HTTP status code
HTTP status code: 400 Bad Request
What does 400 Bad Request mean in HTTP?
The server cannot or will not process the request because it sees a fault on the client's side, such as malformed syntax or invalid message framing.
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What it means in trading API work
A trading API usually sends 400 when a field is missing, misspelt or out of range: a size below the minimum, a price with too many decimal places, an instrument name it does not know. The body of the reply normally carries the API's own error code, and that code is what names the fault.
This part is this site's own explanation, written for people learning to call a trading API. It is not part of the source cited above and it describes no particular broker.
What should I do about 400 Bad Request?
Read the body of the reply and look up the API's own code. Compare what you sent with the documented request. Do not send the same request again unchanged, because it will be refused in the same way.
This page is an independent explanation in plain words. The definition itself is in the source it was read from, which is the authority if the two ever differ.