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[GH-ISSUE #2850] [langgraph]: Documentation Error in Agentic RAG Tutorial #2689

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opened 2026-06-05 17:26:18 -04:00 by yindo · 0 comments
yindo commented 2026-06-05 17:26:18 -04:00
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Originally created by @tirelyl on GitHub (Feb 28, 2026).
Original GitHub issue: https://github.com/langchain-ai/docs/issues/2850

Type of issue

issue / bug

Language

JavaScript

Description

Documentation Error: gradeDocuments node returns string instead of state update object in Agentic RAG tutorial

Summary

The Build a custom RAG agent with LangGraph tutorial contains a critical error in the gradeDocuments node implementation. The node is defined to return a string ("generate" or "rewrite"), but StateGraph nodes must return objects or Command instances to update state. This code will throw an InvalidUpdateError at runtime.

Location

Section 4: "Grade documents" in the Agentic RAG tutorial

Current (Incorrect) Code

const gradeDocuments: GraphNode<typeof State> = async (state) => {
  const model = new ChatOpenAI({
    model: "gpt-4.1",
    temperature: 0,
  }).withStructuredOutput(gradeDocumentsSchema);

  const score = await prompt.pipe(model).invoke({
    question: state.messages.at(0)?.content,
    context: state.messages.at(-1)?.content,
  });

  if (score.binaryScore === "yes") {
    return "generate";  // ❌ ERROR: Nodes cannot return primitives
  }
  return "rewrite";     // ❌ ERROR: Nodes cannot return primitives
}

And the conditional edge tries to read from state.messages:

.addConditionalEdges(
  "gradeDocuments",
  (state) => {
    const lastMessage = state.messages.at(-1);
    return lastMessage.content === "generate" ? "generate" : "rewrite";  // ❌ This won't work
  }
)

Why This is Wrong

  1. StateGraph nodes must return objects or Commands: According to the source code in libs/langgraph-core/src/graph/state.ts (lines 1379-1389), the _getUpdates function validates node return values and throws an InvalidUpdateError if a primitive type is returned:
} else {
  const typeofInput = Array.isArray(input) ? "array" : typeof input;
  throw new InvalidUpdateError(
    `Expected node "${nodeKey.toString()}" to return an object or an array containing at least one Command object, received ${typeofInput}`,
    {
      lc_error_code: "INVALID_GRAPH_NODE_RETURN_VALUE",
    }
  );
}

  1. No automatic wrapping: LangGraph does not automatically wrap primitive return values into state updates. The string will not magically appear in state.messages[].content.

  2. Confusion between nodes and conditional edges: The documentation appears to confuse node functions (which update state) with conditional edge routing functions (which return strings to indicate the next node).

Originally created by @tirelyl on GitHub (Feb 28, 2026). Original GitHub issue: https://github.com/langchain-ai/docs/issues/2850 ### Type of issue issue / bug ### Language JavaScript ### Description Documentation Error: `gradeDocuments` node returns string instead of state update object in Agentic RAG tutorial ### Summary The [Build a custom RAG agent with LangGraph](https://docs.langchain.com/oss/javascript/langgraph/agentic-rag) tutorial contains a critical error in the `gradeDocuments` node implementation. The node is defined to return a string (`"generate"` or `"rewrite"`), but StateGraph nodes must return objects or Command instances to update state. This code will throw an `InvalidUpdateError` at runtime. ### Location Section 4: "Grade documents" in the Agentic RAG tutorial ### Current (Incorrect) Code ```typescript const gradeDocuments: GraphNode<typeof State> = async (state) => { const model = new ChatOpenAI({ model: "gpt-4.1", temperature: 0, }).withStructuredOutput(gradeDocumentsSchema); const score = await prompt.pipe(model).invoke({ question: state.messages.at(0)?.content, context: state.messages.at(-1)?.content, }); if (score.binaryScore === "yes") { return "generate"; // ❌ ERROR: Nodes cannot return primitives } return "rewrite"; // ❌ ERROR: Nodes cannot return primitives } ``` And the conditional edge tries to read from `state.messages`: ```typescript .addConditionalEdges( "gradeDocuments", (state) => { const lastMessage = state.messages.at(-1); return lastMessage.content === "generate" ? "generate" : "rewrite"; // ❌ This won't work } ) ``` ### Why This is Wrong 1. **StateGraph nodes must return objects or Commands**: According to the source code in `libs/langgraph-core/src/graph/state.ts` (lines 1379-1389), the `_getUpdates` function validates node return values and throws an `InvalidUpdateError` if a primitive type is returned: ```typescript } else { const typeofInput = Array.isArray(input) ? "array" : typeof input; throw new InvalidUpdateError( `Expected node "${nodeKey.toString()}" to return an object or an array containing at least one Command object, received ${typeofInput}`, { lc_error_code: "INVALID_GRAPH_NODE_RETURN_VALUE", } ); } ``` 2. **No automatic wrapping**: LangGraph does not automatically wrap primitive return values into state updates. The string will not magically appear in `state.messages[].content`. 3. **Confusion between nodes and conditional edges**: The documentation appears to confuse node functions (which update state) with conditional edge routing functions (which return strings to indicate the next node).
yindo added the langgraphexternal labels 2026-06-05 17:26:18 -04:00
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Reference: langchain-ai/docs#2689
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