fix(frontend): avoid render-time subtask context mutation

frontend/src/core/tasks/derive.ts

@@ -0,0 +1,47 @@
+import type { Message } from "@langchain/langgraph-sdk";
+
+import { extractTextFromMessage } from "@/core/messages/utils";
+
+import { parseSubtaskResult } from "./subtask-result";
+import type { Subtask } from "./types";
+
+export function buildSubtaskMapFromMessages(
+  messages: Message[],
+): Record<string, Subtask> {
+  const tasks: Record<string, Subtask> = {};
+
+  for (const message of messages) {
+    if (message.type === "ai") {
+      for (const toolCall of message.tool_calls ?? []) {
+        if (toolCall.name !== "task" || !toolCall.id) {
+          continue;
+        }
+
+        tasks[toolCall.id] = {
+          id: toolCall.id,
+          status: "in_progress",
+          subagent_type: String(toolCall.args?.subagent_type ?? ""),
+          description: String(toolCall.args?.description ?? ""),
+          prompt: String(toolCall.args?.prompt ?? ""),
+        };
+      }
+      continue;
+    }
+
+    if (message.type !== "tool" || !message.tool_call_id) {
+      continue;
+    }
+
+    const task = tasks[message.tool_call_id];
+    if (!task) {
+      continue;
+    }
+
+    tasks[message.tool_call_id] = {
+      ...task,
+      ...parseSubtaskResult(extractTextFromMessage(message)),
+    };
+  }
+
+  return tasks;
+}