From experience I can tell you that you just need to do it manually. And keep maintaing the spec manually. That keeps you aware of it. It is your contract with the outside world. You don't want tools to automatically update it everytime you push. On the contrary, you want tools to slap you hard every time you push and you forgot to update the spec. Now you're going to think hard why did you change the spec and if you maybe need to bump to /api/v2/*
As far as generating the first spec from an existing API I think there was shown here on HN a while ago a tool that can generate the spec from you just "browsing" the API with developer tools or something. Maybe even from a HAR file IIRC.
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From experience I can tell you that you just need to do it manually. And keep maintaing the spec manually. That keeps you aware of it. It is your contract with the outside world. You don't want tools to automatically update it everytime you push. On the contrary, you want tools to slap you hard every time you push and you forgot to update the spec. Now you're going to think hard why did you change the spec and if you maybe need to bump to /api/v2/*
As far as generating the first spec from an existing API I think there was shown here on HN a while ago a tool that can generate the spec from you just "browsing" the API with developer tools or something. Maybe even from a HAR file IIRC.