diff --git a/nostrdb/src/nostrdb.c b/nostrdb/src/nostrdb.c
index 0792a170..f24ecd7e 100644
--- a/nostrdb/src/nostrdb.c
+++ b/nostrdb/src/nostrdb.c
@@ -671,6 +671,12 @@ ndb_filter_elements_data(const struct ndb_filter *filter, int offset)
 	return data;
 }
 
+struct ndb_filter_custom *
+ndb_filter_get_custom_element(const struct ndb_filter *filter, const struct ndb_filter_elements *els)
+{
+	return (struct ndb_filter_custom *)ndb_filter_elements_data(filter, els->elements[0]);
+}
+
 unsigned char *
 ndb_filter_get_id_element(const struct ndb_filter *filter, const struct ndb_filter_elements *els, int index)
 {
@@ -754,6 +760,7 @@ static const char *ndb_filter_field_name(enum ndb_filter_fieldtype field)
 	case NDB_FILTER_LIMIT: return "limit";
 	case NDB_FILTER_SEARCH: return "search";
 	case NDB_FILTER_RELAYS: return "relays";
+	case NDB_FILTER_CUSTOM: return "custom";
 	}
 
 	return "unknown";
@@ -821,6 +828,10 @@ static int ndb_filter_add_element(struct ndb_filter *filter, union ndb_filter_el
 	offset = filter->data_buf.p - filter->data_buf.start;
 
 	switch (current->field.type) {
+	case NDB_FILTER_CUSTOM:
+		if (!cursor_push(&filter->data_buf, (unsigned char *)&el, sizeof(el)))
+			return 0;
+		break;
 	case NDB_FILTER_IDS:
 	case NDB_FILTER_AUTHORS:
 		if (!cursor_push(&filter->data_buf, (unsigned char *)el.id, 32))
@@ -861,6 +872,7 @@ static int ndb_filter_add_element(struct ndb_filter *filter, union ndb_filter_el
 		case NDB_ELEMENT_INT:
 			// ints are not allowed in tag filters
 		case NDB_ELEMENT_UNKNOWN:
+		case NDB_ELEMENT_CUSTOM:
 			return 0;
 		}
 		// push a pointer of the string in the databuf as an element
@@ -925,6 +937,7 @@ int ndb_filter_add_str_element_len(struct ndb_filter *filter, const char *str, i
 	case NDB_FILTER_IDS:
 	case NDB_FILTER_AUTHORS:
 	case NDB_FILTER_KINDS:
+	case NDB_FILTER_CUSTOM:
 		return 0;
 	case NDB_FILTER_SEARCH:
 		if (current->count == 1) {
@@ -951,6 +964,41 @@ int ndb_filter_add_str_element(struct ndb_filter *filter, const char *str)
 	return ndb_filter_add_str_element_len(filter, str, strlen(str));
 }
 
