diff --git a/src/features/background-agent/background-task-notification-template.test.ts b/src/features/background-agent/background-task-notification-template.test.ts
new file mode 100644
index 000000000..5555e909a
--- /dev/null
+++ b/src/features/background-agent/background-task-notification-template.test.ts
@@ -0,0 +1,130 @@
+import { describe, expect, test } from "bun:test"
+import { buildBackgroundTaskNotificationText } from "./background-task-notification-template"
+
+describe("buildBackgroundTaskNotificationText", () => {
+  describe("#given one task still running after a completed task notification", () => {
+    test("#when building the partial notification #then it preserves the existing completed-task format", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "task-1",
+          description: "Index repo",
+          status: "completed",
+        },
+        duration: "42s",
+        statusText: "COMPLETED",
+        allComplete: false,
+        remainingCount: 1,
+        completedTasks: [],
+      })
+
+      // when
+      const expectedNotification = `<system-reminder>
+[BACKGROUND TASK COMPLETED]
+**ID:** \`task-1\`
+**Description:** Index repo
+**Duration:** 42s
+
+**1 task still in progress.** You WILL be notified when ALL complete.
+Do NOT poll - continue productive work.
+
+Use \`background_output(task_id="task-1")\` to retrieve this result when ready.
+</system-reminder>`
+
+      // then
+      expect(notification).toBe(expectedNotification)
+    })
+  })
+
+  describe("#given one task still running after a failed task notification", () => {
+    test("#when building the partial notification #then it preserves the existing failure format", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "task-2",
+          description: "Summarize logs",
+          status: "error",
+          error: "Timed out",
+        },
+        duration: "3m 4s",
+        statusText: "ERROR",
+        allComplete: false,
+        remainingCount: 2,
+        completedTasks: [],
+      })
+
+      // when
+      const expectedNotification = `<system-reminder>
+[BACKGROUND TASK ERROR]
+**ID:** \`task-2\`
+**Description:** Summarize logs
+**Duration:** 3m 4s
+**Error:** Timed out
+
+**2 tasks still in progress.** You WILL be notified when ALL complete.
+**ACTION REQUIRED:** This task failed. Check the error and decide whether to retry, cancel remaining tasks, or continue.
+
+Use \`background_output(task_id="task-2")\` to retrieve this result when ready.
+</system-reminder>`
+
+      // then
+      expect(notification).toBe(expectedNotification)
+    })
+  })
+
+  describe("#given all sibling tasks completed with mixed outcomes", () => {
+    test("#when building the final notification #then it preserves the existing summary format", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "task-3",
+          description: "Fallback task",
+          status: "error",
+          error: "Denied",
+        },
+        duration: "10s",
+        statusText: "ERROR",
+        allComplete: true,
+        remainingCount: 0,
+        completedTasks: [
+          {
+            id: "task-1",
+            description: "Index repo",
+            status: "completed",
+          },
+          {
+            id: "task-2",
+            description: "Summarize logs",
+            status: "cancelled",
+            error: "User aborted",
+          },
+          {
+            id: "task-3",
+            description: "Fallback task",
+            status: "error",
+            error: "Denied",
+          },
+        ],
+      })
+
+      // when
+      const expectedNotification = `<system-reminder>
+[ALL BACKGROUND TASKS FINISHED - 2 FAILED]
+
+**Completed:**
+- \`task-1\`: Index repo
+
+**Failed:**
+- \`task-2\`: Summarize logs [CANCELLED] - User aborted
+- \`task-3\`: Fallback task [ERROR] - Denied
+
+Use \`background_output(task_id="<id>")\` to retrieve each result.
+
+**ACTION REQUIRED:** 2 task(s) failed. Check errors above and decide whether to retry or proceed.
+</system-reminder>`
+
+      // then
+      expect(notification).toBe(expectedNotification)
+    })
+  })
+})
diff --git a/src/features/background-agent/background-task-notification-template.ts b/src/features/background-agent/background-task-notification-template.ts
index e2e74cc78..240efe9c0 100644
--- a/src/features/background-agent/background-task-notification-template.ts
+++ b/src/features/background-agent/background-task-notification-template.ts
@@ -1,14 +1,21 @@
-import type { BackgroundTask } from "./types"
+import type { BackgroundTaskStatus } from "./types"
 
 export type BackgroundTaskNotificationStatus = "COMPLETED" | "CANCELLED" | "INTERRUPTED" | "ERROR"
 
+export interface BackgroundTaskNotificationTask {
+  id: string
+  description: string
+  status: BackgroundTaskStatus
+  error?: string
+}
+
 export function buildBackgroundTaskNotificationText(input: {
-  task: BackgroundTask
+  task: BackgroundTaskNotificationTask
   duration: string
   statusText: BackgroundTaskNotificationStatus
   allComplete: boolean
   remainingCount: number
-  completedTasks: BackgroundTask[]
+  completedTasks: BackgroundTaskNotificationTask[]
 }): string {
   const { task, duration, statusText, allComplete, remainingCount, completedTasks } = input
 
