Avoid messages for response-only updates

agent/response.go

@@ -146,6 +146,10 @@ func (resp *Response) ToUpdates() []*ResponseUpdate {
 		if createdAt.IsZero() {
 			createdAt = resp.CreatedAt
 		}
+		role := msg.Role
+		if role == "" {
+			role = message.RoleAssistant
+		}
 		updates = append(updates, &ResponseUpdate{
 			RawRepresentation:    msg.RawRepresentation,
 			AdditionalProperties: msg.AdditionalProperties,
@@ -154,7 +158,7 @@ func (resp *Response) ToUpdates() []*ResponseUpdate {
 			ResponseID:           resp.ID,
 			FinishReason:         resp.FinishReason,
 			AuthorName:           msg.AuthorName,
-			Role:                 msg.Role,
+			Role:                 role,
 			CreatedAt:            createdAt,
 			Contents:             msg.Contents,
 		})
@@ -179,25 +183,28 @@ func (resp *Response) Update(update *ResponseUpdate) {
 	if update == nil {
 		return
 	}
-	msg := resp.targetMessage(update)
-	// Some members on ResponseUpdate map to members of Message.
-	// Incorporate those into the latest message; in cases where the message
-	// stores a single value, prefer the latest update's value over anything
-	// stored in the message.
-	msg.AuthorName = cmp.Or(update.AuthorName, msg.AuthorName)
-	msg.Role = cmp.Or(update.Role, msg.Role)
-	msg.ID = cmp.Or(update.MessageID, msg.ID)
-	if !isValidCreatedAt(msg.CreatedAt) && isValidCreatedAt(update.CreatedAt) {
-		msg.CreatedAt = update.CreatedAt
-	}
-	msg.Contents = append(msg.Contents, update.Contents...)
-	if update.AdditionalProperties != nil {
-		if msg.AdditionalProperties == nil {
-			msg.AdditionalProperties = make(map[string]any)
+	// A response-level metadata update must not create an empty message.
+	if update.MessageID != "" || update.AuthorName != "" || update.Role != "" || len(update.Contents) > 0 || update.RawRepresentation != nil {
+		msg := resp.targetMessage(update)
+		// Some members on ResponseUpdate map to members of Message.
+		// Incorporate those into the latest message; in cases where the message
+		// stores a single value, prefer the latest update's value over anything
+		// stored in the message.
+		msg.AuthorName = cmp.Or(update.AuthorName, msg.AuthorName)
+		msg.Role = cmp.Or(update.Role, msg.Role)
+		msg.ID = cmp.Or(update.MessageID, msg.ID)
+		if !isValidCreatedAt(msg.CreatedAt) && isValidCreatedAt(update.CreatedAt) {
+			msg.CreatedAt = update.CreatedAt
+		}
+		msg.Contents = append(msg.Contents, update.Contents...)
+		if update.AdditionalProperties != nil {
+			if msg.AdditionalProperties == nil {
+				msg.AdditionalProperties = make(map[string]any)
+			}
+			maps.Copy(msg.AdditionalProperties, update.AdditionalProperties)
 		}
-		maps.Copy(msg.AdditionalProperties, update.AdditionalProperties)
+		msg.RawRepresentation = appendRawRepresentation(msg.RawRepresentation, update.RawRepresentation)
 	}
-	msg.RawRepresentation = appendRawRepresentation(msg.RawRepresentation, update.RawRepresentation)
 
 	// Other members on a ResponseUpdate map to members of the response.
 	// Update the response object with those, preferring the values from later updates.

agent/response_test.go

@@ -963,6 +963,86 @@ func TestResponse_ToUpdates_WithAdditionalPropertiesOnlyProducesSingleUpdate(t *
 	}
 }
 
+func TestResponse_ToUpdates_RoundTripDoesNotCreateMessageForResponseMetadata(t *testing.T) {
+	tests := []struct {
+		name         string
+		response     *agent.Response
+		wantToken    string
+		wantProperty any
+	}{
+		{
+			name:      "continuation token",
+			response:  &agent.Response{ContinuationToken: "token-123"},
+			wantToken: "token-123",
+		},
+		{
+			name:         "additional properties",
+			response:     &agent.Response{AdditionalProperties: map[string]any{"key": "value"}},
+			wantProperty: "value",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var collected agent.Response
+			for _, update := range tt.response.ToUpdates() {
+				collected.Update(update)
+			}
+
+			if len(collected.Messages) != 0 {
+				t.Fatalf("expected no messages after round trip, got %d", len(collected.Messages))
+			}
+			if collected.ContinuationToken != tt.wantToken {
+				t.Errorf("continuation token = %q, want %q", collected.ContinuationToken, tt.wantToken)
+			}
+			if got := collected.AdditionalProperties["key"]; got != tt.wantProperty {
+				t.Errorf("additional property = %v, want %v", got, tt.wantProperty)
+			}
+		})
+	}
+}
+
+func TestResponse_ToUpdates_RoundTripPreservesMetadataOnlyMessages(t *testing.T) {
+	createdAt := time.Date(2026, time.September, 20, 12, 0, 0, 0, time.UTC)
+	tests := []struct {
+		name    string
+		message *message.Message
+	}{
+		{
+			name: "additional properties",
+			message: &message.Message{
+				AdditionalProperties: map[string]any{"key": "value"},
+			},
+		},
+		{
+			name: "created at",
+			message: &message.Message{
+				CreatedAt: createdAt,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			original := &agent.Response{Messages: []*message.Message{tt.message}}
+			var collected agent.Response
+			for _, update := range original.ToUpdates() {
+				collected.Update(update)
+			}
+
+			if len(collected.Messages) != 1 {
+				t.Fatalf("message count = %d, want 1", len(collected.Messages))
+			}
+			if got := collected.Messages[0].AdditionalProperties["key"]; got != tt.message.AdditionalProperties["key"] {
+				t.Errorf("additional property = %v, want %v", got, tt.message.AdditionalProperties["key"])
+			}
+			if got := collected.Messages[0].CreatedAt; !got.Equal(tt.message.CreatedAt) {
+				t.Errorf("created at = %v, want %v", got, tt.message.CreatedAt)
+			}
+		})
+	}
+}
+
 func TestResponse_String(t *testing.T) {
 	msg := func(texts ...string) *message.Message {
 		var contents message.Contents