diff --git a/go/delivery/builder.go b/go/delivery/builder.go
index faf2eff..fd414b8 100644
--- a/go/delivery/builder.go
+++ b/go/delivery/builder.go
@@ -165,7 +165,8 @@ type ToolchainUsed struct {
 // BuilderBuild is the value of a builder build key: its State (queued, building, done or failed), By the builder
 // instance that holds it, Step what it is doing or did last, Manifest the digest of the module's manifest (the key of
 // the indexer's manifests bucket), Artifacts when done, Toolchains by language when it built or is building (a done
-// build names them, so two builders that differ are seen: ADR 0405 clause 2), and Error when failed.
+// build names them, so two builders that differ are seen: ADR 0405 clause 2), Error when failed, and Waits when queued for a
+// package version.
 type BuilderBuild struct {
 	V          int                      `json:"v"`
 	State      string                   `json:"state"`
@@ -175,6 +176,10 @@ type BuilderBuild struct {
 	Artifacts  []BuiltArtifact          `json:"artifacts,omitempty"`
 	Toolchains map[string]ToolchainUsed `json:"toolchains,omitempty"`
 	Error      string                   `json:"error,omitempty"`
+	// Waits is, on a queued key only, the package version `<name>@<version>` the module's lock names and the registry
+	// does not yet hold: the key is not claimed until that package's trunk key is done and published (novox/hq ADR 0500
+	// clause 6). A reader that does not know the field reads the key as queued.
+	Waits string `json:"waits,omitempty"`
 }
 
 func (b BuilderBuild) check() error {
@@ -207,6 +212,14 @@ func (b BuilderBuild) check() error {
 			return fmt.Errorf("delivery: the reference %q of the %s %q %s", a.Reference, a.Kind, a.Name, p)
 		}
 	}
+	if b.Waits != "" {
+		if b.State != BuildQueued {
+			return fmt.Errorf("delivery: a %s build waits for %q; only a queued key waits for a package", b.State, b.Waits)
+		}
+		if p := (BuiltArtifact{Kind: "package", Reference: b.Waits}).referenceProblem(); p != "" {
+			return fmt.Errorf("delivery: the package a key waits for, %q, %s", b.Waits, p)
+		}
+	}
 	switch b.State {
 	case BuildQueued, BuildBuilding:
 		if len(b.Artifacts) > 0 || b.Error != "" {
diff --git a/go/delivery/builder_reference_test.go b/go/delivery/builder_reference_test.go
index ab8e26b..b8f804a 100644
--- a/go/delivery/builder_reference_test.go
+++ b/go/delivery/builder_reference_test.go
@@ -96,3 +96,23 @@ func TestOnlyAPushFailureIsBuiltAgainBeforeTheFingerprintChanges(t *testing.T) {
 	}
 }
 
+// A key whose module's lock names a version of the group's package not yet published waits, queued, saying which
+// (ADR 0500 clause 6): an older reader reads it as queued, and only a queued key waits.
+func TestAKeyWaitsQueuedForAPackageVersion(t *testing.T) {
+	b := BuilderBuild{State: BuildQueued, Manifest: digestOf('4'), Waits: "@novox/mesh-sdk@0.1.15"}
+	data, err := EncodeBuilderBuild(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	back, err := DecodeBuilderBuild(data)
+	if err != nil || back.Waits != b.Waits || back.State != BuildQueued {
+		t.Fatalf("decoded %+v, %v", back, err)
+	}
+	building := BuilderBuild{State: BuildBuilding, By: "x", Waits: "@novox/mesh-sdk@0.1.15"}
+	if _, err := EncodeBuilderBuild(building); err == nil {
+		t.Fatal("a building key that waits was accepted")
+	}
+	if _, err := EncodeBuilderBuild(BuilderBuild{State: BuildQueued, Waits: "mesh-sdk"}); err == nil {
+		t.Fatal("a wait naming no version was accepted")
+	}
+}