The Harness as a Library — Subagents and Session Store

> A harness you can import: built-in tools, subagents for context isolation, hooks, W3C trace propagation, session persistence. The Claude Agent SDK is the reference example — the library form of the Claude Code harness — and Claude Managed Agents is the hosted alternative for long-running async work.

Type: Learn + Build
Languages: Python (stdlib)
Prerequisites: Phase 14 · 01 (Agent Loop), Phase 14 · 10 (Skill Libraries)
Time: ~75 minutes

Learning Objectives

• Explain the difference between the Anthropic Client SDK (raw API) and the Claude Agent SDK (harness shape).
• Describe subagents — parallelization and context isolation — and when to reach for them.
• Name the Python SDK’s session store surface (append, load, list_sessions, delete, list_subkeys) and the role of --session-mirror.
• Implement a stdlib harness with built-in tools, subagent spawning with isolated context, lifecycle hooks, and a session store.

The Problem

A raw LLM API gets you one round-trip. A production agent needs tool execution, MCP servers, lifecycle hooks, subagent spawning, session persistence, trace propagation. Claude Agent SDK ships this shape as a library — the same harness Claude Code uses, exposed for custom agents.

The Concept

Client SDK vs Agent SDK

• Client SDK (anthropic). Raw Messages API. You own the loop, the tools, the state.
• Agent SDK (claude-agent-sdk). Built-in tool execution, MCP connections, hooks, subagent spawning, session store. The Claude Code loop as a library.

Built-in tools

The SDK ships 10+ tools out of the box: file read/write, shell, grep, glob, web fetch, more. Custom tools register via the standard tool-schema interface.

Subagents

Two purposes documented by Anthropic:

1. Parallelization. Run independent work concurrently. “Find the test file for each of these 20 modules” is 20 parallel subagent tasks.
2. Context isolation. Subagents use their own context window; only results return to the orchestrator. The orchestrator’s budget is preserved.

Python SDK recent additions: list_subagents(), get_subagent_messages() for reading subagent transcripts.

Session store

Protocol parity with TypeScript:

• append(session_id, message) — add a turn.
• load(session_id) — restore conversation.
• list_sessions() — enumerate.
• delete(session_id) — with cascade to subagent sessions.
• list_subkeys(session_id) — list subagent keys.

--session-mirror (CLI flag) mirrors the transcript to an external file as it streams, for debugging.

Hooks

Lifecycle hooks you can register:

• PreToolUse, PostToolUse — gate or audit tool calls.
• SessionStart, SessionEnd — set up and tear down.
• UserPromptSubmit — act on user input before the model sees it.
• PreCompact — run before context compaction.
• Stop — cleanup on agent exit.
• Notification — side-channel alerts.

Hooks are how pro-workflow (Phase 14 curriculum reference) and similar systems add cross-cutting behavior.

W3C trace context

OTel spans active on the caller propagate into the CLI subprocess via W3C trace context headers. The whole multi-process trace shows up as one trace in your backend.

Claude Managed Agents

The hosted alternative (beta header managed-agents-2026-04-01). Long-running async work, built-in prompt caching, built-in compaction. Trade control for managed infrastructure.

Where this pattern goes wrong

• Subagent over-spawn. Spawning 100 subagents for 100 tiny tasks. Overhead dominates. Batch instead.
• Hook creep. Every team adds hooks; startup time balloons. Review hooks quarterly.
• Session bloat. Sessions accumulate; size grows. Use list_sessions + expiry policy.

ae-subagent-isolation

Build It

code/main.py implements the SDK shape in stdlib:

• Tool, ToolRegistry with built-in read_file, write_file, list_dir.
• Subagent — private context, isolated run, results returned.
• SessionStore — append, load, list, delete, list_subkeys.
• Hooks — pre_tool_use, post_tool_use, session_start, session_end.
• A demo: main agent spawns 3 subagents in parallel (each isolated), aggregates results, persists session.

Run it:

python3 code/main.py

The trace shows subagent context isolation (orchestrator context size stays bounded), hook execution, and session persistence.

Use It

• Claude Agent SDK for Claude-first products that want the Claude Code harness shape.
• Claude Managed Agents for hosted long-running async work.
• OpenAI Agents SDK (Lesson 16) for OpenAI-first counterparts.
• LangGraph + custom tools if you want the graph-shaped state machine instead.

Ship It

outputs/skill-claude-agent-scaffold.md scaffolds a Claude Agent SDK app with subagents, hooks, session store, MCP server attachment, and W3C trace propagation.

Exercises

1. Add a subagent spawner that batches 20 tasks into groups of 5 parallel subagents. Measure orchestrator context size vs one-per-task.
2. Implement a PreToolUse hook that rate-limits write_file calls (5 per minute per session). Trace the behavior.
3. Wire list_subkeys to render a subagent tree. What does deep nesting look like?
4. Port the toy to the real claude-agent-sdk Python package. What changes about tool registration?
5. Read the Claude Managed Agents docs. When would you switch from self-hosted to managed?

Key Terms

| Term | What people say | What it actually means |
|——|—————-|————————|
| Agent SDK | “Claude Code as a library” | Harness shape: tools, MCP, hooks, subagents, session store |
| Subagent | “Child agent” | Separate context, own budget; results bubble up |
| Session store | “Conversation DB” | Persist, load, list, delete turns with subagent cascade |
| Hook | “Lifecycle callback” | Pre/post tool, session, prompt submit, compact, stop |
| W3C trace context | “Cross-process trace” | Parent span propagates into CLI subprocess |
| Managed Agents | “Hosted harness” | Anthropic-hosted long-running async work |
| --session-mirror | “Transcript mirror” | Writes session turns to an external file as they stream |
| MCP server | “Tool surface” | External tool/resource source attached to the agent |

Further Reading

• Claude Agent SDK overview — the library form of Claude Code
• Anthropic, Building agents with the Claude Agent SDK — production patterns
• Claude Managed Agents overview — hosted alternative
• OpenAI Agents SDK — counterpart

Course content adapted from AI Engineering from Scratch by Rohit Ghumare (MIT License)