What worked for me (Mac + MCP Server is in Python + using uv):

npx @modelcontextprotocol/inspector /opt/homebrew/bin/uv --directory /Users/my_home_directory/src/tutorials/mcp/weather run weather.py

One of the messages printed on the console during the Inspector's startup was:
Open inspector with token pre-filled:
http://localhost:6274/?MCP_PROXY_AUTH_TOKEN=a_sixty_four_character_token_that_changes_with_each_inspector_restart

I copy-pasted that in Safari. The Inspector was able to connect to the MCP server after that.