The endpoint /iserver/auth/status will return the status of your Web API brokerage session via its "authenticated" field. A response containing "authenticated": "false" indicates that no Web API brokerage session currently exists for the requesting username, while "true" indicates that you are able to access /iserver endpoints and make use of trading and market data functionality.
How can I tell if I have an active brokerage session in the Web API?
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Why are my requests to /iserver endpoints failing, while others succeed?
What is a brokerage session?
Where can I find a list of Web API endpoints?
Where can I find the Web API reference guide?