langgraph-cli dev server starts but returns 404 on root and /docs, Studio fails to connect

[nicobytes]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangGraph.js documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph.js rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package).

Example Code

This is my graph:

import {
  MessagesAnnotation,
  StateGraph,
  START,
  END,
} from '@langchain/langgraph';

const mockLlm = (_: typeof MessagesAnnotation.State) => {
  return { messages: [{ role: 'ai', content: 'hello world' }] };
};

export const graph = new StateGraph(MessagesAnnotation)
  .addNode('mock_llm', mockLlm)
  .addEdge(START, 'mock_llm')
  .addEdge('mock_llm', END)
  .compile();

Error Message and Stack Trace (if applicable)

Description

I am trying to run a simple LangGraph application using the CLI development server. The terminal indicates that the server has started successfully and registered my graph. However, when I try to access the API documentation (/docs) or connect via LangGraph Studio, I get connection errors and 404 responses.

Steps to Reproduce:

1. Define a simple graph
2. Run the dev server using npx @langchain/langgraph-cli dev (or via npm script).
3. The terminal shows Server running at ::1:2024.
4. Navigate to http://localhost:2024/docs or open the LangGraph Studio URL provided in the terminal.

System Info

Node version: v24.11.1
Operating system: darwin arm64
Package manager: npm
Package manager version: N/A

@langchain/core -> @1.0.5, , @"1.0.5", @"^1.0.5", @"^1.0.1", @"^0.3.59, @"^1.0.0"
langsmith -> @0.3.79, , @"~0.3.74", @"^0.3.33", @"^0.3.64"
zod -> @4.1.12, , @"^4.1.12", @"^3.25.76, @"^3.25, @"^3.25.32, @3.25.76, @"^3.19.1"
@langchain/langgraph-checkpoint-postgres -> @1.0.0, , @"1.0.0"
@langchain/langgraph-checkpoint -> @1.0.0, , @"^1.0.0", @"~0.0.16
@langchain/langgraph-cli -> @1.0.3, , @"1.0.3"
@langchain/langgraph-api -> @1.0.3, , @"1.0.3"
@langchain/langgraph-sdk -> @1.0.0, , @"~1.0.0", @"~0.0.16
@langchain/langgraph-ui -> @1.0.3, , @"1.0.3"
@langchain/langgraph -> @1.0.2, , @"1.0.2", @"^1.0.0", @"^0.2.57
@langchain/openai -> @1.1.2, , @"1.1.2"
langchain -> @1.0.5, , @"1.0.5"