+int ndb_filter_add_custom_filter_element(struct ndb_filter *filter, ndb_filter_callback_fn *cb, void *ctx)
+{
+	union ndb_filter_element el;
+	struct ndb_filter_elements *current;
+	struct ndb_filter_custom custom;
+
+	custom.cb = cb;
+	custom.ctx = ctx;
+
+	if (!(current = ndb_filter_current_element(filter)))
+		return 0;
+
+	switch (current->field.type) {
+	case NDB_FILTER_CUSTOM:
+		break;
+	case NDB_FILTER_IDS:
+	case NDB_FILTER_AUTHORS:
+	case NDB_FILTER_TAGS:
+	case NDB_FILTER_SEARCH:
+	case NDB_FILTER_RELAYS:
+	case NDB_FILTER_KINDS:
+	case NDB_FILTER_SINCE:
+	case NDB_FILTER_UNTIL:
+	case NDB_FILTER_LIMIT:
+		return 0;
+	}
+
+	if (!ndb_filter_set_elem_type(filter, NDB_ELEMENT_CUSTOM))
+		return 0;
+
+	el.custom_filter = custom;
+
+	return ndb_filter_add_element(filter, el);
+}
+
 int ndb_filter_add_int_element(struct ndb_filter *filter, uint64_t integer)
 {
 	union ndb_filter_element el;
@@ -964,6 +1012,7 @@ int ndb_filter_add_int_element(struct ndb_filter *filter, uint64_t integer)
 	case NDB_FILTER_TAGS:
 	case NDB_FILTER_SEARCH:
 	case NDB_FILTER_RELAYS:
+	case NDB_FILTER_CUSTOM:
 		return 0;
 	case NDB_FILTER_KINDS:
 	case NDB_FILTER_SINCE:
@@ -996,6 +1045,7 @@ int ndb_filter_add_id_element(struct ndb_filter *filter, const unsigned char *id
 	case NDB_FILTER_KINDS:
 	case NDB_FILTER_SEARCH:
 	case NDB_FILTER_RELAYS:
+	case NDB_FILTER_CUSTOM:
 		return 0;
 	case NDB_FILTER_IDS:
 	case NDB_FILTER_AUTHORS:
@@ -1079,6 +1129,7 @@ static int ndb_tag_filter_matches(struct ndb_filter *filter,
 			case NDB_ELEMENT_INT:
 				// int elements int tag queries are not supported
 			case NDB_ELEMENT_UNKNOWN:
+			case NDB_ELEMENT_CUSTOM:
 				return 0;
 			}
 		}
@@ -1166,6 +1217,7 @@ static int ndb_filter_matches_with(struct ndb_filter *filter,
 	int i, j;
 	struct ndb_filter_elements *els;
 	struct search_id_state state;
+	struct ndb_filter_custom *custom;
 
 	state.filter = filter;
 
@@ -1244,6 +1296,12 @@ static int ndb_filter_matches_with(struct ndb_filter *filter,
 			// the search index will be walked for these kinds
 			// of queries.
 			continue;
+		case NDB_FILTER_CUSTOM:
+			custom = ndb_filter_get_custom_element(filter, els);
+			if (custom->cb(custom->ctx, note))
+				continue;
+			break;
+
 		case NDB_FILTER_LIMIT:
 cont:
 			continue;
@@ -1297,6 +1355,7 @@ static int ndb_filter_field_eq(struct ndb_filter *a_filt,
 	const char *a_str, *b_str;
 	unsigned char *a_id, *b_id;
 	uint64_t a_int, b_int;
+	struct ndb_filter_custom *a_custom, *b_custom;
 
 	if (a_field->count != b_field->count)
 		return 0;
@@ -1318,6 +1377,11 @@ static int ndb_filter_field_eq(struct ndb_filter *a_filt,
 
 	for (i = 0; i < a_field->count; i++) {
 		switch (a_field->field.elem_type) {
+		case NDB_ELEMENT_CUSTOM:
+			a_custom = ndb_filter_get_custom_element(a_filt, a_field);
+			b_custom = ndb_filter_get_custom_element(b_filt, b_field);
+			if (memcmp(a_custom, b_custom, sizeof(*a_custom)))
+				return 0;
 		case NDB_ELEMENT_UNKNOWN:
 			return 0;
 		case NDB_ELEMENT_STRING:
@@ -1373,6 +1437,7 @@ void ndb_filter_end_field(struct ndb_filter *filter)
 		// TODO: generic tag search sorting
 		break;
 	case NDB_FILTER_SINCE:
+	case NDB_FILTER_CUSTOM:
 	case NDB_FILTER_UNTIL:
 	case NDB_FILTER_LIMIT:
 	case NDB_FILTER_SEARCH:
@@ -6412,6 +6477,9 @@ static int cursor_push_json_elem_array(struct cursor *cur,
 	for (i = 0; i < elems->count; i++) {
 
