Describe the bug
Unable to run a minimal MCP server and testing it with mcp inspector.
I tried different url in mcp inspector same problem:
http://127.0.0.1:8000/
http://127.0.0.1:8000/mcp
http://127.0.0.1:8000/mcp/

=============================================

Created a minimal test server name test_server.py

#!/usr/bin/env python3
"""
Minimal FastMCP test server to debug connection issues
"""

import sys
import logging
from mcp.server.fastmcp import FastMCP

Configure logging

logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger("test-server")

Create FastMCP server

app = FastMCP("Test Server")

@app.tool()
def test_tool() -> str:
"""A simple test tool"""
return "Hello from FastMCP!"

if name == "main":
print("Starting FastMCP server...", file=sys.stderr)
try:
app.run() # Use default settings
except Exception as e:
print(f"Error starting server: {e}", file=sys.stderr)
raise

=============================================
Started it using
mcp dev test_server.py
DEBUG:mcp.server.lowlevel.server:Initializing server 'Test Server'
DEBUG:mcp.server.lowlevel.server:Registering handler for ListToolsRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for CallToolRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for ListResourcesRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for ReadResourceRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for PromptListRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for GetPromptRequest
DEBUG:mcp.server.lowlevel.server:Registering handler for ListResourceTemplatesRequest
Starting MCP inspector...
⚙️ Proxy server listening on port 6277
🔍 MCP Inspector is up and running at http://127.0.0.1:6274 🚀
Received POST message for sessionId undefined
New streamable-http connection
Query parameters: [Object: null prototype] {
url: 'http://127.0.0.1:8000/',
transportType: 'streamable-http'
}
Connected to Streamable HTTP transport
Connected MCP client to backing server transport
Created streamable web app transport 39d882dd-0ae5-49c1-a8b2-7cd8a774b28c
Error from MCP server: Error: Error POSTing to endpoint (HTTP 404): {"detail":"Not Found"}
at StreamableHTTPClientTransport.send (file:///usr/local/lib/node_modules/@modelcontextprotocol/inspector/node_modules/@modelcontextprotocol/sdk/dist/esm/client/streamableHttp.js:265:23)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)

=============================================
pip show mcp
Name: mcp
Version: 1.9.2
Summary: Model Context Protocol SDK
Home-page: https://modelcontextprotocol.io
Author: Anthropic, PBC.
Author-email:
License: MIT
Location: .../venv/lib/python3.12/site-packages
Requires: anyio, httpx, httpx-sse, pydantic, pydantic-settings, python-multipart, sse-starlette, starlette, uvicorn
Required-by:

To Reproduce

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• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

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