Make sure that response format is the same for single ID and several IDs (vectored IDs). It's one of REST principals: GET is a "safe method" (nullipotent) and should produce no side-effects.
For endpoints that support vectored IDs in GET
request I usually use response format similar to OpenSearch i.e.
GET user/1,3
where user/1 exists and user/3 does not exist returns 200 with
{
"startIndex": 0,
"count": 1,
"itemsPerPage": 1,
"entry": [
{ /* user/1 */}
]
}
GET user/3
where user/3 does not exist returns 200 with
{
"startIndex": 0,
"count": 0,
"itemsPerPage": 0,
"entry": []
}