UI can't connect to local/http FastMCP instance (Firefox only)

[rkz]

Inspector Version

0.21.1

Describe the bug

Inspector's UI cannot connect to a basic MCP server scaffolded using FastMCP through HTTP transport. However, the CLI works.

To Reproduce:

1. Create a local MCP at server.py:

# /// script
# requires-python = ">=3.10"
# dependencies = [
#     "fastmcp>=0.1.0",
#     "starlette>=0.35.0",
# ]
# ///

from fastmcp import FastMCP
from starlette.middleware.cors import CORSMiddleware
from starlette.middleware import Middleware

mcp = FastMCP("MCP Server")

@mcp.tool
def greet(name: str) -> str:
    return f"Hello, {name}!"

if __name__ == "__main__":
    mcp.run(
        transport="http",
        middleware=[
            Middleware(
                CORSMiddleware,
                allow_origins=["*"],
                allow_credentials=True,
                allow_methods=["*"],
                allow_headers=["*"],
            ),
        ],
    )

2. Install uv and run uv run server.py.

3. Run Inspector CLI:

npx -y @modelcontextprotocol/inspector --cli http://localhost:8000/mcp --transport http --method tools/call --tool-name greet --tool-arg name=John

It works.

4. Run Inspector UI and try to connect. It fails claiming a session ID is missing during the second notifications/initialized call. However a session ID was actually sent by the server earlier:

Environment:

• OS: macOS
• Browser: Firefox (only - working with Chrome)