# s12: Task System — Break Big Goals into Small Tasks [English](README.md) · [中文](README.zh.md) · [日本語](README.ja.md) s01 → ... → s10 → s11 → `s12` → [s13](../s13_background_tasks/) → s14 → ... → s18 → s19 > *"Break big goals into small tasks, order them, persist"* — File-persisted task graph, the foundation for multi-agent collaboration. > > **Harness Layer**: Tasks — Persisted goals, recoverable progress. --- ## The Problem The agent receives a project: set up a database, write APIs, add tests. It uses s05's TodoWrite to create a checklist, then starts writing the API first, gets halfway through and realizes there are no database tables, goes back to fix them; when adding tests, discovers the API interface signatures have changed again... You can't build the roof before laying the foundation. Tasks have ordering. Task prerequisites can be represented as a Directed Acyclic Graph (DAG); this chapter records them with `blockedBy`. s05's TodoWrite is an execution checklist for the current task, kept in session memory. What you need here is a **task system**: each task is a JSON file, tasks have `blockedBy` dependencies, and they persist across sessions on disk. --- ## The Solution ![Task System Overview](images/task-system-overview.en.svg) This chapter adds 5 task tools, persistence in the `.tasks/` directory, and `blockedBy` dependency checks. TodoWrite vs Task System: | | TodoWrite (s05) | Task System (s12) | |---|---|---| | Role | Execution checklist for the current task | Recoverable task system | | Storage | In-process / session state | `.tasks/{id}.json` | | Dependencies | None | `blockedBy` / `blocks` graph | | Lifecycle | Current session / current task | Cross-session | | Coordination | No task claiming | `owner` / claim | | Status | pending / in_progress / completed | pending / in_progress / completed | | Granularity | The agent's own steps | Tasks that can be claimed, tracked, and unblocked | | Update contract | Replace the whole checklist | Create/get/update/list individual records | --- ## How It Works ![Task DAG](images/task-dag.en.svg) ### Task: Data Structure Each task is a JSON file, stored in the `.tasks/` directory: ```python @dataclass class Task: id: str subject: str description: str status: str # pending | in_progress | completed owner: str | None # Agent name (multi-agent scenarios) blockedBy: list[str] # List of dependency task IDs ``` IDs are generated with `timestamp + random hex`. ### create_task: Create Tasks ```python def create_task(subject: str, description: str = "", blockedBy: list[str] | None = None) -> Task: task = Task( id=f"task_{int(time.time())}_{random_hex(4)}", subject=subject, description=description, status="pending", owner=None, blockedBy=blockedBy or [], ) save_task(task) return task ``` Automatically calls `save_task` on creation to write `.tasks/{id}.json`. `blockedBy` declares dependencies, for example "write API" has `blockedBy: ["task_schema"]`. ### can_start: Dependency Check A task can only start after all its `blockedBy` dependencies are **completed**: ```python def can_start(task_id: str) -> bool: task = load_task(task_id) for dep_id in task.blockedBy: if not _task_path(dep_id).exists(): return False # missing dependency = blocked dep = load_task(dep_id) if dep.status != "completed": return False return True ``` `can_start` is a prerequisite check for `claim_task`: if any `blockedBy` dependency is not completed, the task cannot be claimed. Missing dependencies are treated as blocked, avoiding crashes from referencing wrong IDs. ### claim_task: Claim a Task When the agent starts working on a task, it calls `claim_task`: sets `owner`, changes status from `pending` → `in_progress`. The `owner` field records who is working on the task, preventing duplicate claims in multi-agent scenarios: ```python def claim_task(task_id: str, owner: str = "agent") -> str: task = load_task(task_id) if task.status != "pending": return f"Task {task_id} is {task.status}, cannot claim" if not can_start(task_id): deps = [d for d in task.blockedBy if load_task(d).status != "completed"] return f"Blocked by: {deps}" task.owner = owner task.status = "in_progress" save_task(task) return f"Claimed {task_id} ({task.subject})" ``` If the task is already claimed by someone else (`status != "pending"`), or dependencies aren't met (`can_start` returns False), the claim is rejected. ### complete_task: Complete and Unblock When a task is done, set it to `completed`. Simultaneously scan all other tasks to find downstream tasks that were **just unblocked**: ```python def complete_task(task_id: str) -> str: task = load_task(task_id) task.status = "completed" save_task(task) # Find newly unblocked downstream tasks unblocked = [t.subject for t in list_tasks() if t.status == "pending" and t.blockedBy and can_start(t.id)] msg = f"Completed {task_id} ({task.subject})" if unblocked: msg += f"\nUnblocked: {', '.join(unblocked)}" return msg ``` After completing "schema", `can_start` returns True for "endpoints" and "docs"; they can begin. ### get_task: View Full Details `list_tasks` only shows a one-line summary. `get_task` returns the full task JSON, including description and dependency details. When recovering across sessions, the agent needs to read the full description to continue work: ```python def get_task(task_id: str) -> str: task = load_task(task_id) return json.dumps(asdict(task), indent=2) ``` ### State Machine: Two Actions, Three States ``` pending ──claim──→ in_progress ──complete──→ completed ``` Here `claim` / `complete` are actions, while `pending` / `in_progress` / `completed` are states: - **claim_task**: `pending` → `in_progress`. Sets owner, begins work. - **complete_task**: `in_progress` → `completed`. Marks the task done and unblocks downstream. ### Putting It Together ```python # Create tasks with dependencies schema = create_task("setup database schema") endpoints = create_task("create API endpoints", blockedBy=[schema.id]) tests = create_task("write tests", blockedBy=[endpoints.id]) docs = create_task("write docs", blockedBy=[schema.id]) # Agent claims the first available task claim_task(schema.id) # ✓ Claimed (no dependencies) complete_task(schema.id) # ✓ Completed → unblocks endpoints, docs claim_task(endpoints.id) # ✓ Claimed (schema completed) complete_task(endpoints.id) # ✓ Completed → unblocks tests claim_task(docs.id) # ✓ Claimed (schema completed) complete_task(docs.id) # ✓ Completed claim_task(tests.id) # ✓ Claimed (endpoints completed) complete_task(tests.id) # ✓ Completed ``` Each `create_task` writes a JSON file, each `claim_task` / `complete_task` updates the file. Across sessions, the `.tasks/` directory persists — the agent reads the files to recover progress. --- ## Changes from s11 | Component | Before (s11) | After (s12) | |-----------|-------------|-------------| | Task management | None | Task dataclass + 5 tools | | New types | — | Task (id, subject, description, status, owner, blockedBy) | | Storage | No persistence | `.tasks/{id}.json` cross-session | | Dependencies | None | `blockedBy` graph + `can_start` check | | Tools | bash, read_file, write_file (3) | + create_task, list_tasks, get_task, claim_task, complete_task (8) | | Lifecycle | — | pending → in_progress → completed (no release rollback) | --- ## Try It ```sh cd learn-claude-code python s12_task_system/code.py ``` Try these prompts: 1. `Create tasks: setup database schema, create API endpoints (depends on schema), write tests (depends on endpoints), write docs (depends on schema)` 2. `List all tasks and their statuses` 3. `Claim the first unblocked task and complete it` 4. `List tasks again — which ones are now unblocked?` What to observe: Are JSON files generated in the `.tasks/` directory? After completing a task, are the blocked tasks unblocked? --- ## What's Next The task graph is in place. But some tasks take a long time — like running full test suites or deploying to a server. The agent calls the LLM billed by token, it can't afford to wait on a slow operation. s13 Background Tasks → Slow operations go to the background. The agent continues processing other tasks, and gets notified when the background work is done.