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Async subagents let a supervisor agent launch background tasks that return immediately, so the supervisor can continue interacting with the user while subagents work concurrently. The supervisor can check progress, send follow-up instructions, or cancel tasks at any point. This builds on subagents, which run synchronously and block the supervisor until completion. Use async subagents when tasks are long-running, parallelizable, or need mid-flight steering. On an Agent Server, the supervisor also learns when a task finishes, fails, or needs input, so it does not have to check.
Async subagents communicate with any server that implements the Agent Protocol. You can use LangSmith Deployments, or self-host any Agent Protocol-compatible server. Each subagent runs independently of the supervisor, which controls them through the SDK to launch, check, update, and cancel.

When to use async subagents

Configure async subagents

Define async subagents as a list of AsyncSubAgent specs, each pointing to an Agent Protocol server:
For LangGraph-based deployments, register all graphs in the same langgraph.json for co-deployed setups:

Use the async subagent tools

The AsyncSubAgentMiddleware which is included in the Deep Agents stack when async subagents are configured, gives the supervisor five tools: The supervisor’s LLM calls these tools like any other tool. The middleware handles thread creation, run management, and state persistence automatically.

Understand the lifecycle

A typical interaction follows this sequence:
  • Launch creates a new thread on the server and starts a run with input determined by the launch mode. Isolated mode sends only the task description; fork mode also sends the parent’s effective conversation history. The thread ID is returned as the task ID. The supervisor reports this ID to the user and does not poll for completion.
  • Check fetches the current run status. If the run succeeded, it retrieves the thread state to extract the subagent’s final output. If still running, it reports that to the user.
  • Update creates a new run on the same thread. By default it interrupts the previous run, and the subagent restarts with the full conversation history plus the new instructions. The task ID stays the same.
  • Cancel calls runs.cancel() on the server and marks the task as "cancelled".
  • List iterates over all tracked tasks. For non-terminal tasks, it fetches live status from the server in parallel. Terminal statuses (success, error, cancelled) are returned from cache.
To let follow-ups wait, set updateStrategy: "enqueue". Update then queues the new run behind the current one, and the task is pending until it starts. Cancelling the task also cancels earlier runs that are still active. If the current run pauses for input or approval before the follow-up starts, the follow-up still starts and that request is dropped.
updateStrategy requires deepagents>=1.16.0. It is in beta; APIs and behavior may change between releases.
If a subagent paused for input or approval, check and list report it as waiting. Updating a waiting task drops its pending request, and cancelling one only marks it as "cancelled", since it has no active run. See Handle subagents waiting for input. When running on an Agent Protocol server, subagents automatically update the supervisor as they complete. If using a single deployment for the supervisor and subagents, subagents will wake the supervisor if it is idle; otherwise it receives the update at the start of its next run.
Task notifications require deepagents>=1.16.0. They are in beta; APIs and behavior may change between releases.
Notifications are returned to the supervisor as a HumanMessage. They can include images and other multimodal media. The message carries a TaskEvent (task_id, agent_name, status, run_id) in additional_kwargs.deepagents_notification, so your app can show notifications differently from user messages:

Deliver results through your app

webhook and notifyParent require deepagents>=1.16.0. notifyParent is in beta; APIs and behavior may change between releases.
Set webhook to have the Agent Server call your app whenever a run of the subagent finishes. The call includes the run’s thread ID, run ID, status, and metadata, including parent_thread_id and subagent_name. To deliver results through your app instead of waking the supervisor, also turn off parent notifications. The supervisor still receives the subagent’s updates while it runs.

Handle subagents waiting for input

The waiting status requires deepagents>=1.16.0. It is in beta; APIs and behavior may change between releases.
When a subagent pauses for input or approval with interrupt() or human-in-the-loop, the check and list tools report it as waiting with its pending interrupts. The supervisor cannot answer or approve on the user’s behalf. The request is for the user. Your app answers it on the subagent’s own thread. The task ID is the subagent’s thread ID:
When the subagent finishes after the answer, the supervisor receives a notification as usual. If a subagent needs information from the supervisor rather than the user, have it ask in its result. The supervisor replies with update_async_task.

Understand state management

Task metadata is stored in a dedicated state channel (asyncTasks) on the supervisor’s graph, separate from the message history. This is critical because deep agents compact their message history when the context window fills up. If task IDs were only in tool messages, they would be lost during compaction. The dedicated channel ensures the supervisor can always recall its tasks through list_async_tasks, even after multiple rounds of summarization. Each tracked task records the task ID, agent name, thread ID, run ID, status, and timestamps (createdAt, checkedAt, updatedAt).

