Building Agents: Client SDK vs Agent SDK vs Managed Agents.

Anthropic exposes three distinct layers for building a Claude-powered agent, and this objective tests picking the right one for a given deployment, not memorizing one API. The Client SDK / Messages API is direct model access: you write the while response.stop_reason == "tool_use" loop yourself and execute every tool call in your own code. The Claude Agent SDK is "the same tools, agent loop, and context management that power Claude Code, programmable in Python and TypeScript" - Claude autonomously drives tool execution inside a process you still own and deploy. Claude Managed Agents is a beta, hosted REST API where Anthropic runs both the agent loop and the sandbox; your application only sends events and streams back results over SSE.

These layers share primitives - a model call, tool definitions, optional MCP servers - but differ in who owns the loop and where state lives. The Agent SDK's session state is JSONL on your own filesystem; Managed Agents' session state is an Anthropic-hosted event log, persisted server-side and fetchable in full. That distinction drives almost every other tradeoff on this page: what you can self-host, whether hooks apply, and whether the deployment can meet Zero Data Retention.

Client SDK vs Agent SDK is a one-line contrast the docs make explicit. With the Client SDK: response = client.messages.create(...); while response.stop_reason == "tool_use": result = your_tool_executor(...); response = client.messages.create(tool_result=result, ...) - you own every iteration. With the Agent SDK: async for message in query(prompt="Fix the bug in auth.py", options=ClaudeAgentOptions(allowed_tools=["Read","Edit","Bash"])): print(message) - Claude reads the file, finds the bug, and edits it without you writing a dispatch loop. Built-in tools ship with zero tool-execution code: Read, Write, Edit, Bash, Monitor (watch a background script and react to each output line), Glob, Grep, WebSearch, WebFetch, and AskUserQuestion.

Subagents, MCP, permissions, and sessions are Agent SDK constructs configured through `ClaudeAgentOptions`. Subagents are declared with AgentDefinition(description=..., prompt=..., tools=[...]) inside an agents={} dict, and must be invoked via the Agent tool - so "Agent" has to appear in allowed_tools or the subagent never fires; messages emitted inside a subagent's context carry a parent_tool_use_id for tracing. MCP servers wire in as mcp_servers={"playwright": {"command": "npx", "args": ["@playwright/mcp@latest"]}} - external tool servers become callable inside the same loop with no custom glue. allowed_tools pre-approves specific tools; permission_mode="acceptEdits" is another supported mode for auto-approving file edits. Sessions resume via options=ClaudeAgentOptions(resume=session_id), where session_id is captured from the first query's SystemMessage with subtype == "init" - full prior context carries forward without you re-sending history.

Hooks are deterministic actions independent of model compliance, and they run through two structurally different mechanisms that share event names but not a wire contract - conflating them is the single most common mistake on this topic. Lifecycle events span once-per-session (SessionStart, SessionEnd), once-per-turn (UserPromptSubmit, Stop), per-tool-call (PreToolUse, PostToolUse, PostToolUseFailure, PermissionRequest, plus more), subagent (SubagentStart, SubagentStop), and compaction (PreCompact, PostCompact) categories, but not every event is registerable both ways. Claude Agent SDK hooks are in-process Python/TypeScript callbacks registered via HookMatcher(matcher="Edit|Write", hooks=[callback]) inside ClaudeAgentOptions/options.hooks. Each callback receives (input_data, tool_use_id, context) and returns a structured dict directly to the SDK - there is no process boundary, no exit code, and no stdout parsing. Top-level fields (systemMessage, continue) apply to every event; a PreToolUse callback blocks or allows the call by setting hookSpecificOutput.permissionDecision to "allow", "deny", or "ask" (the TypeScript SDK also supports "defer"; the Python SDK is allow/deny/ask), and returning {} allows the operation unchanged. Claude Code command hooks are a separate mechanism: shell commands configured in .claude/settings.json that Claude Code launches as external processes and that report back over stdout plus the process exit code - exit 0 means stdout is parsed as JSON, exit 2 is the blocking signal (stderr is fed back to Claude as an error message and stdout is ignored), and any other exit code is a non-blocking error. StopFailure is a documented Claude Code hook event (fires when a turn ends on an API error) but is not one of the Agent SDK's in-process callback events - it isn't something an SDK HookMatcher callback receives, so don't cite it as SDK-native. Never blend the two: an in-process SDK callback never has an "exit code," and a Claude Code shell command hook never returns a Python dict. For the day-to-day PreToolUse/PostToolUse enforcement pattern (deterministic gate vs. audit trail), see the dedicated hooks page - see Study Next.

Managed Agents is a different runtime with its own core concepts and its own governance mechanism, not hooks. Its four building blocks are Agent (model + system prompt + tools + MCP servers + skills), Environment (an Anthropic-managed cloud sandbox, or a self-hosted sandbox on your own infrastructure), Session (a running agent instance), and Events (the messages exchanged between your app and the agent). Self-hosted sandboxes move tool execution onto infrastructure you control, but orchestration stays on Anthropic's side under a shared-responsibility model - Anthropic still secures the control plane: session/queue integrity and multi-tenant isolation. All Managed Agents calls require the managed-agents-2026-04-01 beta header, and because sessions are stateful by design (server-persisted conversation history, sandbox state, and outputs), Managed Agents is not currently eligible for Zero Data Retention or HIPAA BAA coverage - a hard constraint for regulated workloads, distinct from anything about hooks or tool access. Per-tool governance on a Managed Agents session runs through always_allow/always_ask policies set at agent-configuration time, not PreToolUse callbacks - see the auth and access control page for that distinction in depth.

Migration is a named, expected path, not an edge case. "A common path is to prototype with the Agent SDK locally, then move to Managed Agents for production" once a workload needs to run unattended for hours with no process babysitting it, or needs Anthropic to own sandbox/session persistence. The same tools and MCP servers typically carry over; what changes is who runs the loop and where the state lives.