 		switch (elems->field.elem_type)  {
+		case NDB_ELEMENT_CUSTOM:
+			// can't serialize custom functions
+			break;
 		case NDB_ELEMENT_STRING:
 			str = ndb_filter_get_string_element(filter, elems, i);
 			if (!cursor_push_jsonstr(cur, str))
@@ -6464,6 +6532,9 @@ int ndb_filter_json(const struct ndb_filter *filter, char *buf, int buflen)
 	for (i = 0; i < filter->num_elements; i++) {
 		elems = ndb_filter_get_elements(filter, i);
 		switch (elems->field.type) {
+		case NDB_FILTER_CUSTOM:
+			// nothing to encode these as
+			break;
 		case NDB_FILTER_IDS:
 			if (!cursor_push_str(c, "\"ids\":"))
 				return 0;
@@ -7325,6 +7396,9 @@ static int ndb_filter_parse_json(struct ndb_json_parser *parser,
 
 		// we parsed a top-level field
 		switch(field) {
+		case NDB_FILTER_CUSTOM:
+			// can't really parse these yet
+			break;
 		case NDB_FILTER_AUTHORS:
 		case NDB_FILTER_IDS:
 			if (!ndb_filter_parse_json_ids(parser, filter)) {
diff --git a/nostrdb/src/nostrdb.h b/nostrdb/src/nostrdb.h
index 172d829e..fff4af14 100644
--- a/nostrdb/src/nostrdb.h
+++ b/nostrdb/src/nostrdb.h
@@ -2,6 +2,7 @@
 #define NOSTRDB_H
 
 #include <inttypes.h>
+#include <stdbool.h>
 #include "win.h"
 #include "cursor.h"
 
@@ -48,6 +49,7 @@ struct ndb_t {
 };
 
 struct ndb_str {
+	// NDB_PACKED_STR, NDB_PACKED_ID
 	unsigned char flag;
 	union {
 		const char *str;
@@ -163,8 +165,9 @@ enum ndb_filter_fieldtype {
 	NDB_FILTER_LIMIT   = 7,
 	NDB_FILTER_SEARCH  = 8,
 	NDB_FILTER_RELAYS  = 9,
+	NDB_FILTER_CUSTOM  = 10,
 };
-#define NDB_NUM_FILTERS 7
+#define NDB_NUM_FILTERS 10
 
 // when matching generic tags, we need to know if we're dealing with
 // a pointer to a 32-byte ID or a null terminated string
@@ -173,6 +176,7 @@ enum ndb_generic_element_type {
 	NDB_ELEMENT_STRING  = 1,
 	NDB_ELEMENT_ID      = 2,
 	NDB_ELEMENT_INT     = 3,
+	NDB_ELEMENT_CUSTOM  = 4,
 };
 
 enum ndb_search_order {
@@ -250,10 +254,18 @@ struct ndb_filter_string {
 	int len;
 };
 
+typedef bool ndb_filter_callback_fn(void *, struct ndb_note *);
+
+struct ndb_filter_custom {
+	void *ctx;
+	ndb_filter_callback_fn *cb;
+};
+
 union ndb_filter_element {
 	struct ndb_filter_string string;
 	const unsigned char *id;
 	uint64_t integer;
+	struct ndb_filter_custom custom_filter;
 };
 
 struct ndb_filter_field {
@@ -551,6 +563,7 @@ int ndb_filter_init_with(struct ndb_filter *filter, int pages);
 int ndb_filter_add_id_element(struct ndb_filter *, const unsigned char *id);
 int ndb_filter_add_int_element(struct ndb_filter *, uint64_t integer);
 int ndb_filter_add_str_element(struct ndb_filter *, const char *str);
+int ndb_filter_add_custom_filter_element(struct ndb_filter *filter, ndb_filter_callback_fn *cb, void *ctx);
 int ndb_filter_eq(const struct ndb_filter *, const struct ndb_filter *);
 
 /// is `a` a subset of `b`