LangSmith Studio URL Redirects to Error Page

Issue Description

When running langgraph dev to start LangSmith Studio, users may encounter a CORS error with the message "TypeError: Failed to fetch". The browser console shows an error similar to:

Access to fetch at 'http://127.0.0.1:2024/assistants/[assistant-id]' from origin 'https://your-langsmith-domain.com' 

has been blocked by CORS policy: Response to preflight request doesn't pass access control check: 

The 'Access-Control-Allow-Origin' header has a value 'http://127.0.0.1:11434' that is not equal to 

the supplied origin.

Symptoms

- LangSmith Studio redirects to an error page

- CORS policy blocking requests

- Unexpected reference to port 11434 in CORS headers

- Issue is not consistently reproducible across different environments

Root Cause

This issue is commonly caused by browser extensions that interfere with local development, particularly:

- Ollama Chrome extensions or similar AI model extensions

- Extensions that proxy or modify HTTP requests

- Extensions that set custom CORS headers

The extension redirects requests to its own port (e.g., 11434 for Ollama), causing CORS mismatches with LangSmith Studio.

Solution

Quick Fix: Disable Conflicting Browser Extensions

1. Identify conflicting extensions: Check for Ollama, AI model, or development proxy extensions

2. Disable or uninstall the extension

3. Restart your browser

4. Run langgraph dev again

Alternative Troubleshooting Steps

If disabling extensions doesn't resolve the issue, try these steps:

1. Configure CORS in langgraph.json

Add explicit CORS configuration to your langgraph.json file:

{
  "graphs": {
    "your_graph": "./path/to/graph.py:graph"
  },
  "http": {
    "cors": {
      "allow_origins": ["https://your-langsmith-domain.com"],
      "allow_credentials": true,
      "allow_methods": ["*"],
      "allow_headers": ["*"]
    }
  }
}

2. Clear Browser Cache

Old CORS responses may be cached by the browser:

- Open browser settings

- Clear browsing data (cache and cookies)

- Restart browser and try again

3. Specify Studio URL Explicitly

Run langgraph dev with an explicit studio URL:

langgraph dev --studio-url https://your-langsmith-domain.com

4. Check for Port Conflicts

Look in your configuration files for anything that might redirect requests to unexpected ports (like 11434):

- Check .env files

- Review langgraph.json configuration

- Verify no other services are running on port 2024

Prevention

- Be aware of browser extensions that modify HTTP requests when doing local development

- Keep a clean development browser profile without extensions

- Document any required extensions for your development environment

Additional Notes

- This issue may appear intermittently depending on which browser extensions are active

- Different users may experience this in different environments based on their browser configuration

- The issue is specific to the browser environment and doesn't affect the LangGraph server itself

Keywords: LangSmith Studio, LangGraph Studio, CORS error, TypeError Failed to fetch, Ollama extension, browser extension conflict, port 11434, langgraph dev