fix(telemetry): use max instead of sum for streaming token usage aggregation

agentscope-extensions/agentscope-extensions-studio/src/main/java/io/agentscope/core/tracing/telemetry/StreamChatResponseAggregator.java

@@ -27,7 +27,6 @@
 import io.agentscope.core.model.ChatUsage;
 import java.util.ArrayList;
 import java.util.List;
-import java.util.concurrent.atomic.AtomicInteger;
 
 /**
  * An aggregator for streaming {@link ChatResponse}.
@@ -41,9 +40,9 @@ final class StreamChatResponseAggregator {
     private final ThinkingAccumulator thinkingAcc = new ThinkingAccumulator();
     private final ToolCallsAccumulator toolCallsAcc = new ToolCallsAccumulator();
 
-    // Usage
-    private final AtomicInteger inputTokens = new AtomicInteger(0);
-    private final AtomicInteger outputTokens = new AtomicInteger(0);
+    // Usage: take the max value from all chunks, since providers report cumulative totals
+    private int inputTokens;
+    private int outputTokens;
     private double time;
 
     private String finishReason;
@@ -73,8 +72,8 @@ public void append(ChatResponse chunk) {
 
         ChatUsage usage = chunk.getUsage();
         if (usage != null) {
-            inputTokens.addAndGet(usage.getInputTokens());
-            outputTokens.addAndGet(usage.getOutputTokens());
+            inputTokens = Math.max(inputTokens, usage.getInputTokens());
+            outputTokens = Math.max(outputTokens, usage.getOutputTokens());
             time = usage.getTime();
         }
 
@@ -95,8 +94,8 @@ public ChatResponse getResponse() {
                 .content(contentBlocks)
                 .usage(
                         ChatUsage.builder()
-                                .inputTokens(inputTokens.get())
-                                .outputTokens(outputTokens.get())
+                                .inputTokens(inputTokens)
+                                .outputTokens(outputTokens)
                                 .time(time)
                                 .build())
                 .finishReason(finishReason)

agentscope-extensions/agentscope-extensions-studio/src/test/java/io/agentscope/core/tracing/telemetry/StreamChatResponseAggregatorTest.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2024-2026 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.agentscope.core.tracing.telemetry;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import io.agentscope.core.message.TextBlock;
+import io.agentscope.core.model.ChatResponse;
+import io.agentscope.core.model.ChatUsage;
+import java.util.List;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+@DisplayName("StreamChatResponseAggregator Tests")
+class StreamChatResponseAggregatorTest {
+
+    @Test
+    @DisplayName("Cumulative usage should take max, not sum")
+    void testCumulativeUsageTakesMax() {
+        StreamChatResponseAggregator agg = StreamChatResponseAggregator.create();
+
+        for (int i = 1; i <= 5; i++) {
+            agg.append(
+                    ChatResponse.builder()
+                            .id("test-id")
+                            .content(List.of(TextBlock.builder().text("chunk" + i).build()))
+                            .usage(
+                                    ChatUsage.builder()
+                                            .inputTokens(100)
+                                            .outputTokens(i * 20)
+                                            .time(i * 0.5)
+                                            .build())
+                            .finishReason(i == 5 ? "stop" : null)
+                            .build());
+        }
+
+        ChatResponse response = agg.getResponse();
+        assertEquals("test-id", response.getId());
+        assertEquals(100, response.getUsage().getInputTokens());
+        assertEquals(100, response.getUsage().getOutputTokens());
+        assertEquals("stop", response.getFinishReason());
+    }
+
+    @Test
+    @DisplayName("Only last chunk carries usage (OpenAI style)")
+    void testOnlyLastChunkHasUsage() {
+        StreamChatResponseAggregator agg = StreamChatResponseAggregator.create();
+
+        for (int i = 0; i < 3; i++) {
+            agg.append(
+                    ChatResponse.builder()
+                            .id("openai-id")
+                            .content(List.of(TextBlock.builder().text("part" + i).build()))
+                            .build());
+        }
+
+        agg.append(
+                ChatResponse.builder()
+                        .id("openai-id")
+                        .usage(ChatUsage.builder().inputTokens(200).outputTokens(150).build())
+                        .finishReason("stop")
+                        .build());
+
+        ChatResponse response = agg.getResponse();
+        assertEquals(200, response.getUsage().getInputTokens());
+        assertEquals(150, response.getUsage().getOutputTokens());
+    }
+}