Marvin has a public API which lets you create tasks, get basic info about your account, start time tracking and more. Docs are here.
But most of this isn't helpful if you have cloud sync disabled! To that end, the Marvin desktop app is bundled with an optional local API server which serves a subset of the public API. This will let you create tasks, get basic info about your account, etc. by querying a server running within the desktop app.
How to enable and configure?
You can enable the local API server by opening the API strategy settings in the desktop app. Remember: if you have cloud sync disabled, you should be using the desktop app with automatic backups enabled (in order to avoid data loss)!
Click the checkbox to enable the API server, and optionally change the port and host where the server should listen. Then click out of the dialog, and within one minute the local API server will be started.
How to use?
Check out the API documentation to learn how to use the API and which endpoints are supported by the local API server.
You might also consider using the Marvin CLI (command-line interface) to simplify use of both the Public API and the Desktop Local API.