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Nick Hollon
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StreamingHandler now yields SubgraphRunStream handles for each nested Pregel as it spawns. Each handle is a BaseRunStream wrapping a mini-mux built from the same transformer factories as the root — so sub.values, sub.messages, sub.subgraphs are populated by standard ValuesTransformer / MessagesTransformer / SubgraphTransformer instances at that subagent's scope. No routing or ChatModelStream assembly duplicated across transformers. Key pieces: - StreamLifecycleHandler (pregel/_lifecycle.py): callback handler attached at pregel stream / astream sites when "lifecycle" is in stream_modes. Emits started / running / completed / failed / interrupted events per nested Pregel via metadata-based detection (langgraph_checkpoint_ns + name != langgraph_node, excluding __start__/__end__ sentinels). Carries trigger_call_id from the parent task id. Root subgraph terminal state is emitted eagerly at __init__ and by SubgraphTransformer.finalize / fail. - StreamMux.make_child(scope) + factory-based construction. Mux takes a factory list; make_child produces a mini-mux at a new scope with fresh instances. bind_pump / bind_apump cascade through children so any subagent cursor drives the root pump. - StreamTransformer.scope (base attribute) + scope_exact class flag. Mux skips process() for out-of-scope events when scope_exact=True (default), so user transformers get scope-filtered events with no boilerplate. SubgraphTransformer opts out to receive cross-scope events for forwarding. - BaseRunStream shared base for GraphRunStream, AsyncGraphRunStream, and SubgraphRunStream. Provides extensions, native attrs, raw __iter__ / __aiter__, interleave. Root subclasses own the graph iterator and pump; SubgraphRunStream adds lifecycle metadata (path, status, error, checkpoint, trigger_call_id, graph_name). - SubgraphTransformer (stream/transformers.py) is now a thin discovery + forwarding dispatcher. On lifecycle.started at its scope + 1, it creates a SubgraphRunStream via parent_mux.make_child(ns). It forwards each event matching a direct-child's path into that child's mini-mux. Terminal lifecycle closes the mini-mux. - StreamMode literal extended with "lifecycle". pregel/remote.py filters it out of the SDK stream-mode list since wire-protocol support isn't landed yet. Tests: 16 new in test_stream_subgraph_transformer.py (unit + sync/async end-to-end including error + grandchild); existing Values/Messages namespace filter tests migrated through mux.push to reflect the new scope_exact contract.
Low-level orchestration framework for building stateful agents.
Trusted by companies shaping the future of agents – including Klarna, Replit, Elastic, and more – LangGraph is a low-level orchestration framework for building, managing, and deploying long-running, stateful agents.
pip install -U langgraph
If you're looking to quickly build agents with LangChain's create_agent (built on LangGraph), check out the LangChain Agents documentation.
Note
Looking for the JS/TS library? Check out LangGraph.js and the JS docs.
Why use LangGraph?
LangGraph provides low-level supporting infrastructure for any long-running, stateful workflow or agent:
- Durable execution — Build agents that persist through failures and can run for extended periods, automatically resuming from exactly where they left off.
- Human-in-the-loop — Seamlessly incorporate human oversight by inspecting and modifying agent state at any point during execution.
- Comprehensive memory — Create truly stateful agents with both short-term working memory for ongoing reasoning and long-term persistent memory across sessions.
- Debugging with LangSmith — Gain deep visibility into complex agent behavior with visualization tools that trace execution paths, capture state transitions, and provide detailed runtime metrics.
- Production-ready deployment — Deploy sophisticated agent systems confidently with scalable infrastructure designed to handle the unique challenges of stateful, long-running workflows.
Tip
For developing, debugging, and deploying AI agents and LLM applications, see LangSmith.
LangGraph ecosystem
While LangGraph can be used standalone, it also integrates seamlessly with any LangChain product, giving developers a full suite of tools for building agents.
To improve your LLM application development, pair LangGraph with:
- Deep Agents (new!) – Build agents that can plan, use subagents, and leverage file systems for complex tasks.
- LangChain – Provides integrations and composable components to streamline LLM application development.
- LangSmith – Helpful for agent evals and observability. Debug poor-performing LLM app runs, evaluate agent trajectories, gain visibility in production, and improve performance over time.
- LangSmith Deployment – Deploy and scale agents effortlessly with a purpose-built deployment platform for long-running, stateful workflows. Discover, reuse, configure, and share agents across teams – and iterate quickly with visual prototyping in LangSmith Studio.
Documentation
- docs.langchain.com – Comprehensive documentation, including conceptual overviews and guides
- reference.langchain.com/python/langgraph – API reference docs for LangGraph packages
- LangGraph Quickstart – Get started building with LangGraph
- Chat LangChain – Chat with the LangChain documentation and get answers to your questions
Discussions: Visit the LangChain Forum to connect with the community and share all of your technical questions, ideas, and feedback.
Additional resources
- Guides – Quick, actionable code snippets for topics such as streaming, adding memory & persistence, and design patterns (e.g. branching, subgraphs, etc.).
- LangChain Academy – Learn the basics of LangGraph in our free, structured course.
- Case studies – Hear how industry leaders use LangGraph to ship AI applications at scale.
- Contributing Guide – Learn how to contribute to LangChain projects and find good first issues.
- Code of Conduct – Our community guidelines and standards for participation.
Acknowledgements
LangGraph is inspired by Pregel and Apache Beam. The public interface draws inspiration from NetworkX. LangGraph is built by LangChain Inc, the creators of LangChain, but can be used without LangChain.