diff --git a/backend/app/services/__init__.py b/backend/app/services/__init__.py
index e69de29..3de9897 100644
--- a/backend/app/services/__init__.py
+++ b/backend/app/services/__init__.py
@@ -0,0 +1,40 @@
+"""业务逻辑层"""
+from app.services.product_service import (
+    get_product_by_serial,
+    get_product,
+    create_product,
+    update_product,
+    get_all_products,
+)
+from app.services.task_service import (
+    get_task,
+    get_top_level_tasks,
+    create_task,
+    update_task,
+    complete_task,
+    receive_task,
+    reject_task,
+    transfer_task,
+    create_subtask,
+    get_all_tasks,
+)
+
+__all__ = [
+    # Product
+    "get_product_by_serial",
+    "get_product",
+    "create_product",
+    "update_product",
+    "get_all_products",
+    # Task
+    "get_task",
+    "get_top_level_tasks",
+    "create_task",
+    "update_task",
+    "complete_task",
+    "receive_task",
+    "reject_task",
+    "transfer_task",
+    "create_subtask",
+    "get_all_tasks",
+]
diff --git a/backend/app/services/product_service.py b/backend/app/services/product_service.py
new file mode 100644
index 0000000..1030ad8
--- /dev/null
+++ b/backend/app/services/product_service.py
@@ -0,0 +1,161 @@
+"""产品服务 — 业务逻辑层：扫码查询、CRUD"""
+from __future__ import annotations
+import uuid
+from fastapi import HTTPException, status
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+from app.models.product import Product
+from app.models.production_order import ProductionOrder
+from app.models.task import Task
+from app.schemas.product import ProductCreate, ProductUpdate, ProductResponse, ProductScanResponse
+from app.schemas.task import TaskSummaryResponse, TaskResponse
+
+
+def _task_to_response(task: Task) -> TaskResponse:
+    """将 Task ORM 对象递归转为 TaskResponse（含子任务树）"""
+    return TaskResponse(
+        id=task.id,
+        product_id=task.product_id,
+        parent_task_id=task.parent_task_id,
+        task_name=task.task_name,
+        assignee_id=task.assignee_id,
+        status=task.status,
+        notify_parent_on_complete=task.notify_parent_on_complete,
+        is_rework=task.is_rework,
+        reject_reason=task.reject_reason,
+        received_at=task.received_at,
+        completed_at=task.completed_at,
+        created_at=task.created_at,
+        child_tasks=[_task_to_response(c) for c in task.child_tasks],
+    )
+
+
+async def _load_task_tree(db: AsyncSession, product_id: uuid.UUID) -> list[TaskResponse]:
+    """递归加载产品下的完整任务树"""
+    # 先取顶层任务
+    result = await db.execute(
+        select(Task)
+        .options(selectinload(Task.child_tasks))
+        .where(
+            Task.product_id == product_id,
+            Task.parent_task_id.is_(None),
+        )
+        .order_by(Task.created_at)
+    )
+    top_tasks = result.scalars().all()
+
+    # 递归加载每层子任务
+    async def _load_children(t: Task):
+        for child in t.child_tasks:
+            child_result = await db.execute(
+                select(Task)
+                .options(selectinload(Task.child_tasks))
+                .where(Task.id == child.id)
+            )
+            refreshed = child_result.scalar_one()
+            t.child_tasks[t.child_tasks.index(child)] = refreshed
+            await _load_children(refreshed)
+
+    for task in top_tasks:
+        await _load_children(task)
+
+    return [_task_to_response(t) for t in top_tasks]
+
+
+async def get_product_by_serial(db: AsyncSession, serial_number: str) -> ProductScanResponse:
+    """扫码查询：根据 16 位序列号查出产品 + 所属订单 + 完整任务树"""
