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217 lines
7.9 KiB
Markdown
217 lines
7.9 KiB
Markdown
# s13: Background Tasks — 慢操作放后台
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[English](README.md) · [中文](README.zh.md) · [日本語](README.ja.md)
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s01 → ... → s11 → s12 → `s13` → [s14](../s14_cron_scheduler/) → s15 → ... → s20 → s21
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> *"慢操作丢后台, agent 继续处理"* — 后台线程跑命令, 完成后注入通知。
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>
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> **Harness 层**: 后台 — 异步执行, 不阻塞主循环。
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---
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## 问题
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你用过洗衣机吗？把衣服扔进去，按下启动，然后去做饭、回消息或看论文。30 分钟后洗衣机"滴滴滴"提醒你：好了。你不会站在洗衣机前面干等 30 分钟。
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Agent 的 bash 工具也一样。`pip install torch` 要 10 分钟，`npm run build` 要 3 分钟。这些命令一跑，Agent 就在等 bash 工具返回，没法利用这段时间处理别的任务。
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读文件是毫秒级，不等。`git status` 一秒内返回，不等。但 `npm install`？分钟级。Agent 等 10 分钟什么都不做，而 LLM 按 token 计费，空转就是浪费。
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---
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## 解决方案
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本章把慢操作放入后台线程，Agent 继续运行循环；任务完成后，结果以通知形式注入对话。
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同步 vs 后台：
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| | 同步 (s12) | 后台 (s13) |
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|---|---|---|
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| 慢操作 | Agent 干等 | 后台线程执行 |
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| Agent 空闲 | 是 | 否，继续处理 |
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| 结果 | 立即返回 | 下轮注入通知 |
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| 判断标准 | — | `run_in_background` 参数（模型显式请求），启发式兜底 |
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---
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## 工作原理
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### should_run_background: 显式请求优先，启发式兜底
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模型通过 bash 工具的 `run_in_background` 参数显式请求后台执行。如果模型没有指定，则使用关键词启发式判断：
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```python
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def is_slow_operation(tool_name: str, tool_input: dict) -> bool:
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"""Fallback heuristic: commands likely to take > 30s."""
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if tool_name != "bash":
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return False
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cmd = tool_input.get("command", "").lower()
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slow_keywords = ["install", "build", "test", "deploy", "compile",
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"docker build", "pip install", "npm install",
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"cargo build", "pytest", "make"]
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return any(kw in cmd for kw in slow_keywords)
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def should_run_background(tool_name: str, tool_input: dict) -> bool:
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"""Model explicit request takes priority; fallback to heuristic."""
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if tool_input.get("run_in_background"):
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return True
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return is_slow_operation(tool_name, tool_input)
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```
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### start_background_task: 后台执行与生命周期
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把工具调用包装成 worker 函数，扔到 daemon 线程里执行。每个后台任务有唯一 ID，状态存在 `background_tasks` 字典里：
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```python
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_bg_counter = 0
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background_tasks: dict[str, dict] = {} # bg_id → {tool_use_id, command, status}
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background_results: dict[str, str] = {} # bg_id → output
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background_lock = threading.Lock()
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def start_background_task(block) -> str:
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"""Run tool in a daemon thread. Returns background task ID."""
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global _bg_counter
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_bg_counter += 1
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bg_id = f"bg_{_bg_counter:04d}"
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def worker():
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result = execute_tool(block)
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with background_lock:
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background_tasks[bg_id]["status"] = "completed"
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background_results[bg_id] = result
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with background_lock:
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background_tasks[bg_id] = {
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"tool_use_id": block.id,
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"command": block.input.get("command", ""),
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"status": "running",
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}
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thread = threading.Thread(target=worker, daemon=True)
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thread.start()
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return bg_id
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```
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`start_background_task()` 返回 `bg_id`。`daemon=True` 确保 Agent 进程退出时线程一起退出。
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### collect_background_results: 通知收集
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后台任务完成后，收集结果并格式化为 `<task_notification>` 通知：
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```python
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def collect_background_results() -> list[str]:
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"""Collect completed results as task_notification messages."""
