diff --git a/src/tools/background-task/create-background-output.blocking.test.ts b/src/tools/background-task/create-background-output.blocking.test.ts
new file mode 100644
index 000000000..82de143e9
--- /dev/null
+++ b/src/tools/background-task/create-background-output.blocking.test.ts
@@ -0,0 +1,112 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+import type { ToolContext } from "@opencode-ai/plugin/tool"
+import type { BackgroundTask } from "../../features/background-agent"
+import type { BackgroundOutputClient, BackgroundOutputManager } from "./clients"
+import { createBackgroundOutput } from "./create-background-output"
+
+const projectDir = "/Users/yeongyu/local-workspaces/oh-my-opencode"
+
+const mockContext = {
+  sessionID: "test-session",
+  messageID: "test-message",
+  agent: "test-agent",
+  directory: projectDir,
+  worktree: projectDir,
+  abort: new AbortController().signal,
+  metadata: () => {},
+  ask: async () => {},
+} as unknown as ToolContext
+
+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: "running",
+    ...overrides,
+  }
+}
+
+function createMockClient(): BackgroundOutputClient {
+  return {
+    session: {
+      messages: async () => ({ data: [] }),
+    },
+  }
+}
+
+describe("createBackgroundOutput block=true polling", () => {
+  test("returns terminal error output when task fails during blocking wait", async () => {
+    // #given
+    let pollCount = 0
+    const task = createTask({ status: "running" })
+    const manager: BackgroundOutputManager = {
+      getTask: (id: string) => {
+        if (id !== task.id) return undefined
+
+        pollCount += 1
+        if (pollCount >= 2) {
+          task.status = "error"
+          task.error = "task failed"
+        }
+
+        return task
+      },
+    }
+
+    const tool = createBackgroundOutput(manager, createMockClient())
+
+    // #when
+    const output = await tool.execute(
+      {
+        task_id: task.id,
+        block: true,
+        timeout: 3000,
+        full_session: false,
+      },
+      mockContext
+    )
+
+    // #then
+    expect(pollCount).toBeGreaterThanOrEqual(2)
+    expect(output).toContain("Status | **error**")
+    expect(output).not.toContain("Timed out waiting")
+  })
+
+  test("returns latest output with timeout note when task stays running", async () => {
+    // #given
+    let pollCount = 0
+    const task = createTask({ status: "running" })
+    const manager: BackgroundOutputManager = {
+      getTask: (id: string) => {
+        if (id !== task.id) return undefined
+        pollCount += 1
+        return task
+      },
+    }
+
+    const tool = createBackgroundOutput(manager, createMockClient())
+
+    // #when
+    const output = await tool.execute(
+      {
+        task_id: task.id,
+        block: true,
+        timeout: 10,
+      },
+      mockContext
+    )
+
+    // #then
+    expect(pollCount).toBeGreaterThanOrEqual(2)
+    expect(output).toContain("# Full Session Output")
+    expect(output).toContain("Timed out waiting")
+    expect(output).toContain("still running")
+  })
+})
diff --git a/src/tools/background-task/create-background-output.ts b/src/tools/background-task/create-background-output.ts
index 78593a884..e12cfa9aa 100644
--- a/src/tools/background-task/create-background-output.ts
+++ b/src/tools/background-task/create-background-output.ts
@@ -33,6 +33,14 @@ function formatResolvedTitle(task: BackgroundTask): string {
   return `${label} - ${task.description}`
 }
 
+function isTaskActiveStatus(status: BackgroundTask["status"]): boolean {
+  return status === "pending" || status === "running"
+}
+
+function appendTimeoutNote(output: string, timeoutMs: number): string {
+  return `${output}\n\n> **Timed out waiting** after ${timeoutMs}ms. Task is still running; showing latest available output.`
+}
+
 export function createBackgroundOutput(manager: BackgroundOutputManager, client: BackgroundOutputClient): ToolDefinition {
   return tool({
     description: BACKGROUND_OUTPUT_DESCRIPTION,
@@ -83,7 +91,9 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
 
         let resolvedTask = task
 
-        if (shouldBlock && (task.status === "pending" || task.status === "running")) {
+        let didTimeoutWhileActive = false
+
+        if (shouldBlock && isTaskActiveStatus(task.status)) {
           const startTime = Date.now()
           while (Date.now() - startTime < timeoutMs) {
             await delay(1000)
@@ -93,30 +103,39 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
               return `Task was deleted: ${args.task_id}`
             }
 
-            if (currentTask.status !== "pending" && currentTask.status !== "running") {
-              resolvedTask = currentTask
+            resolvedTask = currentTask
+
+            if (!isTaskActiveStatus(currentTask.status)) {
               break
             }
           }
 
-          const finalCheck = manager.getTask(args.task_id)
-          if (finalCheck) {
-            resolvedTask = finalCheck
+          if (isTaskActiveStatus(resolvedTask.status)) {
+            const finalCheck = manager.getTask(args.task_id)
+            if (finalCheck) {
+              resolvedTask = finalCheck
+            }
+          }
+
+          if (isTaskActiveStatus(resolvedTask.status)) {
+            didTimeoutWhileActive = true
           }
         }
 
-        const isActive = resolvedTask.status === "pending" || resolvedTask.status === "running"
+        const isActive = isTaskActiveStatus(resolvedTask.status)
         const includeThinking = isActive || (args.include_thinking ?? false)
         const includeToolResults = isActive || (args.include_tool_results ?? false)
 
         if (fullSession) {
-          return await formatFullSession(resolvedTask, client, {
+          const output = await formatFullSession(resolvedTask, client, {
             includeThinking,
             messageLimit: args.message_limit,
             sinceMessageId: args.since_message_id,
             includeToolResults,
             thinkingMaxChars: args.thinking_max_chars,
           })
+
+          return didTimeoutWhileActive ? appendTimeoutNote(output, timeoutMs) : output
         }
 
         if (resolvedTask.status === "completed") {
@@ -127,7 +146,8 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
           return formatTaskStatus(resolvedTask)
         }
 
-        return formatTaskStatus(resolvedTask)
+        const statusOutput = formatTaskStatus(resolvedTask)
+        return didTimeoutWhileActive ? appendTimeoutNote(statusOutput, timeoutMs) : statusOutput
       } catch (error) {
         return `Error getting output: ${error instanceof Error ? error.message : String(error)}`
       }