Preserve reported token usage presence

json_contract_test.go

@@ -12,6 +12,36 @@ import (
 	"gitea.maximumdirect.net/eric/promptkit"
 )
 
+func TestGenerateResponseUsagePresenceJSON(t *testing.T) {
+	value := promptkit.GenerateResponse{
+		Content: "ok",
+		Usage:   promptkit.TokenUsage{TotalTokens: 5},
+		UsagePresence: &promptkit.TokenUsagePresence{
+			PromptTokens: true, CompletionTokens: true, TotalTokens: true,
+			CachedTokens: true, CacheWriteTokens: true,
+		},
+	}
+	payload, err := json.Marshal(value)
+	if err != nil {
+		t.Fatal(err)
+	}
+	object := decodeJSONObject(t, payload)
+	if _, ok := object["usage_presence"]; !ok {
+		t.Fatalf("missing usage_presence: %s", payload)
+	}
+	var decoded promptkit.GenerateResponse
+	if err := json.Unmarshal(payload, &decoded); err != nil || !reflect.DeepEqual(decoded, value) {
+		t.Fatalf("round trip = %+v, %v", decoded, err)
+	}
+	payload, err = json.Marshal(promptkit.GenerateResponse{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := decodeJSONObject(t, payload)["usage_presence"]; ok {
+		t.Fatalf("nil usage_presence was serialized: %s", payload)
+	}
+}
+
 func TestPreparedRunJSONContractRoundTripsAllFields(t *testing.T) {
 	value := fullyPopulatedPreparedRun()
 	payload, err := json.Marshal(value)