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with background_lock:
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ready_ids = [bid for bid, task in background_tasks.items()
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if task["status"] == "completed"]
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notifications = []
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for bg_id in ready_ids:
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with background_lock:
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task = background_tasks.pop(bg_id)
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output = background_results.pop(bg_id, "")
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notifications.append(
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f"<task_notification>\n"
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f" <task_id>{bg_id}</task_id>\n"
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f" <status>completed</status>\n"
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f" <command>{task['command']}</command>\n"
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f" <summary>{output[:200]}</summary>\n"
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f"</task_notification>")
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return notifications
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```
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通知不复用原始 `tool_use_id`。原始 tool call 已经用占位 `tool_result` 回复了，后台完成是独立事件，用 `task_notification` 格式注入。这符合 Messages API 的工具配对语义：一个 `tool_use` 只对应一个 `tool_result`。
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### 循环中的集成
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agent_loop 里，工具执行分两条路，通知和结果合并为一条 user 消息：
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```python
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results = []
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for block in response.content:
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if block.type != "tool_use":
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continue
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if should_run_background(block.name, block.input):
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bg_id = start_background_task(block)
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results.append({"type": "tool_result",
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"tool_use_id": block.id,
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"content": f"[Background task {bg_id} started] "
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f"Result will be available when complete."})
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else:
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output = execute_tool(block)
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results.append({"type": "tool_result",
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"tool_use_id": block.id, "content": output})
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# 通知和工具结果合入同一条 user 消息
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user_content = []
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bg_notifications = collect_background_results()
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if bg_notifications:
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for notif in bg_notifications:
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user_content.append({"type": "text", "text": notif})
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user_content.extend(results)
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messages.append({"role": "user", "content": user_content})
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```
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慢操作先回一个带 `bg_id` 的占位 tool_result，LLM 知道这个命令还在跑，可以先做别的事。后台完成后，通知作为独立 text block 和当前轮的 tool_result 一起组成 user 消息。
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### 合起来跑
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```
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Turn 1:
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LLM → bash "npm install" (run_in_background=true)
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→ start_background_task → bg_0001
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→ tool_result: "[Background task bg_0001 started]..."
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→ LLM: "OK, I'll check later. Let me also read the config."
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Turn 2:
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LLM → read_file "package.json" (fast, sync)
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→ tool_result: file content
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→ collect: bg_0001 done! inject <task_notification>
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→ LLM sees: config file + install notification in one message
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```
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Agent 没干等，npm install 跑后台的时候，它去读了配置文件。
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---
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## 相对 s12 的变更
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| 组件 | 之前 (s12) | 之后 (s13) |
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|------|-----------|-----------|
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| 执行模型 | 全部同步 | 慢操作后台线程 + 通知注入 |
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| bash schema | `command` | `command` + `run_in_background` |
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| 新函数 | — | `should_run_background`, `is_slow_operation`, `start_background_task`, `collect_background_results` |
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| 新类型 | — | `background_tasks: dict`, `background_results: dict`, `background_lock: Lock` |
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| 通知格式 | — | `<task_notification>`（不复用 tool_use_id） |
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| 循环行为 | 工具串行执行 | 慢操作异步，快操作同步，通知每轮收集 |
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| 工具 | 8 (s12) | 8（不变，执行策略变了） |
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---
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## 试一下
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```sh
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cd learn-claude-code
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python s13_background_tasks/code.py
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```
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试试这些 prompt：
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1. `Run pip list in the background and find all Python files in this directory`
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2. `Run npm install (use run_in_background) and while waiting, read package.json`
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3. `Create a task to setup the project, then run pip list in the background`
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观察重点：慢操作有没有被送到后台？`bg_id` 是否返回？后台通知有没有以 `<task_notification>` 格式注入？
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---
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## 接下来
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后台任务解决了"慢操作不阻塞"。但如果想定时做某件事呢？比如"每天早上 9 点跑测试"、"每 5 分钟检查一次服务器状态"。
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s14 Cron Scheduler → 给 Agent 装一个闹钟。
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<!-- translation-sync: zh@v1, en@v1, ja@v1 -->
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