diff --git a/go/clients/nodeengineclient/client.go b/go/clients/nodeengineclient/client.go
index 0b1939f..b39982e 100644
--- a/go/clients/nodeengineclient/client.go
+++ b/go/clients/nodeengineclient/client.go
@@ -10,10 +10,11 @@ import (
 	"errors"
 	"fmt"
 	"regexp"
+	"strings"
 	"time"
 
 	seat "git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
-	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"git.novox.be/novox/mesh-sdk/go/keys"
 )
 
 const (
@@ -56,7 +57,7 @@ var ErrUnknownVersion = errors.New("node-engine: unknown compose-candidate answe
 var nameOK = regexp.MustCompile(`^[-/_=a-zA-Z0-9]+$`)
 
 func checkAsk(a seat.ComposeCandidateInput) error {
-	if _, _, err := delivery.ParseCompositionKey(a.Candidate); err != nil {
+	if _, _, err := ParseCompositionKey(a.Candidate); err != nil {
 		return fmt.Errorf("node-engine: a compose-candidate ask names no candidate: %w", err)
 	}
 	return nil
@@ -89,7 +90,7 @@ func CheckAnswer(a seat.ComposeCandidateAnswer) error {
 	if a.Node == "" || !nameOK.MatchString(a.Node) {
 		return fmt.Errorf("node-engine: the answer's node name %q is empty or holds a dot, a wildcard, whitespace or a character a key refuses", a.Node)
 	}
-	if _, _, err := delivery.ParseCompositionKey(a.Candidate); err != nil {
+	if _, _, err := ParseCompositionKey(a.Candidate); err != nil {
 		return err
 	}
 	if a.Took < 0 {
@@ -181,3 +182,37 @@ func DecodeAnswer(data []byte) (seat.ComposeCandidateAnswer, error) {
 	}
 	return a, CheckAnswer(a)
 }
+
+var heads12 = regexp.MustCompile(`^[0-9a-f]{12}$`)
+
+// CompositionKey is the key of a candidate the node is asked to compose: the group's check at its heads, `<escaped
+// group>.<heads hash>`, the group escaped as a key part and the heads hash 12 lowercase hex digits (the checker's verdict
+// key of the same check).
+func CompositionKey(group, headsHash string) (string, error) {
+	if group == "" {
+		return "", errors.New("node-engine: the group name is empty")
+	}
+	if !heads12.MatchString(headsHash) {
+		return "", fmt.Errorf("node-engine: %q is not a heads hash, 12 lowercase hex digits", headsHash)
+	}
+	return keys.EscapeName(group) + "." + headsHash, nil
+}
+
+// ParseCompositionKey is the inverse of CompositionKey; it refuses a key that is not one, naming it.
+func ParseCompositionKey(key string) (group, headsHash string, err error) {
+	i := strings.LastIndex(key, ".")
+	if i < 0 {
+		return "", "", fmt.Errorf("node-engine: %q is not a composition key, <group>.<heads hash>", key)
+	}
+	part, headsHash := key[:i], key[i+1:]
+	if !heads12.MatchString(headsHash) {
+		return "", "", fmt.Errorf("node-engine: %q is not a composition key: %q is not a heads hash, 12 lowercase hex digits", key, headsHash)
+	}
+	if part == "" || strings.Contains(part, ".") {
+		return "", "", fmt.Errorf("node-engine: %q is not a composition key: the group part is empty or holds a dot", key)
+	}
+	if group, err = keys.UnescapeName(part); err != nil {
+		return "", "", fmt.Errorf("node-engine: %q is not a composition key: %w", key, err)
+	}
+	return group, headsHash, nil
+}
diff --git a/go/clients/nodeengineclient/client_test.go b/go/clients/nodeengineclient/client_test.go
index 542b207..1586968 100644
--- a/go/clients/nodeengineclient/client_test.go
+++ b/go/clients/nodeengineclient/client_test.go
@@ -8,13 +8,12 @@ import (
 	"testing"
 
 	seat "git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
-	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
 
 // The verb compose-candidate's ask and answer (ADR 0593 §7, §9): each round-trips, and a reader refuses what it could
 // not take, naming it.
 func TestComposeCandidateRoundTripsAndRefuses(t *testing.T) {
-	key, err := delivery.CompositionKey("mesh-sdk#7", "0123456789ab")
+	key, err := CompositionKey("mesh-sdk#7", "0123456789ab")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -95,3 +94,22 @@ func TestComposeCandidateRoundTripsAndRefuses(t *testing.T) {
 		}
 	}
 }
+
+// A composition key round-trips its group and heads hash, and refuses what is not one.
+func TestCompositionKeyRoundTripsAndRefuses(t *testing.T) {
+	key, err := CompositionKey("fix/1.2", "0123456789ab")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, h, err := ParseCompositionKey(key); err != nil || g != "fix/1.2" || h != "0123456789ab" {
+		t.Fatalf("round trip: %q %q %v", g, h, err)
+	}
+	for _, bad := range []string{"", "nogroup", "g.0123456789AB", "g.short", ".0123456789ab", "a.b.0123456789ab"} {
+		if _, _, err := ParseCompositionKey(bad); err == nil {
+			t.Errorf("%q was taken for a composition key", bad)
+		}
+	}
+	if _, err := CompositionKey("", "0123456789ab"); err == nil {
+		t.Error("a key with no group was made")
+	}
+}