inspector v2 fails to connect to MCP server at https://localhost

[symdeb]

Which version line?

v2 — current (@modelcontextprotocol/inspector@latest)

Which client?

Web

Inspector version

2.10.0

Node version

v23.11.1

Operating system (and browser, for the web client)

Ubuntu 26.04

Transport

Streamable HTTP

MCP server under inspection

1. nvm use 21
   npx @modelcontextprotocol/inspector
2. Add server: https://localhost/api/mcp/mcp

Steps to reproduce

1. nvm use 21
   npx @modelcontextprotocol/inspector
2. Add server: https://localhost/api/mcp/mcp
3. Connect
4. Error observed (No any HTTP requests enter the OAUTH2 or MCP server )

Brave
OAuth authorization failed for "localhost"
Remote fetch failed (401): {"error":"Unauthorized","message":"Authentication required. Use the x-mcp-remote-auth header with Bearer token."}
http 2025 & 2026 : OK

Chrome:
Error Fetch failed
http 2025 & 2026 : OK

Firefox:
Known issue that it hangs (see other posts)

Vivaldi
Error Fetch failed
http 2025 & 2026 : OK

Note: when using http:
Error Fetch failed
http 2025 & 2026 : OK

Expected behavior

Connect to the MCP server at localhost

Actual behavior

Does not connect , failed

Logs, errors, or screenshots

No response

Already prototyped a fix?

No response

Before you submit

• I searched existing issues and this is not a duplicate.
• This is not a security vulnerability report (those go through the private advisory process).