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/features/background-agent/background-task-notification-template.test.ts

@@ -154,6 +154,76 @@ Use \`background_output(task_id="<id>")\` to retrieve each result.
     })
   })
 
+  describe("#given a completed task with retry attempt history", () => {
+    test("#when building the final notification #then it renders the spec-aligned balanced attempt timeline", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "task-3",
+          description: "Fallback task",
+          status: "completed",
+          attempts: [
+            {
+              attemptID: "att-1",
+              attemptNumber: 1,
+              sessionID: "ses-primary",
+              providerID: "genai-proxy-openai",
+              modelID: "gpt-5.4-mini",
+              status: "error",
+              error: "Forbidden: Selected provider is forbidden",
+            },
+            {
+              attemptID: "att-2",
+              attemptNumber: 2,
+              sessionID: "ses-fallback",
+              providerID: "anthropic",
+              modelID: "claude-haiku-4.5",
+              status: "completed",
+            },
+          ],
+        },
+        duration: "10s",
+        statusText: "COMPLETED",
+        allComplete: true,
+        remainingCount: 0,
+        completedTasks: [
+          {
+            id: "task-3",
+            description: "Fallback task",
+            status: "completed",
+            attempts: [
+              {
+                attemptID: "att-1",
+                attemptNumber: 1,
+                sessionID: "ses-primary",
+                providerID: "genai-proxy-openai",
+                modelID: "gpt-5.4-mini",
+                status: "error",
+                error: "Forbidden: Selected provider is forbidden",
+              },
+              {
+                attemptID: "att-2",
+                attemptNumber: 2,
+                sessionID: "ses-fallback",
+                providerID: "anthropic",
+                modelID: "claude-haiku-4.5",
+                status: "completed",
+              },
+            ],
+          },
+        ],
+      })
+
+      // then
+      expect(notification).toContain("[ALL BACKGROUND TASKS COMPLETE]")
+      expect(notification).toContain("- `task-3`: Fallback task")
+      expect(notification).toContain("Background task attempts:")
+      expect(notification).toContain("  - Attempt 1 — ERROR — genai-proxy-openai/gpt-5.4-mini — ses-primary")
+      expect(notification).toContain("    Error: Forbidden: Selected provider is forbidden")
+      expect(notification).toContain("  - Attempt 2 — COMPLETED — anthropic/claude-haiku-4.5 — ses-fallback")
+    })
+  })
+
   describe("#given a single task notification with undefined description", () => {
     test("#when building the partial notification #then it uses task ID as fallback", () => {
       // given

src/features/background-agent/background-task-notification-template.ts

@@ -1,4 +1,4 @@
-import type { BackgroundTaskStatus } from "./types"
+import type { BackgroundTaskAttempt, BackgroundTaskStatus } from "./types"
 
 export type BackgroundTaskNotificationStatus = "COMPLETED" | "CANCELLED" | "INTERRUPTED" | "ERROR"
 
@@ -7,6 +7,55 @@ export interface BackgroundTaskNotificationTask {
   description: string
   status: BackgroundTaskStatus
   error?: string
+  attempts?: BackgroundTaskAttempt[]
+}
+
+function formatAttemptModel(attempt: BackgroundTaskAttempt): string {
+  if (attempt.providerID && attempt.modelID) {
+    return `${attempt.providerID}/${attempt.modelID}`
+  }
+
+  if (attempt.modelID) {
+    return attempt.modelID
+  }
+
+  if (attempt.providerID) {
+    return attempt.providerID
+  }
+
+  return "unknown-model"
+}
+
+function formatAttemptTimeline(task: BackgroundTaskNotificationTask): string {
+  if (!task.attempts || task.attempts.length <= 1) {
+    return ""
+  }
+
+  const lines = task.attempts
+    .map((attempt) => {
+      const attemptLines = [
+        `  - Attempt ${attempt.attemptNumber} — ${attempt.status.toUpperCase()} — ${formatAttemptModel(attempt)} — ${attempt.sessionID ?? "unknown"}`,
+      ]
+
+      if (attempt.status !== "completed" && attempt.error) {
+        attemptLines.push(`    Error: ${attempt.error}`)
+      }
+
+      return attemptLines.join("\n")
+    })
+    .join("\n")
+
+  return `Background task attempts:\n${lines}`
+}
+
+function formatTaskSummaryLine(task: BackgroundTaskNotificationTask): string {
+  const baseLine = `- \`${task.id}\`: ${task.description || task.id}`
+  const statusSuffix = task.status === "completed"
+    ? ""
+    : ` [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}`
+  const timeline = formatAttemptTimeline(task)
+
+  return `${baseLine}${statusSuffix}${timeline ? `\n${timeline}` : ""}`
 }
 
 export function buildBackgroundTaskNotificationText(input: {
@@ -27,10 +76,10 @@ export function buildBackgroundTaskNotificationText(input: {
     const failedTasks = completedTasks.filter((t) => t.status !== "completed")
 
     const succeededText = succeededTasks.length > 0
-      ? succeededTasks.map((t) => `- \`${t.id}\`: ${safeDescription(t)}`).join("\n")
+      ? succeededTasks.map((t) => formatTaskSummaryLine(t)).join("\n")
       : ""
     const failedText = failedTasks.length > 0
-      ? failedTasks.map((t) => `- \`${t.id}\`: ${safeDescription(t)} [${t.status.toUpperCase()}]${t.error ? ` - ${t.error}` : ""}`).join("\n")
+      ? failedTasks.map((t) => formatTaskSummaryLine(t)).join("\n")
       : ""
 
     const hasFailures = failedTasks.length > 0
@@ -46,7 +95,7 @@ export function buildBackgroundTaskNotificationText(input: {
       body += `\n**Failed:**\n${failedText}\n`
     }
     if (!body) {
-      body = `- \`${task.id}\`: ${safeDescription(task)} [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}\n`
+      body = `${formatTaskSummaryLine(task)}\n`
     }
 
     return `<system-reminder>