diff --git a/manifest/manifest.go b/manifest/manifest.go
index 731bbb4..2d40e8e 100644
--- a/manifest/manifest.go
+++ b/manifest/manifest.go
@@ -61,18 +61,29 @@ func (m *Manifest) SlotNames() []string {
 
 // DefaultDaemon returns the name, spec, and true for the default daemon.
 // A daemon is the default if it has Default:true, or if it is the only daemon
-// in the map. Returns empty values and false if no default can be determined.
+// in the map. If multiple daemons have Default:true, returns false (ambiguous).
+// Returns empty values and false if no default can be determined.
 func (m *Manifest) DefaultDaemon() (string, DaemonSpec, bool) {
 	if len(m.Daemons) == 0 {
 		return "", DaemonSpec{}, false
 	}
 
-	// Look for an explicit default.
+	// Look for an explicit default; reject ambiguous multiple defaults.
+	var defaultName string
+	var defaultSpec DaemonSpec
 	for name, spec := range m.Daemons {
 		if spec.Default {
-			return name, spec, true
+			if defaultName != "" {
+				// Multiple defaults — ambiguous.
+				return "", DaemonSpec{}, false
+			}
+			defaultName = name
+			defaultSpec = spec
 		}
 	}
+	if defaultName != "" {
+		return defaultName, defaultSpec, true
+	}
 
 	// If exactly one daemon exists, treat it as the implicit default.
 	if len(m.Daemons) == 1 {
diff --git a/manifest/manifest_test.go b/manifest/manifest_test.go
index bcf551b..04a61fb 100644
--- a/manifest/manifest_test.go
+++ b/manifest/manifest_test.go
@@ -132,6 +132,28 @@ func TestManifest_DefaultDaemon_Bad_NoDaemons(t *testing.T) {
 	assert.Empty(t, spec.Binary)
 }
 
+func TestManifest_DefaultDaemon_Bad_MultipleDefaults(t *testing.T) {
+	m := Manifest{
+		Daemons: map[string]DaemonSpec{
+			"api":    {Binary: "./bin/api", Default: true},
+			"worker": {Binary: "./bin/worker", Default: true},
+		},
+	}
+	_, _, ok := m.DefaultDaemon()
+	assert.False(t, ok)
+}
+
+func TestManifest_DefaultDaemon_Bad_MultipleNoneDefault(t *testing.T) {
+	m := Manifest{
+		Daemons: map[string]DaemonSpec{
+			"api":    {Binary: "./bin/api"},
+			"worker": {Binary: "./bin/worker"},
+		},
+	}
+	_, _, ok := m.DefaultDaemon()
+	assert.False(t, ok)
+}
+
 func TestManifest_DefaultDaemon_Good_SingleImplicit(t *testing.T) {
 	m := Manifest{
 		Daemons: map[string]DaemonSpec{