Will clarify that: a) The content of the URL excluding the brace-encoded content must already be properly URL-encoded b) The braces themselves and the FHIRPath content within them are NOT url-encoded c) The FHIRPath expressions embedded in the query must resolve to a string (though possibly either an empty string or empty collection which will be treated as an empty string) d) The result of substituting the result of the FHIRPath expression must be a valid URL.  I.e. the FHIRPath must take care of escaping string content when necessary (usually using the '.encode('urlbase64')' function).  Will add the following examples: Observation?{{'subject=Patient/' %patient.id '&'}}code=http%3A%2F%2Fexample.org%2FCodeSystem%2Fdemo%7Cvalue With the explanation that this will filter by the patient if there is a patient.id in context, and otherwise will only filter by code. Observation?code:text= %codesearch.encode('urlbase64')