449 Retry With
Meaning
A Microsoft extension indicating the request should be retried after doing the appropriate action. Typically, it is returned when the server cannot fulfill the request because the client needs to provide more information.
When to Use
Used in Microsoft IIS (especially in scenarios like Microsoft Azure or older services) when the server expects the client to retry the request with certain conditions met (for example, after providing required information or changing something in the request).
Example
In Microsoft IIS, 449 Retry With might be seen if a request is missing some required conditions; the response implies the client should "retry with" the necessary information. For instance, if an API requires a certain parameter and it's missing, IIS could respond with 449 (instead of a 400).
When Not to Use
Do not use outside Microsoft-specific environments. This is not an official HTTP status code. In standard usage, you'd typically send a 400 Bad Request or similar, possibly with guidance in the response body for the client.
Source
Unofficial (Microsoft IIS)