+    result = await db.execute(
+        select(Product)
+        .options(
+            selectinload(Product.order),
+            selectinload(Product.parent_product),
+        )
+        .where(Product.serial_number == serial_number)
+    )
+    product = result.scalar_one_or_none()
+    if not product:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"未找到序列号为 {serial_number} 的产品",
+        )
+
+    # 获取顶层任务摘要（兼容旧接口）
+    top_tasks_result = await db.execute(
+        select(Task)
+        .where(
+            Task.product_id == product.id,
+            Task.parent_task_id.is_(None),
+        )
+        .order_by(Task.created_at)
+    )
+    top_tasks = top_tasks_result.scalars().all()
+
+    # 获取完整任务树（递归嵌套，供前端渲染十字矩阵树状图）
+    task_tree = await _load_task_tree(db, product.id)
+
+    return ProductScanResponse(
+        id=product.id,
+        serial_number=product.serial_number,
+        order_id=product.order_id,
+        order_no=product.order.order_no if product.order else "",
+        material_id=product.material_id,
+        parent_product_id=product.parent_product_id,
+        current_location_id=product.current_location_id,
+        status=product.status,
+        created_at=product.created_at,
+        top_level_tasks=[
+            TaskSummaryResponse.model_validate(t) for t in top_tasks
+        ],
+        task_tree=task_tree,
+    )
+
+
+async def get_product(db: AsyncSession, product_id: uuid.UUID) -> Product:
+    """获取产品，不存在则 404"""
+    result = await db.execute(
+        select(Product)
+        .options(selectinload(Product.order))
+        .where(Product.id == product_id)
+    )
+    product = result.scalar_one_or_none()
+    if not product:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"产品不存在: {product_id}",
+        )
+    return product
+
+
+async def create_product(db: AsyncSession, data: ProductCreate) -> ProductResponse:
+    """创建产品"""
+    product = Product(**data.model_dump())
+    db.add(product)
+    await db.commit()
+    await db.refresh(product)
+    return ProductResponse.model_validate(product)
+
+
+async def update_product(db: AsyncSession, product_id: uuid.UUID, data: ProductUpdate) -> ProductResponse:
+    """更新产品"""
+    product = await get_product(db, product_id)
+    update_data = data.model_dump(exclude_unset=True)
+    for field, value in update_data.items():
+        setattr(product, field, value)
+    await db.commit()
+    await db.refresh(product)
+    return ProductResponse.model_validate(product)
+
+
+async def get_all_products(db: AsyncSession, skip: int = 0, limit: int = 50) -> list[ProductResponse]:
+    """获取产品列表"""
+    result = await db.execute(
+        select(Product)
+        .options(selectinload(Product.order))
+        .offset(skip)
+        .limit(limit)
+        .order_by(Product.created_at.desc())
+    )
+    products = result.scalars().all()
+    return [ProductResponse.model_validate(p) for p in products]
diff --git a/backend/app/services/task_service.py b/backend/app/services/task_service.py
new file mode 100644
index 0000000..692cec1
--- /dev/null
+++ b/backend/app/services/task_service.py
@@ -0,0 +1,593 @@
+"""任务服务 — 核心业务逻辑：接收、驳回返工、裂变转交、无限嵌套子任务"""
+from __future__ import annotations
+import uuid
+from datetime import datetime, timezone
+from fastapi import HTTPException, status
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+from app.models.task import Task, TASK_STATUS_PENDING, TASK_STATUS_WIP, TASK_STATUS_COMPLETED, TASK_STATUS_REJECTED
+from app.models.product import Product
