fix: dedupe token usage aggregation by message id (#2770)

frontend/src/core/messages/usage.ts

@@ -28,7 +28,12 @@ export function getUsageMetadata(message: Message): TokenUsage | null {
 }
 
 /**
- * Accumulate token usage across all AI messages in a thread.
+ * Accumulate token usage across AI messages.
+ *
+ * UI rendering may place the same AI message in more than one group, such as
+ * when a message contains both reasoning and final answer content. Token usage
+ * is attached to the AI message itself, so a message id should only contribute
+ * once to any aggregate.
  */
 export function accumulateUsage(messages: Message[]): TokenUsage | null {
   const cumulative: TokenUsage = {
@@ -37,14 +42,25 @@ export function accumulateUsage(messages: Message[]): TokenUsage | null {
     totalTokens: 0,
   };
   let hasUsage = false;
+  const countedMessageIds = new Set<string>();
+
   for (const message of messages) {
     const usage = getUsageMetadata(message);
-    if (usage) {
-      hasUsage = true;
-      cumulative.inputTokens += usage.inputTokens;
-      cumulative.outputTokens += usage.outputTokens;
-      cumulative.totalTokens += usage.totalTokens;
+    if (!usage) {
+      continue;
     }
+
+    if (message.id) {
+      if (countedMessageIds.has(message.id)) {
+        continue;
+      }
+      countedMessageIds.add(message.id);
+    }
+
+    hasUsage = true;
+    cumulative.inputTokens += usage.inputTokens;
+    cumulative.outputTokens += usage.outputTokens;
+    cumulative.totalTokens += usage.totalTokens;
   }
   return hasUsage ? cumulative : null;
 }

frontend/tests/unit/core/messages/usage.test.ts

@@ -0,0 +1,81 @@
+import type { Message } from "@langchain/langgraph-sdk";
+import { expect, test } from "vitest";
+
+import { accumulateUsage } from "@/core/messages/usage";
+import {
+  getAssistantTurnUsageMessages,
+  getMessageGroups,
+} from "@/core/messages/utils";
+
+test("accumulates each AI message usage only once by message id", () => {
+  const aiMessage = {
+    id: "ai-1",
+    type: "ai",
+    content: "Answer",
+    usage_metadata: { input_tokens: 10, output_tokens: 5, total_tokens: 15 },
+  } as Message;
+
+  expect(accumulateUsage([aiMessage, aiMessage])).toEqual({
+    inputTokens: 10,
+    outputTokens: 5,
+    totalTokens: 15,
+  });
+});
+
+test("counts later usage-bearing snapshots for the same AI message id", () => {
+  const earlySnapshot = {
+    id: "ai-1",
+    type: "ai",
+    content: "Streaming...",
+  } as Message;
+  const completedSnapshot = {
+    id: "ai-1",
+    type: "ai",
+    content: "Complete answer",
+    usage_metadata: { input_tokens: 10, output_tokens: 5, total_tokens: 15 },
+  } as Message;
+
+  expect(accumulateUsage([earlySnapshot, completedSnapshot])).toEqual({
+    inputTokens: 10,
+    outputTokens: 5,
+    totalTokens: 15,
+  });
+});
+
+test("keeps header and per-turn aggregation consistent for duplicated UI groups", () => {
+  const messages = [
+    {
+      id: "human-1",
+      type: "human",
+      content: "Explain this",
+    },
+    {
+      id: "ai-1",
+      type: "ai",
+      content: "<think>checking context</think>Final answer",
+      usage_metadata: { input_tokens: 20, output_tokens: 7, total_tokens: 27 },
+    },
+  ] as Message[];
+
+  const groups = getMessageGroups(messages);
+  const usageMessagesByGroupIndex = getAssistantTurnUsageMessages(groups);
+  const turnUsageMessages = usageMessagesByGroupIndex.at(-1);
+
+  expect(groups.map((group) => group.type)).toEqual([
+    "human",
+    "assistant:processing",
+    "assistant",
+  ]);
+  expect(turnUsageMessages?.map((message) => message.id)).toEqual([
+    "ai-1",
+    "ai-1",
+  ]);
+  expect(accumulateUsage(messages)).toEqual(
+    accumulateUsage(turnUsageMessages!),
+  );
+  expect(accumulateUsage(turnUsageMessages!)).toEqual({
+    inputTokens: 20,
+    outputTokens: 7,
+    totalTokens: 27,
+  });
+});