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

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,
+  });
+});