+from app.models.task_log import TaskLog
+from app.schemas.task import (
+    TaskCreate,
+    TaskUpdate,
+    TaskCompleteRequest,
+    TaskRejectRequest,
+    TaskTransferRequest,
+    SubtaskCreate,
+    TaskResponse,
+    TaskCompleteResponse,
+    TaskTransferResponse,
+    TaskSummaryResponse,
+    TaskListResponse,
+)
+
+# 特殊位置常量
+VIRTUAL_WAREHOUSE = "virtual_warehouse"
+
+
+# ============================================================
+# 内部辅助函数
+# ============================================================
+
+async def _get_task_or_404(db: AsyncSession, task_id: uuid.UUID) -> Task:
+    """获取任务，不存在则 404"""
+    result = await db.execute(
+        select(Task)
+        .options(
+            selectinload(Task.child_tasks),
+            selectinload(Task.parent_task),
+            selectinload(Task.product),
+        )
+        .where(Task.id == task_id)
+    )
+    task = result.scalar_one_or_none()
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"任务不存在: {task_id}",
+        )
+    return task
+
+
+async def _get_task_with_children_recursive(db: AsyncSession, task_id: uuid.UUID) -> Task:
+    """递归加载任务及其所有子孙任务"""
+    result = await db.execute(
+        select(Task)
+        .options(selectinload(Task.child_tasks))
+        .where(Task.id == task_id)
+    )
+    task = result.scalar_one_or_none()
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"任务不存在: {task_id}",
+        )
+
+    # 递归加载每一层子任务
+    async def _load_children(t: Task):
+        for child in t.child_tasks:
+            child_result = await db.execute(
+                select(Task)
+                .options(selectinload(Task.child_tasks))
+                .where(Task.id == child.id)
+            )
+            refreshed_child = child_result.scalar_one()
+            t.child_tasks[t.child_tasks.index(child)] = refreshed_child
+            await _load_children(refreshed_child)
+
+    await _load_children(task)
+    return task
+
+
+def _to_response(task: Task) -> TaskResponse:
+    """将 Task ORM 对象转为递归 TaskResponse"""
+    return TaskResponse(
+        id=task.id,
+        product_id=task.product_id,
+        parent_task_id=task.parent_task_id,
+        task_name=task.task_name,
+        assignee_id=task.assignee_id,
+        status=task.status,
+        notify_parent_on_complete=task.notify_parent_on_complete,
+        is_rework=task.is_rework,
+        reject_reason=task.reject_reason,
+        received_at=task.received_at,
+        completed_at=task.completed_at,
+        created_at=task.created_at,
+        child_tasks=[_to_response(c) for c in task.child_tasks],
+    )
+
+
+async def _check_all_critical_children_completed(
+    db: AsyncSession, task_id: uuid.UUID
+) -> tuple[bool, list[str]]:
+    """
+    检查指定任务下所有 notify_parent_on_complete=True 的子任务是否都已完成。
+    返回 (是否全部完成, 未完成的子任务名称列表)。
+    """
+    result = await db.execute(
+        select(Task).where(
+            Task.parent_task_id == task_id,
+            Task.notify_parent_on_complete.is_(True),
+        )
+    )
+    critical_children = result.scalars().all()
+
+    incomplete = [
+        child.task_name
+        for child in critical_children
+        if child.status != TASK_STATUS_COMPLETED
+    ]
+    return len(incomplete) == 0, incomplete
+
+
+async def _create_task_log(
+    db: AsyncSession,
+    task_id: uuid.UUID,
+    action_type: str,
+    operator_id: str | None = None,
+    remark: str | None = None,
+) -> TaskLog:
+    """创建任务操作日志"""
+    log = TaskLog(
+        task_id=task_id,
