# Langchain Agents Langgraph Code > Use when building a custom-graph LangGraph agent — when `create_agent(...)` + middleware isn't enough and you need explicit StateGraph control (multi-graph workflows, custom routing, non-standard state schemas, parallel branches). For the common case, use `create_agent` first (see middleware skill). - Skill: `tomevault-io/langchain-agents-langgraph-code` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/langchain-agents-langgraph-code` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/langchain-agents-langgraph-code/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/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. --- > Source: [cwijayasundara/agent_cli_langchain](https://github.com/cwijayasundara/agent_cli_langchain) — distributed by [TomeVault](https://tomevault.io).