diff --git a/src/features/background-agent/background-task-notification-template.test.ts b/src/features/background-agent/background-task-notification-template.test.ts
index 5555e909a..37b416570 100644
--- a/src/features/background-agent/background-task-notification-template.test.ts
+++ b/src/features/background-agent/background-task-notification-template.test.ts
@@ -127,4 +127,52 @@ Use \`background_output(task_id="<id>")\` to retrieve each result.
       expect(notification).toBe(expectedNotification)
     })
   })
+
+  describe("#given all tasks completed with undefined descriptions", () => {
+    test("#when building the final notification #then it uses task ID as fallback instead of 'undefined'", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "bg_abc123",
+          description: undefined as unknown as string,
+          status: "completed",
+        },
+        duration: "5s",
+        statusText: "COMPLETED",
+        allComplete: true,
+        remainingCount: 0,
+        completedTasks: [
+          { id: "bg_abc123", description: undefined as unknown as string, status: "completed" },
+          { id: "bg_def456", description: undefined as unknown as string, status: "completed" },
+        ],
+      })
+
+      // then
+      expect(notification).not.toContain(": undefined")
+      expect(notification).toContain("bg_abc123")
+      expect(notification).toContain("bg_def456")
+    })
+  })
+
+  describe("#given a single task notification with undefined description", () => {
+    test("#when building the partial notification #then it uses task ID as fallback", () => {
+      // given
+      const notification = buildBackgroundTaskNotificationText({
+        task: {
+          id: "bg_xyz789",
+          description: undefined as unknown as string,
+          status: "completed",
+        },
+        duration: "3s",
+        statusText: "COMPLETED",
+        allComplete: false,
+        remainingCount: 2,
+        completedTasks: [],
+      })
+
+      // then
+      expect(notification).not.toContain("undefined")
+      expect(notification).toContain("bg_xyz789")
+    })
+  })
 })
diff --git a/src/features/background-agent/background-task-notification-template.ts b/src/features/background-agent/background-task-notification-template.ts
index 240efe9c0..ad6769fac 100644
--- a/src/features/background-agent/background-task-notification-template.ts
+++ b/src/features/background-agent/background-task-notification-template.ts
@@ -19,6 +19,7 @@ export function buildBackgroundTaskNotificationText(input: {
 }): string {
   const { task, duration, statusText, allComplete, remainingCount, completedTasks } = input
 
+  const safeDescription = (t: BackgroundTaskNotificationTask): string => t.description || t.id
   const errorInfo = task.error ? `\n**Error:** ${task.error}` : ""
 
   if (allComplete) {
@@ -26,10 +27,10 @@ export function buildBackgroundTaskNotificationText(input: {
     const failedTasks = completedTasks.filter((t) => t.status !== "completed")
 
     const succeededText = succeededTasks.length > 0
-      ? succeededTasks.map((t) => `- \`${t.id}\`: ${t.description}`).join("\n")
+      ? succeededTasks.map((t) => `- \`${t.id}\`: ${safeDescription(t)}`).join("\n")
       : ""
     const failedText = failedTasks.length > 0
-      ? failedTasks.map((t) => `- \`${t.id}\`: ${t.description} [${t.status.toUpperCase()}]${t.error ? ` - ${t.error}` : ""}`).join("\n")
+      ? failedTasks.map((t) => `- \`${t.id}\`: ${safeDescription(t)} [${t.status.toUpperCase()}]${t.error ? ` - ${t.error}` : ""}`).join("\n")
       : ""
 
     const hasFailures = failedTasks.length > 0
@@ -45,7 +46,7 @@ export function buildBackgroundTaskNotificationText(input: {
       body += `\n**Failed:**\n${failedText}\n`
     }
     if (!body) {
-      body = `- \`${task.id}\`: ${task.description} [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}\n`
+      body = `- \`${task.id}\`: ${safeDescription(task)} [${task.status.toUpperCase()}]${task.error ? ` - ${task.error}` : ""}\n`
     }
 
     return `<system-reminder>
@@ -62,7 +63,7 @@ Use \`background_output(task_id="<id>")\` to retrieve each result.${hasFailures
   return `<system-reminder>
 [BACKGROUND TASK ${statusText}]
 **ID:** \`${task.id}\`
-**Description:** ${task.description}
+**Description:** ${safeDescription(task)}
 **Duration:** ${duration}${errorInfo}
 
 **${remainingCount} task${remainingCount === 1 ? "" : "s"} still in progress.** You WILL be notified when ALL complete.
diff --git a/src/tools/delegate-task/tools.test.ts b/src/tools/delegate-task/tools.test.ts
index ac2fc1b25..33e338c1b 100644
--- a/src/tools/delegate-task/tools.test.ts
+++ b/src/tools/delegate-task/tools.test.ts
@@ -1366,6 +1366,35 @@ describe("sisyphus-task", () => {
       )).rejects.toThrow("Invalid arguments: 'run_in_background' parameter is REQUIRED")
     })
 
+    test("#given category without description #when executing #then throws required parameter error", async () => {
+      // given
+      const { createDelegateTask } = require("./tools")
+      const mockManager = { launch: async () => ({}) }
+      const mockClient = {
+        app: { agents: async () => ({ data: [] }) },
+        config: { get: async () => ({ data: { model: SYSTEM_DEFAULT_MODEL } }) },
+        session: {
+          create: async () => ({ data: { id: "test-session" } }),
+          prompt: async () => ({ data: {} }),
+          promptAsync: async () => ({ data: {} }),
+          messages: async () => ({ data: [] }),
+        },
+      }
+      const tool = createDelegateTask({ manager: mockManager, client: mockClient })
+
+      // when
+      // then
+      await expect(tool.execute(
+        {
+          prompt: "Do something",
+          category: "quick",
+          run_in_background: false,
+          load_skills: [],
+        },
+        { sessionID: "parent-session", messageID: "parent-message", agent: "sisyphus", abort: new AbortController().signal }
+      )).rejects.toThrow("Invalid arguments: 'description' parameter is REQUIRED")
+    })
+
     test("#given explicit run_in_background=false #when executing #then sync execution succeeds", async () => {
       // given
       const { createDelegateTask } = require("./tools")
diff --git a/src/tools/delegate-task/tools.ts b/src/tools/delegate-task/tools.ts
index c149f8025..46822bfcd 100644
--- a/src/tools/delegate-task/tools.ts
+++ b/src/tools/delegate-task/tools.ts
@@ -122,6 +122,9 @@ export function createDelegateTask(options: DelegateTaskToolOptions): ToolDefini
         title: args.description,
       })
 
+      if (!args.description || typeof args.description !== "string") {
+        throw new Error(`Invalid arguments: 'description' parameter is REQUIRED. Provide a short (3-5 words) task description.`)
+      }
       if (args.run_in_background === undefined) {
         throw new Error(`Invalid arguments: 'run_in_background' parameter is REQUIRED. Specify run_in_background=false for task delegation, or run_in_background=true for parallel exploration.`)
       }