LangGraph — every error, with the fix.
The complete fix database for LangGraph in 2026. Fifteen deep guides covering the errors developers actually hit: graph construction, state schemas, checkpointing, agents, streaming, human-in-the-loop, and LangGraph Platform deploys. Every page has tested Python code, real error messages, and the three fixes that solve 90% of cases.
Install & import
Every code sample on this hub targets LangGraph 0.2+ with Python 3.11+.
pip install "langgraph>=0.2.60" langchain-anthropic langchain-openai
pip install "langgraph-checkpoint-postgres" "langgraph-checkpoint-sqlite"Graph Construction
The errors you hit when building the graph: node registration, edge wiring, and getting to a clean compile.
State Management
Everything about the shape of your state and how updates merge — schemas, reducers, and message accumulation.
Checkpointing & Persistence
Saving and resuming state across invocations. MemorySaver, SqliteSaver, Postgres, and thread-level retrieval.
Agents, Tools & Structured Output
The prebuilt ReAct agent, tool round-trips, and forcing structured JSON out of the graph.
Streaming, Interrupts & Deployment
Streaming events to the client, pausing for human input, and shipping to LangGraph Platform.
Quick reference — where to look first
| If you see... | Start here |
|---|---|
ValueError: Node ... not found | #121 · Node & edge configuration |
InvalidUpdateError | #124 · State schema errors |
| Messages disappearing between nodes | #125 · Reducer errors |
ValueError: Missing configuration key 'thread_id' | #127 · Checkpointer setup |
psycopg.errors.UndefinedTable | #128 · Postgres checkpointer |
get_state returns None | #129 · Thread & retrieval |
| Tool call ID / ordering rejection from provider | #131 · ToolNode & tool_calls |
| No streaming chunks arriving | #133 · Streaming modes |
| Graph doesn't pause at interrupt | #134 · HITL interrupts |
| LangGraph Platform build failure | #135 · Platform & subgraphs |