Find a supervisor’s subagent threads

Thread tagging requires deepagents>=1.16.0.
start_async_task creates each subagent thread with metadata: { parent_thread_id }, the supervisor’s thread ID. Use it to find a supervisor’s subagent threads, or to read the supervisor’s thread in a subagent’s graph factory:

Choose a transport

Same-deployment (omit url)

When a subagent spec omits the url field, the LangGraph SDK talks to the Agent Server that is running this graph, using the address that server provides. Elsewhere, the SDK defaults to http://localhost:8123. For LangGraph-based deployments, register both graphs in the same langgraph.json. Deployed Agent Servers provide this address from version 0.18.0 (langgraph dev already does). On earlier versions, set url to the deployment URL.

HTTP transport (remote)

Add a url field to switch to HTTP transport, where SDK calls go over the network to a remote Agent Protocol server:
For LangGraph deployments, authentication is handled by the LangGraph SDK using LANGSMITH_API_KEY (or LANGGRAPH_API_KEY) from environment variables. Self-hosted Agent Protocol servers may use a different authentication mechanism. Use HTTP transport when subagents need independent scaling, different resource profiles, or are maintained by a different team.

Choose a launch mode

The mode field on each AsyncSubAgent spec controls what conversation context the subagent receives when a task starts. "isolated" is the right choice for most tasks. Use "fork" when the subagent needs established context — for example, a background drafting agent that should write in the style and voice already set up in the conversation.

Fork mode

Fork mode requires deepagents>=1.16.0. It is experimental and may change in a future release.
Set mode: "fork" on any AsyncSubAgent spec:
At launch, start_async_task sends a snapshot of the parent’s effective conversation history, followed by the task description. If the parent has compacted its history, the snapshot includes that compacted context, not every original message. Later messages from the parent are not forwarded to the child. The subagent starts on a new server thread. Fork mode sends conversation messages as run input; it does not copy the parent’s checkpoint history or all parent state. Metadata can associate the new thread with its supervisor, but that association does not itself transfer conversation context. Prompt-cache reuse. To get cache hits on the forked conversation prefix, configure the remote agent with the same model, system prompt, tools, and prompt-processing middleware as the parent. Fork mode copies conversation history; it does not synchronize those settings. Matching configuration is necessary but not sufficient: the rendered prefix must match, and cache hits depend on the provider. Offloaded media. If the parent offloads media with offloadBinaryContent, fork mode sends references to it rather than the media itself. The subagent must be a deep agent that shares the parent’s backend to see the media; other deep agents get a note that it’s unavailable, and other graphs can’t use these messages.

Choose a deployment topology

Single deployment

A single deployment means all agents are co-deployed on the same Agent Server. Omit url so each graph reaches the others through that server. For LangGraph-based deployments, register all graphs in one langgraph.json. This is the recommended starting point — one server to manage.

Split deployment

Supervisor on one server, subagents on another via HTTP transport. Use when subagents need different compute profiles or independent scaling.

Hybrid

In a hybrid deployment, some subagents omit url (same Agent Server), others set url for a remote server:

Best practices

Size the worker pool for local development

When running locally with langgraph dev, increase the worker pool to accommodate concurrent subagent runs. Each active run occupies a worker slot. A supervisor with 3 concurrent subagent tasks requires 4 slots (1 supervisor + 3 subagents). Under-provisioning causes launches to queue.

Write clear subagent descriptions

The supervisor uses descriptions to decide which subagent to launch. Be specific and action-oriented:

Trace with thread IDs

When using LangGraph-based deployments, every async subagent run is a standard LangGraph run, fully visible in LangSmith. The supervisor’s trace shows tool calls for launch, check, update, cancel, and list. Each subagent run appears as a separate trace, linked by thread ID. Use the thread ID (task ID) to correlate supervisor orchestration traces with subagent execution traces.

Troubleshooting

Supervisor polls immediately after launch

Problem: The supervisor calls check in a loop right after launching, turning async execution into blocking. Solution: The middleware injects system prompt rules to prevent this. If polling persists, reinforce the behavior in your supervisor’s system prompt:

Supervisor reports stale status

Problem: The supervisor references a task status from earlier in conversation history instead of making a fresh check call. Solution: The middleware prompt instructs the model that “task statuses in conversation history are always stale.” If this still occurs, add explicit instructions to always call check or list before reporting status.

Task ID lookup failures

Problem: The supervisor truncates or reformats the task ID, causing check or cancel to fail. Solution: The middleware prompt instructs the model to always use the full task ID. If truncation persists, this is typically a model-specific issue — try a different model or add “always show the full task_id, never truncate or abbreviate it” to your system prompt.

Subagent launches queue instead of running

Problem: Launching a subagent hangs or takes a long time to start. Solution: The worker pool is likely exhausted. Increase the pool size with --n-jobs-per-worker. See Size the worker pool.

Reference implementation

The async-deep-agents repository contains working examples in both Python and TypeScript that deploy to LangSmith Deployments. It demonstrates a supervisor with researcher and coder subagents running as background tasks.