+        operator_id=operator_id,
+        action_type=action_type,
+        remark=remark,
+    )
+    db.add(log)
+    return log
+
+
+# ============================================================
+# 公开 API — 查询
+# ============================================================
+
+async def get_task(db: AsyncSession, task_id: uuid.UUID) -> TaskResponse:
+    """获取任务详情 — 递归包含所有子任务"""
+    task = await _get_task_with_children_recursive(db, task_id)
+    return _to_response(task)
+
+
+async def get_top_level_tasks(db: AsyncSession, product_id: uuid.UUID) -> list[TaskSummaryResponse]:
+    """获取产品的顶层任务列表"""
+    result = await db.execute(
+        select(Task)
+        .where(
+            Task.product_id == product_id,
+            Task.parent_task_id.is_(None),
+        )
+        .order_by(Task.created_at)
+    )
+    tasks = result.scalars().all()
+    return [TaskSummaryResponse.model_validate(t) for t in tasks]
+
+
+async def create_task(db: AsyncSession, data: TaskCreate) -> TaskResponse:
+    """创建任务"""
+    task = Task(**data.model_dump())
+    db.add(task)
+    await db.commit()
+    await db.refresh(task)
+    return _to_response(task)
+
+
+async def update_task(db: AsyncSession, task_id: uuid.UUID, data: TaskUpdate) -> TaskResponse:
+    """更新任务"""
+    task = await _get_task_or_404(db, task_id)
+    update_data = data.model_dump(exclude_unset=True)
+    for field, value in update_data.items():
+        setattr(task, field, value)
+    await db.commit()
+    await db.refresh(task)
+    return _to_response(task)
+
+
+async def get_all_tasks(
+    db: AsyncSession, product_id: uuid.UUID | None = None, skip: int = 0, limit: int = 50
+) -> TaskListResponse:
+    """获取任务列表，可按产品筛选"""
+    stmt = select(Task).options(selectinload(Task.child_tasks))
+    if product_id:
+        stmt = stmt.where(Task.product_id == product_id)
+    stmt = stmt.offset(skip).limit(limit).order_by(Task.created_at.desc())
+
+    result = await db.execute(stmt)
+    tasks = result.scalars().all()
+
+    # 构造嵌套响应时只返回顶层任务
+    all_tasks = [_to_response(t) for t in tasks if t.parent_task_id is None]
+    return TaskListResponse(tasks=all_tasks, total=len(all_tasks))
+
+
+# ============================================================
+# 核心业务 1：确认接收 (PENDING → WIP)
+# ============================================================
+
+async def receive_task(
+    db: AsyncSession, task_id: uuid.UUID, operator_id: str | None = None
+) -> TaskResponse:
+    """
+    操作员确认接收任务。
+
+    校验：只有状态为 PENDING 的任务可接收。
+    动作：状态改为 WIP，记录 received_at 为当前时间。
+    """
+    task = await _get_task_or_404(db, task_id)
+
+    # 校验：只有 PENDING 状态可接收
+    if task.status != TASK_STATUS_PENDING:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"只有待接收(PENDING)状态的任务可接收，当前状态: {task.status}",
+        )
+
+    now = datetime.now(timezone.utc)
+    task.status = TASK_STATUS_WIP
+    task.received_at = now
+
+    await _create_task_log(
+        db, task_id,
+        action_type="receive",
+        operator_id=operator_id,
+        remark=f"操作员确认接收任务「{task.task_name}」",
+    )
+
+    await db.commit()
+    await db.refresh(task)
+
+    return _to_response(task)
+
+
+# ============================================================
+# 核心业务 2：品质驳回 (→ REJECTED + 返工闭环)
+# ============================================================
+
+async def reject_task(
+    db: AsyncSession, task_id: uuid.UUID, request: TaskRejectRequest, operator_id: str | None = None
+) -> TaskResponse:
+    """
+    品质驳回：将当前任务标记为 REJECTED，并自动创建返工任务给上一道工序负责人。
