400 response when logging water

[alasdair-crono]

Our Fitbit integration has been in place for many years and we have never experienced issues with water logging.

On March 3rd 2026 we started seeing a lot of 400 responses from requests to the log water endpoint, e.g.

POST https://api.fitbit.com/1/user/<user-id>/foods/log/water.json?date=<date>&amount=2.07&unit=cup

(I have obfuscated the user id and for some reason the forum won't let me include the date)

The response body looks like this:

"error": { "code": 400, "message": "Request contains an invalid argument.", "status": "INVALID_ARGUMENT" }

Some requests are still successful, however, the majority fail with this response. We haven't changed anything in our integration and the request looks well formed, so why have we suddenly started receiving 400 responses. What is the invalid argument that the response mentions?

[alasdair-crono]

This issue was occurring because we were attempting to log water for a date in the future. It's very annoying that you can log food for future dates but not water. This can be tricky to debug because it's based on the user's time zone in their fitbit profile. Please at least change the error response to make it clear what the problem is.

[DorisFitbit]

Hi @alasdair-crono 

Thank you for reporting this issue. Upon review, we were able to reproduce the behavior. I have filed a ticket with our engineering team to work on a fix. I will provide an update as soon as the issue is resolved.