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)_/¯
We use a browser-based agent to learn all the APIs and turn them into skills. Most users will run this in a staging/test account to create all the recipes/blueprints. Our agent is also instructed to not take any destructive actions - but of course LLMs make mistakes (hence the test account :) ).