+
+    防呆闭环逻辑：
+    1. 将当前任务状态改为 REJECTED，记录 reject_reason 和 completed_at。
+    2. 查找当前任务的父任务 (parent_task) 的负责人 (assignee_id)。
+       - 若有父任务：返工任务分配给父任务的 assignee_id。
+       - 若无父任务（顶层任务）：返工任务分配给当前任务自己的 assignee_id。
+    3. 为该负责人新建一个完全一样的任务，但 is_rework=True，status=PENDING。
+    4. 新返工任务挂在同一个 parent_task_id 下（与原任务同级）。
+    """
+    task = await _get_task_or_404(db, task_id)
+
+    # 校验：不能重复驳回已完成/已驳回的任务
+    if task.status in (TASK_STATUS_COMPLETED, TASK_STATUS_REJECTED):
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"任务状态为 {task.status}，无法驳回",
+        )
+
+    now = datetime.now(timezone.utc)
+
+    # --- 1. 标记当前任务为已驳回 ---
+    task.status = TASK_STATUS_REJECTED
+    task.reject_reason = request.reason
+    task.completed_at = now
+
+    await _create_task_log(
+        db, task_id,
+        action_type="reject",
+        operator_id=operator_id,
+        remark=f"品质驳回: {request.reason}",
+    )
+
+    # --- 2. 确定返工任务的负责人（上一道工序的负责人） ---
+    rework_assignee_id: str | None = None
+    if task.parent_task_id:
+        # 有父任务：查父任务的 assignee
+        parent_result = await db.execute(
+            select(Task).where(Task.id == task.parent_task_id)
+        )
+        parent_task = parent_result.scalar_one_or_none()
+        if parent_task:
+            rework_assignee_id = parent_task.assignee_id
+    else:
+        # 无父任务（顶层）：返工给自己
+        rework_assignee_id = task.assignee_id
+
+    # --- 3. 创建返工任务 ---
+    rework_task = Task(
+        product_id=task.product_id,
+        parent_task_id=task.parent_task_id,  # 与原任务同级
+        task_name=task.task_name,
+        assignee_id=rework_assignee_id,
+        status=TASK_STATUS_PENDING,
+        notify_parent_on_complete=task.notify_parent_on_complete,
+        is_rework=True,
+    )
+    db.add(rework_task)
+    await db.flush()
+
+    await _create_task_log(
+        db, rework_task.id,
+        action_type="create",
+        operator_id=operator_id,
+        remark=f"返工任务（驳回自「{task.task_name}」，原因: {request.reason}），分配给 {rework_assignee_id}",
+    )
+
+    await db.commit()
+    await db.refresh(task)
+
+    return _to_response(task)
+
+
+# ============================================================
+# 核心业务 3：完工并裂变转交 (→ COMPLETED + 裂变创建下家任务)
+# ============================================================
+
+async def transfer_task(
+    db: AsyncSession, task_id: uuid.UUID, request: TaskTransferRequest, operator_id: str | None = None
+) -> TaskTransferResponse:
+    """
+    完工并裂变转交：
+
+    动作 1（闭环当前节点）：
+    - 将当前任务状态改为 COMPLETED，记录 completed_at。
+
+    动作 2（解析下家）：
+    - 遍历 next_assignees 列表。
+    - 如果包含 'virtual_warehouse'，则将 Product 的 current_location_id 设为 'virtual_warehouse'。
+    - 为每一个 assignee_id（非 virtual_warehouse）新建一条 Task 记录（状态 PENDING）。
+
+    裂变逻辑：
+    - 如果 len(next_assignees) > 1：多路裂变 → 所有新任务挂到当前任务下（parent_task_id = 当前任务ID）。
+    - 如果当前任务本身就是子任务（有 parent_task_id）：单路转交也挂到同一父任务下。
+    - 否则（顶层单路转交）：新任务与当前任务同级（parent_task_id = None）。
+    - 更新 Product 的 current_location_id 为对应的人员（非仓库）。
+    """
+    task = await _get_task_or_404(db, task_id)
+
+    # 校验：不能重复完成
+    if task.status == TASK_STATUS_COMPLETED:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"任务「{task.task_name}」已经完成，无需重复操作",
+        )
+
+    # 校验：卡点逻辑 — 检查关键子任务
+    all_done, incomplete_names = await _check_all_critical_children_completed(db, task_id)
+    if not all_done:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"请等待相关子任务完成：{', '.join(incomplete_names)}",
+        )
+