@@ -50,14 +57,12 @@ Use \`background_output(task_id="<id>")\` to retrieve each result.${hasFailures
 </system-reminder>`
   }
 
-  const agentInfo = task.category ? `${task.agent} (${task.category})` : task.agent
   const isFailure = statusText !== "COMPLETED"
 
   return `<system-reminder>
 [BACKGROUND TASK ${statusText}]
 **ID:** \`${task.id}\`
 **Description:** ${task.description}
-**Agent:** ${agentInfo}
 **Duration:** ${duration}${errorInfo}
 
 **${remainingCount} task${remainingCount === 1 ? "" : "s"} still in progress.** You WILL be notified when ALL complete.
diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index efbed503f..621700030 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -34,6 +34,10 @@ import {
 import { subagentSessions } from "../claude-code-session-state"
 import { getTaskToastManager } from "../task-toast-manager"
 import { formatDuration } from "./duration-formatter"
+import {
+  buildBackgroundTaskNotificationText,
+  type BackgroundTaskNotificationTask,
+} from "./background-task-notification-template"
 import {
   isAbortedSessionError,
   extractErrorName,
@@ -151,7 +155,7 @@ export class BackgroundManager {
   private queuesByKey: Map<string, QueueItem[]> = new Map()
   private processingKeys: Set<string> = new Set()
   private completionTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
-  private completedTaskSummaries: Map<string, Array<{id: string, description: string, status: string, error?: string}>> = new Map()
+  private completedTaskSummaries: Map<string, BackgroundTaskNotificationTask[]> = new Map()
   private idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private notificationQueueByParent: Map<string, Promise<void>> = new Map()
   private rootDescendantCounts: Map<string, number>
@@ -1632,56 +1636,14 @@ export class BackgroundManager {
         : task.status === "error"
           ? "ERROR"
           : "CANCELLED"
-    const errorInfo = task.error ? `\n**Error:** ${task.error}` : ""
-
-    let notification: string
-    if (allComplete) {
-        const succeededTasks = completedTasks.filter(t => t.status === "completed")
-        const failedTasks = completedTasks.filter(t => t.status !== "completed")
-
-        const succeededText = succeededTasks.length > 0
-          ? succeededTasks.map(t => `- \`${t.id}\`: ${t.description}`).join("\n")
-          : ""
-        const failedText = failedTasks.length > 0
-          ? failedTasks.map(t => `- \`${t.id}\`: ${t.description} [${t.status.toUpperCase()}]${t.error ? ` - ${t.error}` : ""}`).join("\n")
-          : ""
-
-        const hasFailures = failedTasks.length > 0
-        const header = hasFailures
-          ? `[ALL BACKGROUND TASKS FINISHED - ${failedTasks.length} FAILED]`
-          : "[ALL BACKGROUND TASKS COMPLETE]"
-
-        let body = ""
-        if (succeededText) {
-          body += `**Completed:**\n${succeededText}\n`
-        }
-        if (failedText) {
-          body += `\n**Failed:**\n${failedText}\n`
-        }
-        if (!body) {
-          body = `- \`${task.id}\`: ${task.description} [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}\n`
-        }
-
-        notification = `<system-reminder>
-${header}
-
-${body.trim()}
-
-Use \`background_output(task_id="<id>")\` to retrieve each result.${hasFailures ? `\n\n**ACTION REQUIRED:** ${failedTasks.length} task(s) failed. Check errors above and decide whether to retry or proceed.` : ""}
-</system-reminder>`
-    } else {
-      notification = `<system-reminder>
-[BACKGROUND TASK ${statusText}]
-**ID:** \`${task.id}\`
-**Description:** ${task.description}
-**Duration:** ${duration}${errorInfo}
-
-**${remainingCount} task${remainingCount === 1 ? "" : "s"} still in progress.** You WILL be notified when ALL complete.
-${statusText === "COMPLETED" ? "Do NOT poll - continue productive work." : "**ACTION REQUIRED:** This task failed. Check the error and decide whether to retry, cancel remaining tasks, or continue."}
-
-Use \`background_output(task_id="${task.id}")\` to retrieve this result when ready.
-</system-reminder>`
-    }
+    const notification = buildBackgroundTaskNotificationText({
+      task,
+      duration,
+      statusText,
+      allComplete,
+      remainingCount,
+      completedTasks,
+    })
 
       let agent: string | undefined = task.parentAgent
       let model: { providerID: string; modelID: string } | undefined