Send custom field criteria as literal dotted keys filter[customFields.<identifier>], not as nested filter[customFields][<identifier>] keys. Text, email, and URL fields accept a string matched as a case- and accent-insensitive substring, with %, _, and backslash treated literally. Boolean fields accept true, false, 1, or 0. Number, date, and date-time fields accept inclusive filter[customFields.<identifier>][from] and/or filter[customFields.<identifier>][to] bounds; when both are sent, from must not exceed to. Number bounds are decimal strings from -99999999999999.999999 through 99999999999999.999999 with at most six fractional digits and no rounding. Date bounds are YYYY-MM-DD calendar dates. Date-time bounds are ISO 8601 timestamps: offsets are normalized to UTC and an absent offset means UTC. Multiple identifiers combine with AND. Blank or null scalar criteria and ranges with both bounds unfilled have no effect, including for unknown dotted keys. Populated criteria for unknown, deleted, restricted, operator-inapplicable, disabled, or non-filterable fields return 400; malformed values for usable fields return 422. Custom-field criteria never return 404. Operator-scoped fields match only groups whose current operator owner makes the definition applicable.