+    now = datetime.now(timezone.utc)
+
+    # --- 动作 1：闭环当前节点 ---
+    task.status = TASK_STATUS_COMPLETED
+    task.completed_at = now
+
+    await _create_task_log(
+        db, task_id,
+        action_type="complete",
+        operator_id=operator_id,
+        remark=request.note or f"完成任务「{task.task_name}」，转交至下一道工序",
+    )
+
+    # --- 动作 2：解析下家 & 裂变 ---
+    has_warehouse = VIRTUAL_WAREHOUSE in request.next_assignees
+    real_assignees = [a for a in request.next_assignees if a != VIRTUAL_WAREHOUSE]
+
+    # 判断是否需要裂变（树状结构）
+    is_fission = len(real_assignees) > 1
+    is_child_task = task.parent_task_id is not None
+
+    created_tasks: list[Task] = []
+
+    for assignee_id in real_assignees:
+        # 确定新任务的 parent_task_id（裂变逻辑）
+        if is_fission:
+            # 多路裂变：所有新任务挂在当前任务下，形成树状分支
+            new_parent_task_id = task.id
+        elif is_child_task:
+            # 当前任务是子任务，单路转交也保持在同一父任务下
+            new_parent_task_id = task.parent_task_id
+        else:
+            # 顶层单路转交：同级
+            new_parent_task_id = None
+
+        new_task = Task(
+            product_id=task.product_id,
+            parent_task_id=new_parent_task_id,
+            task_name=request.next_task_name,
+            assignee_id=assignee_id,
+            status=TASK_STATUS_PENDING,
+            notify_parent_on_complete=False,
+            is_rework=False,
+        )
+        db.add(new_task)
+        created_tasks.append(new_task)
+
+    # 批量 flush 以生成 ID
+    await db.flush()
+
+    for nt in created_tasks:
+        await _create_task_log(
+            db, nt.id,
+            action_type="create",
+            operator_id=operator_id,
+            remark=f"由任务「{task.task_name}」裂变转交创建，分配给 {nt.assignee_id}",
+        )
+
+    # --- 更新 Product 的 current_location_id ---
+    product_result = await db.execute(
+        select(Product).where(Product.id == task.product_id)
+    )
+    product = product_result.scalar_one_or_none()
+    if product:
+        if has_warehouse and not real_assignees:
+            # 只有仓库，没有实际人员 → 入库
+            product.current_location_id = VIRTUAL_WAREHOUSE
+        elif real_assignees:
+            # 有实际人员 → 指向第一个人员（或可考虑指向多人中的主负责人）
+            product.current_location_id = real_assignees[0]
+
+    await db.commit()
+
+    # --- 构建响应 ---
+    refreshed_task = await _get_task_with_children_recursive(db, task_id)
+    created_task_responses = []
+    for nt in created_tasks:
+        await db.refresh(nt)
+        created_task_responses.append(_to_response(nt))
+
+    assignee_list = ", ".join(real_assignees)
+    location_info = ""
+    if has_warehouse:
+        location_info = "，产品已入库(virtual_warehouse)"
+
+    return TaskTransferResponse(
+        completed_task=_to_response(refreshed_task),
+        created_tasks=created_task_responses,
+        message=(
+            f"任务「{task.task_name}」已完成，"
+            f"已创建 {len(created_tasks)} 个下一道工序任务「{request.next_task_name}」"
+            f"（接收人: {assignee_list}）{location_info}"
+        ),
+    )
+
+
+# ============================================================
+# 保留兼容：旧版 complete_task（单步完成/转交）
+# ============================================================
+
+async def complete_task(
+    db: AsyncSession, task_id: uuid.UUID, request: TaskCompleteRequest
+) -> TaskCompleteResponse:
+    """
+    核心业务：完成任务 + 可选创建下一步任务。
+
+    逻辑：
+    1. 检查当前任务是否已完成（幂等）
+    2. 检查所有 notify_parent_on_complete=True 的子任务是否都已完成
+       → 如果存在未完成的关键子任务，返回 400 错误
+    3. 将当前任务状态改为 completed，记录日志
