Solved
How to test URI query string length?
In various places within the developer documentation (error-codes, REST API exception and error handling) we are advised:
414 will be returned when the URI of a GET request exceeds 8KB. To avoid it, check against the length of your query string to see if it exceeds this limit.
How do I check the length of a URI GET request query string in C# to avoid a 414 error?