learn-claude-code/s20_comprehensive

s20: Comprehensive Agent — All Mechanisms, One Loop

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s01 → ... → s18 → s19 → s20

"Many mechanisms, one loop" — tools, permissions, memory, tasks, teams, and plugins all hang off the same while True.

Harness layer: Comprehensive — put the previous 19 mechanisms back into one runnable system.

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Problem

The first 19 chapters add one mechanism at a time. That is the right way to learn, but a real agent does not run with only one mechanism enabled.

A long-running coding agent needs all of these at once:

• tool dispatch and permission boundaries
• hook extension points
• todo planning and task graphs
• skills, memory, and runtime system prompt assembly
• compaction and error recovery
• background tasks and cron scheduling
• teams, protocols, autonomous claiming
• worktree isolation
• MCP external tool integration

The hard part is not piling up features. The hard part is seeing where each mechanism belongs around the loop. S20 is the endpoint chapter: every component is placed back into one harness.

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Solution

S20 does not invent a new mechanism. It merges the teaching components from the earlier chapters into one complete harness:

user input
  → UserPromptSubmit hooks
  → cron/background notification injection
  → context compact
  → memory + skills + MCP state assemble the system prompt
  → LLM
  → has tool_use block?
      no  → Stop hooks → return
      yes → PreToolUse hooks + permission
          → TOOL_HANDLERS / MCP handlers / background dispatch
          → PostToolUse hooks
          → tool_result / task_notification back to messages
          → next round

The loop is still the same structure: call the model, check whether the response contains a tool_use block, execute tools, append results back to messages. CC source does not directly trust stop_reason == "tool_use"; the actual presence of a tool_use block is the continuation signal. What changed is that the harness around the loop is now complete.

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Where Each Component Sits

Position	Component	Role
Around user input	UserPromptSubmit hooks	Log, inject, or audit user input
Before LLM	cron queue	Inject scheduled prompts into messages
Before LLM	background notifications	Inject completed background work as <task_notification>
Before LLM	compaction pipeline	Budget large outputs, trim history, compact old tool results, summarize when needed
Before LLM	memory / skills / MCP state	Assemble the system prompt so the model sees current capabilities and long-term context
LLM call	error recovery	Retry 429/529, escalate max_tokens, compact on prompt-too-long
Before tool execution	PreToolUse hooks + permission	Block dangerous commands, out-of-bounds writes, destructive MCP tools
Tool dispatch	assemble_tool_pool	Assemble built-in tools and dynamic MCP tools
During tool execution	background dispatch	Move slow bash work into a daemon thread and return a placeholder result
After tool execution	PostToolUse hooks	Large-output warnings, logs, post-processing
Back to loop	tool_result	One tool_result per tool_use, then the next model round
No tool_use this round / on stop	Stop hooks	Stats, cleanup, audit

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What code.py Contains

Tools and Dispatch

The built-in tool pool contains 27 tools:

bash, read_file, write_file, edit_file, glob
todo_write, task, load_skill, compact
create_task, list_tasks, get_task, claim_task, complete_task
schedule_cron, list_crons, cancel_cron
spawn_teammate, send_message, check_inbox
request_shutdown, request_plan, review_plan
create_worktree, remove_worktree, keep_worktree
connect_mcp

assemble_tool_pool() assembles these every round:

BUILTIN_TOOLS + connected MCP tools
BUILTIN_HANDLERS + mcp__server__tool handlers

After connect_mcp("docs"), the next round exposes tools like mcp__docs__search.

Permissions and Hooks

Permission is not hardcoded into the tool execution line. It is a PreToolUse hook:

blocked = trigger_hooks("PreToolUse", block)
if blocked:
    results.append(tool_result(block.id, blocked))
    continue

That means permission, logging, and audit logic all attach to the same hook point. After execution, PostToolUse hooks run.

