Managed Agents — Multiagent Sessions

A coordinator agent can delegate to other agents within one session. All agents share the container and filesystem; each runs in its own thread — a context-isolated event stream with its own conversation history, model, system prompt, tools, MCP servers, and skills (from that agent's own config). Threads are persistent: the coordinator can send a follow-up to a subagent it called earlier and that subagent retains its prior turns.

The SDK sets the managed-agents-2026-04-01 beta header automatically on all client.beta.{agents,sessions}.* calls; no additional header is required for multiagent.

Declare the roster on the coordinator

multiagent is a top-level field on agents.create() / agents.update() — not a tools[] entry. agents lists 1–20 roster entries. Nothing changes on sessions.create() — the roster is resolved from the coordinator's config.

orchestrator = client.beta.agents.create(
    name="Engineering Lead",
    model="{{OPUS_ID}}",
    system="You coordinate engineering work. Delegate code review to the reviewer and test writing to the test agent.",
    tools=[{"type": "agent_toolset_20260401"}],
    multiagent={
        "type": "coordinator",
        "agents": [
            reviewer.id,                                            # bare string — latest version
            {"type": "agent", "id": test_writer.id, "version": 4},  # pinned version
            {"type": "self"},                                       # the coordinator itself
        ],
    },
)

session = client.beta.sessions.create(agent=orchestrator.id, environment_id=env.id)

Roster entry	Shape	Notes
String shorthand	`"agent_abc123"`	References the latest version of a stored agent.
Agent reference	`{type: "agent", id, version?}`	Omit `version` to pin the latest at coordinator save time.
Self	`{type: "self"}`	The coordinator can spawn copies of itself.

Up to 20 unique agents in the roster; the coordinator may spawn multiple copies of each. One level of delegation only — depth > 1 is ignored.

Threads

The session-level event stream is the primary thread — it shows the coordinator's trace plus a condensed view of subagent activity (thread status transitions and cross-thread messages, not every subagent tool call). Drill into a specific subagent via the per-thread endpoints:

Operation	HTTP	SDK (`client.beta.sessions.threads.*`)
List threads	`GET /v1/sessions/{sid}/threads`	`.list(session_id)`
Retrieve one	`GET /v1/sessions/{sid}/threads/{tid}`	`.retrieve(thread_id, session_id=...)`
Archive	`POST /v1/sessions/{sid}/threads/{tid}/archive`	`.archive(thread_id, session_id=...)`
List thread events	`GET /v1/sessions/{sid}/threads/{tid}/events`	`.events.list(thread_id, session_id=...)`
Stream thread events	`GET /v1/sessions/{sid}/threads/{tid}/stream`	`.events.stream(thread_id, session_id=...)`

Each SessionThread carries id, status (running | idle | rescheduling | terminated), agent (a resolved snapshot of the agent config — id, name, model, system, tools, skills, mcp_servers, version), parent_thread_id (null for the primary thread, which is included in the list), archived_at, and optional stats/usage. Session status aggregates thread statuses — if any thread is running, session.status is running. Max 25 concurrent threads. When draining a per-thread stream, break on session.thread_status_idle (and check its stop_reason as you would for the session-level idle).

Multiagent events (on the session stream)

Event	Payload highlights	Meaning
`session.thread_created`	`session_thread_id`, `agent_name`	A new thread was created.
`session.thread_status_running`	`session_thread_id`, `agent_name`	Thread started activity.
`session.thread_status_idle`	`session_thread_id`, `agent_name`, `stop_reason`	Thread is awaiting input. Inspect `stop_reason` (same shape as `session.status_idle.stop_reason`).
`session.thread_status_rescheduled`	`session_thread_id`, `agent_name`	Thread is rescheduling after a retryable error.
`session.thread_status_terminated`	`session_thread_id`, `agent_name`	Thread was archived or hit a terminal error.
`agent.thread_message_sent`	`to_session_thread_id`, `to_agent_name`, `content`	Coordinator sent a follow-up to another thread.
`agent.thread_message_received`	`from_session_thread_id`, `from_agent_name`, `content`	An agent delivered its result to the coordinator.

Tool permissions and custom tools from subagent threads

When a subagent needs your client (an always_ask confirmation, or a custom tool result), the request is cross-posted to the primary thread with session_thread_id identifying the originating thread — so you only need to watch the session stream. Reply with user.tool_confirmation (carrying tool_use_id) or user.custom_tool_result (carrying custom_tool_use_id), and echo the session_thread_id from the originating event (the SDK param type and docstring expect it). The server also routes by the tool-use ID, so the echo is belt-and-suspenders rather than load-bearing — but include it.

for event_id in stop.event_ids:
    pending = events_by_id[event_id]
    confirmation = {
        "type": "user.tool_confirmation",
        "tool_use_id": event_id,
        "result": "allow",
    }
    if pending.session_thread_id is not None:
        confirmation["session_thread_id"] = pending.session_thread_id
    client.beta.sessions.events.send(session.id, events=[confirmation])

The same pattern applies to user.custom_tool_result.

Pitfalls

Don't put the roster on sessions.create() or in tools[]. multiagent is a top-level agent field; update the coordinator, then start a session that references it.
Don't assume shared context. Threads share the filesystem but not conversation history or tools. If the coordinator needs a subagent to act on something, it must say so in the delegated message (or write it to disk).
Depth > 1 is ignored. A subagent's own multiagent roster (if any) doesn't cascade — only the session's coordinator delegates.

For per-language bindings beyond Python, WebFetch https://platform.claude.com/docs/en/managed-agents/multi-agent.md (see shared/live-sources.md).

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