Langchain Agents Langgraph Code

LangGraph: editorial guidance

For API reference (signatures, imports, exhaustive method lists), use the mcpdoc MCP tools: fetch_docs("https://docs.langchain.com/oss/python/langgraph/..."). This skill is the opinions layer; the docs are the facts layer.

When to drop down to raw StateGraph

Most agents do NOT need this. Use create_agent(...) + middleware first (see the langchain-agents-middleware skill). Drop down to StateGraph only when:

• Multiple LLM calls in a single graph with custom routing between them.
• Branches that run in parallel and merge.
• Non-message state (custom dataclasses, dicts, dataframes flowing through nodes).
• Multi-graph workflows where one graph calls another as a subgraph.

If your problem fits "one model, some tools, in a loop" — even if the loop is complex — create_agent is the right tool. Don't reach for StateGraph out of habit.

Things the docs won't warn you about

• A node returning {} is a no-op; return None to signal "no state change" cleanly.
• add_conditional_edges mappings must include END if any branch terminates — leaving it out is a silent bug, not an error.
• compile() is not idempotent across bind_tools — rebind tools, then re-compile.
• langgraph dev reloads on file change; if it stops reloading, the graph likely failed to import — check the terminal for the exception.
• A graph with interrupt() calls but no checkpointer will throw at invoke time, not at compile time. Always pair interrupt() with compile(checkpointer=...).

Production essentials (rules of thumb)

• Always pass a thread_id in config={"configurable": {"thread_id": ...}} for any agent with persistent state. A missing thread_id silently starts a fresh thread.
• InMemorySaver is for dev only. Production = PostgresSaver (multi-instance safe) or SqliteSaver (single-node, low-volume). State dies with the process for InMemorySaver.
• Resume an interrupted thread by passing None as input with the same thread_id. The runtime picks up where the interrupt fired.
• Custom state schemas need reducers. Annotated[list, add_messages] appends; without the reducer, each node replaces the field instead of accumulating.
• For node-level retries on raw StateGraph, use RetryPolicy. For create_agent agents, prefer ToolRetryMiddleware / ModelRetryMiddleware — same effect, cleaner composition.

Doc URLs to fetch with mcpdoc

• https://docs.langchain.com/oss/python/langgraph/graph-api.md — StateGraph, nodes, edges
• https://docs.langchain.com/oss/python/langgraph/durable-execution.md — checkpointers + thread_id
• https://docs.langchain.com/oss/python/langgraph/streaming.md — stream_mode modes
• https://docs.langchain.com/oss/python/langgraph/persistence.md — Postgres / Sqlite checkpointers
• https://docs.langchain.com/oss/python/langgraph/subgraphs.md — multi-graph composition

When the user is mid-task and you need a method signature or import path, fetch from these URLs. Don't guess.