FIS Geometry character limit

Yes, there indeed is a limit, based on common URL length limits.
In terms of “accuracy” - take into account that Sentinel-2 has 10 meter resolution. In Web Mercator the decimals of the coordinates go beyond one-meter, so two orders of magnitude less than the accuracy of the Sentinel-2. I suggest you round coordinates to integers.

Thanks, that is a good suggestion. I am also using a proxy page to handle the FIS requests, but it does not seem to work with the longer request strings, even though it uses a POST method.

Can you send an example of such a call, so that we can debug?

Ok I’ll email you off the forum.