately from your actual code is also a great way to get into situations where your map != your territory.
I'd instead ask: support and use API frameworks that allow you to automatically generate OpenAPI specs, or make a lot of noise to get frameworks that don't support generating specs to support that feature. Don't try to maintain OpenAPI specs without automation :)
In that initial implementation period, it's more time-consuming to have to update a spec nobody uses. Maintaining specs separately from your actual code is also a great way to get into situations where your map != your territory.
I'd instead ask: support and use API frameworks that allow you to automatically generate OpenAPI specs, or make a lot of noise to get frameworks that don't support generating specs to support that feature. Don't try to maintain OpenAPI specs without automation :)