Inspector Fails to Connect to Any MCP Server

[wpfleger96]

Describe the bug Starting in the last few weeks, MCP inspector will fail to connect to any MCP server (local or published to PyPy/NPM) and it used to work great for me.

To Reproduce Steps to reproduce the behavior:

1. Start MCP inspector with PagerDuty MCP server: npx @modelcontextprotocol/inspector uvx mcp_pagerduty
2. Open the inspector in my browser and click the Connect button, and I will get the error:

Connection Error, is your MCP server running?

Expected behavior When I click Connect, I expect Inspector to connect to my MCP server and show me the option to list resources, prompts, tools, etc. but instead I only ever get the connection error

Logs Inspector does not emit any error logs for this behavior that I've found, other than showing me the Connection Error message in my browser

Additional context 1-2 months ago Inspector used to work great for me, I'm not sure what changed. I've tried using older versions of inspector, and even passing a dummy MCP server path like:

npx @modelcontextprotocol/inspector uv run /not/a/real/path

will result in the exact same Connection Error message, whether the MCP actually exists or not. I also receive the Connection Error for both local MCP servers and ones downloaded from PyPi/NPM. I'm now unable to get any version of Inspector to connect to any MCP server.