Planning and Tasks

S20 keeps two planning layers:

• todo_write: lightweight plan for the current session, written to .tasks/current_todos.json
• task graph: cross-session, dependency-aware, claimable task files under .tasks/task_*.json

The first keeps a single agent from drifting. The second supports team coordination.

Subagents and Teams

S20 has two kinds of delegation:

• task: one-shot subagent. It uses an isolated messages[], discards intermediate context, and returns only a final summary.
• spawn_teammate: persistent teammate thread. It communicates through MessageBus, polls the task board while idle, and can claim work autonomously.

One-shot subagents solve context isolation. Persistent teammates solve long-running parallel collaboration.

Memory, Skills, and Prompt

assemble_system_prompt(context) assembles each round from:

• identity and tool guidance
• workspace
• skills catalog
• .memory/MEMORY.md
• connected MCP servers

Skills only put their catalog into the system prompt. Full content is loaded on demand through load_skill(name).

Compaction and Recovery

Before the LLM call, S20 runs the compaction pipeline:

tool_result_budget → snip_compact → micro_compact → compact_history

The model call is wrapped with recovery:

• 429: exponential backoff retry
• 529: exponential backoff, optionally switch to fallback model after repeated failures
• max_tokens: raise max tokens, then request continuation
• prompt too long: reactive compact and retry

Background and Cron

Slow bash work does not block the main loop:

should_run_background → start_background_task → placeholder tool_result
background done → task_notification → next round injects messages

The cron scheduler runs as a daemon thread and checks once per second. The CLI watches cron_queue; when a job fires, it injects [Scheduled] ... and runs one agent turn automatically.

Worktree and MCP

Worktree isolation owns directories:

• create_worktree(name, task_id) creates an isolated branch and directory
• the task worktree field binds a task to that directory
• when a teammate claims a task with a worktree, its bash/read/write tools run in that directory

MCP owns external capability:

• connect_mcp(name) connects a mock server
• assemble_tool_pool() assembles MCP tools into the tool pool
• tool names use mcp__server__tool

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Changes from s19

Component	s19	s20
tool pool	built-in + MCP	built-in + MCP, with s01-s18 tools restored
permission	omitted in teaching body	runs inside PreToolUse hook
hooks	omitted	UserPromptSubmit / PreToolUse / PostToolUse / Stop
todo	omitted	todo_write + reminder
skill	omitted	catalog in system prompt + load_skill
compact	omitted	pre-LLM compaction + compact tool + reactive compact
error recovery	simple try/except	retry / max_tokens / prompt too long
background	omitted	slow-operation thread + task notification
cron	omitted	daemon scheduler + durable jobs
multi-agent	kept	kept; teammates use basic tools in isolated directories
worktree	kept	kept
MCP	new	kept as part of the final tool pool

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Try It

cd learn-claude-code
python s20_comprehensive/code.py

Try:

1. Create a todo list for inspecting this repo, then list Python files
2. Connect to the docs MCP server and search for agent loop
3. Create two tasks, create worktrees for them, then spawn alice and bob. Ask them to submit plans before claiming tasks.
4. remind me of the meeting in 3 minutes.
5. Run npm install in the background and continue reading README.md

Watch for:

• whether each tool call passes through hooks/permission
• whether MCP tools appear on the next round after connect_mcp
• whether slow operations return a background placeholder
• whether cron automatically reminds you when the time arrives
• whether teammates submit plans and pause before approval
• whether teammates can claim tasks after plan approval
• whether teammates switch to the bound worktree directory

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The End Is the Beginning

From s01 to s20, the code gets more capable, but the core remains unchanged:

while True:
    response = LLM(messages, tools)
    if not has_tool_use(response.content):
        return
    results = execute_tools(response.content)
    messages.append(tool_results)

Claude Code's complexity is not "another agent brain." It is the complexity of a mature harness. The model decides and chooses actions; the harness organizes environment, tools, permissions, memory, teams, and external capabilities.

This is the endpoint of the course: many mechanisms, one loop.