feat(background-task): render retry timelines and links

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/background-task/clients.ts

@@ -2,7 +2,7 @@ import type { BackgroundManager } from "../../features/background-agent"
 
 export type BackgroundOutputMessage = {
   id?: string
-  info?: { role?: string; time?: string | { created?: number }; agent?: string }
+  info?: { role?: string; time?: string | { created?: number }; agent?: string; error?: unknown }
   parts?: Array<{
     type?: string
     text?: string

src/tools/background-task/task-result-format.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, test } from "bun:test"
+
+import type { BackgroundTask } from "../../features/background-agent"
+import type { BackgroundOutputClient } from "./clients"
+import { formatTaskResult } from "./task-result-format"
+
+function createTask(overrides: Partial<BackgroundTask> = {}): BackgroundTask {
+  return {
+    id: "task-1",
+    sessionID: "ses-1",
+    parentSessionID: "main-1",
+    parentMessageID: "msg-1",
+    description: "background task",
+    prompt: "do work",
+    agent: "test-agent",
+    status: "completed",
+    startedAt: new Date("2026-01-01T00:00:00.000Z"),
+    completedAt: new Date("2026-01-01T00:00:05.000Z"),
+    ...overrides,
+  }
+}
+
+describe("formatTaskResult", () => {
+  test("returns assistant session errors instead of masking them as success text", async () => {
+    const task = createTask()
+    const client: BackgroundOutputClient = {
+      session: {
+        messages: async () => ({
+          data: [
+            {
+              info: {
+                role: "assistant",
+                time: { created: 1 },
+                error: { data: { message: "Forbidden: Selected provider is forbidden" } },
+              },
+              parts: [],
+            },
+          ],
+        }),
+      },
+    }
+
+    const output = await formatTaskResult(task, client)
+
+    expect(output).toContain("Session error")
+    expect(output).toContain("Forbidden: Selected provider is forbidden")
+  })
+})

src/tools/background-task/task-result-format.ts

@@ -1,4 +1,5 @@
 import type { BackgroundTask } from "../../features/background-agent"
+import { extractErrorMessage } from "../../features/background-agent/error-classifier"
 import { consumeNewMessages } from "../../shared/session-cursor"
 import type { BackgroundOutputClient, BackgroundOutputMessagesResult } from "./clients"
 import { extractMessages, getErrorMessage } from "./session-messages"
@@ -56,6 +57,23 @@ Session ID: ${task.sessionID}
     return timeA.localeCompare(timeB)
   })
 
+  const sessionError = sortedMessages
+    .filter((message) => message.info?.role === "assistant" && message.info?.error)
+    .map((message) => extractErrorMessage(message.info?.error))
+    .find((message): message is string => typeof message === "string" && message.length > 0)
+  if (sessionError) {
+    return `Task Result
+
+Task ID: ${task.id}
+Description: ${task.description}
+Duration: ${formatDuration(task.startedAt ?? new Date(), task.completedAt)}
+Session ID: ${task.sessionID}
+
+---
+
+Session error: ${sessionError}`
+  }
+
   const newMessages = consumeNewMessages(task.sessionID, sortedMessages)
   if (newMessages.length === 0) {
     const duration = formatDuration(task.startedAt ?? new Date(), task.completedAt)