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. 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, listsessions, delete, listsubkeys) 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. 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. 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. 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. Two purposes documented by…
The Harness as a Library — Subagents and Session Store: A harness you can import: built-in tools, subagents for context isolation, hooks, W3C trace…
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