+    4. 如果提供了 next_task_name 和 next_assignee_id，创建下一步任务
+    5. 返回完成结果
+    """
+    task = await _get_task_or_404(db, task_id)
+
+    # 注入子任务数据到当前对象以便后续检查
+    children_result = await db.execute(
+        select(Task).where(Task.parent_task_id == task_id)
+    )
+    task.child_tasks = children_result.scalars().all()
+
+    # --- 1. 幂等检查 ---
+    if task.status == TASK_STATUS_COMPLETED:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"任务「{task.task_name}」已经完成，无需重复操作",
+        )
+
+    # --- 2. 卡点逻辑：检查关键子任务 ---
+    all_done, incomplete_names = await _check_all_critical_children_completed(db, task_id)
+    if not all_done:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"请等待相关子任务完成：{', '.join(incomplete_names)}",
+        )
+
+    now = datetime.now(timezone.utc)
+
+    # --- 3. 标记当前任务为已完成 ---
+    task.status = TASK_STATUS_COMPLETED
+    task.completed_at = now
+    await _create_task_log(
+        db, task_id,
+        action_type="complete",
+        operator_id=request.operator_id,
+        remark=request.remark or f"完成任务: {task.task_name}",
+    )
+
+    # --- 4. 可选：创建下一步任务（转交） ---
+    next_task = None
+    if request.next_task_name and request.next_assignee_id:
+        next_task = Task(
+            product_id=task.product_id,
+            parent_task_id=task.parent_task_id,  # 与已完成任务同级
+            task_name=request.next_task_name,
+            assignee_id=request.next_assignee_id,
+            status=TASK_STATUS_PENDING,
+            notify_parent_on_complete=False,
+        )
+        db.add(next_task)
+        await db.flush()
+        await _create_task_log(
+            db, next_task.id,
+            action_type="create",
+            operator_id=request.operator_id,
+            remark=f"由任务「{task.task_name}」完成后转交创建",
+        )
+
+    await db.commit()
+
+    # --- 5. 构建响应 ---
+    refreshed_task = await _get_task_with_children_recursive(db, task_id)
+    next_task_response = None
+    if next_task:
+        await db.refresh(next_task)
+        next_task_response = _to_response(next_task)
+
+    return TaskCompleteResponse(
+        completed_task=_to_response(refreshed_task),
+        next_task=next_task_response,
+        message=f"任务「{task.task_name}」已完成"
+        + (f"，已创建下一步任务「{request.next_task_name}」" if next_task else ""),
+    )
+
+
+# ============================================================
+# 子任务
+# ============================================================
+
+async def create_subtask(
+    db: AsyncSession, parent_task_id: uuid.UUID, data: SubtaskCreate
+) -> TaskResponse:
+    """
+    在现有任务下创建子任务 — 支持无限层级嵌套。
+    新子任务继承父任务的 product_id。
+    """
+    parent = await _get_task_or_404(db, parent_task_id)
+
+    # 禁止在已完成/已驳回的任务下创建子任务
+    if parent.status in (TASK_STATUS_COMPLETED, TASK_STATUS_REJECTED):
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"无法在状态为 {parent.status} 的任务「{parent.task_name}」下创建子任务",
+        )
+
+    subtask = Task(
+        product_id=parent.product_id,
+        parent_task_id=parent_task_id,
+        task_name=data.task_name,
+        assignee_id=data.assignee_id,
+        status=TASK_STATUS_PENDING,
+        notify_parent_on_complete=data.notify_parent_on_complete,
+    )
+    db.add(subtask)
+    await db.commit()
+    await db.refresh(subtask)
+
+    await _create_task_log(
+        db, subtask.id,
+        action_type="create",
+        operator_id=data.assignee_id,
+        remark=f"创建子任务「{data.task_name}」，父任务: 「{parent.task_name}」",
+    )
+    await db.commit()
+
+    return _to_response(subtask)