diff --git a/go/buscalls/buscalls.go b/go/buscalls/buscalls.go
index 45a818b..3282903 100644
--- a/go/buscalls/buscalls.go
+++ b/go/buscalls/buscalls.go
@@ -10,6 +10,7 @@ package buscalls
 
 import (
 	"encoding/json"
+	"strings"
 	"time"
 )
 
@@ -286,10 +287,18 @@ type QueryRequest struct {
 	Limit  int    `json:"limit,omitempty"`
 }
 
-// IndexEntry is one entry of an index.
+// IndexEntry is the value of one key of a declared index (IndexKey): the instance it points at, and the sequence of
+// the operation it was written from. The owner's watcher and the tool runner's reindex both write it so.
 type IndexEntry struct {
-	Key   string          `json:"key"`
-	Value json.RawMessage `json:"value"`
+	Instance string `json:"instance"`
+	Sequence uint64 `json:"sequence"`
+}
+
+// IndexKey is the key of an instance in a declared index: the values of the index's fields, in its order, joined by
+// ".", then the instance's path with each "/" written "." (`17950.listing.a17`). The owner's watcher and the tool
+// runner's reindex both write it, so a reindex leaves the index as the watcher keeps it.
+func IndexKey(fields []string, instance string) string {
+	return strings.Join(append(append([]string{}, fields...), strings.ReplaceAll(instance, "/", ".")), ".")
 }
 
 // QueryAnswer is query's answer.
diff --git a/go/packages_list_test.go b/go/packages_list_test.go
index 4912c0a..ee965ae 100644
--- a/go/packages_list_test.go
+++ b/go/packages_list_test.go
@@ -110,6 +110,15 @@ func TestMeshSDKHoldsOnlyWhatEveryModuleShares(t *testing.T) {
 	if buscalls.MetaCall != "mesh/call" {
 		t.Errorf("the tool call's id is handed under %q, not mesh/call", buscalls.MetaCall)
 	}
+	// 1966/AC5: an index's key is its fields' values, then the instance's path, joined by "."; its entry names the
+	// instance and the sequence.
+	if k := buscalls.IndexKey([]string{"17950", "volvo"}, "listing/a17"); k != "17950.volvo.listing.a17" {
+		t.Errorf("an index key is %q", k)
+	}
+	entry, _ := json.Marshal(buscalls.IndexEntry{Instance: "listing/a17", Sequence: 3})
+	if string(entry) != `{"instance":"listing/a17","sequence":3}` {
+		t.Errorf("an index entry is %s", entry)
+	}
 	// 1966/AC5: an apply naming the tool call it serves, and its answer with the state behind, round-trip.
 	for _, c := range []struct {
 		name     string