Even with an API spec it wouldn't "just work". You'd still need to handle authentication and have a place to manage which APIs you want the in-app agent to have access to / when to call a given tool.
And believe it or not, even big companies with big eng teams don't have API specs available for their applications ¯\_(ツ)_/¯
And believe it or not, even big companies with big eng teams don't have API specs available for their applications ¯\_(ツ)_/¯