Getting Internal Server Error 500 When trying to connect to Local Server

[badrinathvm]

Hi MCP team,

I have MCP tool called weather.py which is running under the http://127.0.0.1:8000, See below screenshot

I am connecting to http://127.0.0.1:8000 , However when the code is executed i am seeing the Internal error: Server error: 500

Any help is appreciated.

func connectToMCP() async {
        let client = Client(name: "weather", version: "1.0.0")
        
        // Create a streaming HTTP transport
        let transport = HTTPClientTransport(
            endpoint: URL(string: "http://127.0.0.1:8000")!,
            streaming: false  // Enable Server-Sent Events for real-time updates
        )
        
        do {
            try await client.connect(transport: transport)
      
  
            // List of available Tools
            let tools = try await client.listTools()
            print("Available tools: \(tools.tools.map { $0.name })")
        } catch let error {
            print(error.localizedDescription)
        }
    }

On inspecting the server side logs i see like below

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/Users/XXX/Documents/projects/mcp-server-basic/.venv/lib/python3.11/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/XXX/Documents/projects/mcp-server-basic/.venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/XXX/Documents/projects/mcp-server-basic/.venv/lib/python3.11/site-packages/uvicorn/middleware/asgi2.py", line 14, in __call__
    instance = self.app(scope)
               ^^^^^^^^^^^^^^^
TypeError: 'FastMCP' object is not callable
INFO:     127.0.0.1:59075 - "POST / HTTP/1.1" 500 Internal Server Error

[stallent]

which transport are you running in FastMCP

[badrinathvm]

@stallent I was setting the STDIO in MCP inspector for testing. Here's the code for weather.py if it helps.

from typing import Any
import httpx
from mcp.server.fastmcp import FastMCP
from fastapi import FastAPI


# Initialize the FASTMCP server - this is the correct way
mcp = FastMCP(name="weather")  # Make sure to use the name parameter explicitly


# constants 
NWS_API_BASE = "https://api.weather.gov"
USER_AGENT = "weather-app/0.1"

async def make_nws_request(url: str) -> dict[str, Any] | None:
    """Make a request to the NWS API and return the response data with proper error handling"""
    headers = {
        "User-Agent": USER_AGENT,
        "Accept": "application/geo+json"
    }
    async with httpx.AsyncClient() as client:
        try: 
            response = await client.get(url, headers=headers, timeout=30.0)
            response.raise_for_status()
            return response.json()
        except Exception as e:
            mcp.log.error(f"Error fetching data from NWS: {e}")
            return None

def format_alert(feature: dict) -> str:
    """Format a weather alert feature into a readable string"""
    props = feature["properties"]
    return f"""
        Event: {props.get('event', 'Unknown')}
        Area: {props.get('areaDesc', 'Unknown')}
        Severity: {props.get('severity', 'Unknown')}
        Description: {props.get('description', 'No description available')}
        Instructions: {props.get('instruction', 'No specific instructions provided')}
    """
    
@mcp.tool()
async def get_alerts(state: str) -> str:
    """Get current weather alerts for a specific state
    
    Args:
        state (str): The two-letter state code (e.g., 'CA', 'NY')
    """
    
    url = f"{NWS_API_BASE}/alerts/active/area/{state}"
    data = await make_nws_request(url)
    
    if not data or "features" not in data:
        return "No active alerts found for the given state"
    
    if not data["features"]:
        return "No active alerts found for the given state"
    
    alerts = [format_alert(feature) for feature in data["features"]]
    return "\n\n".join(alerts)
    

@mcp.resource("echo://{message}")
def echo_resource(message: str) -> str:
    """Echo message as a resource"""
    return f"Resource Echo: {message}"

[stallent]

You must spin up your server using the same transport type you want to use on the client. FastMCP defaults to stdio, so i assume when you used stdio in MCP Inspector it worked. And you should be able to use the stdio client transport in this framework to connect to it via stdio. If you want to use a streamable http transport on both sides. (see docs https://gofastmcp.com/servers/fastmcp#running-the-server) on how to start it up with the proper transport. If you use the "streamable-http" transport on the fastmcp server side, you should be able to connect to it with HTTPClientTransport in the swift client. If that doesn't work, switch back to the MCP Inspector and ensure you can connect to your server using the streamable transport setting in MCP Inspector (instead of stdio). If that does not work, then there is an issue in your setup. If it DOES work via the streamable transport in MCP Inspector but does not work HTTPClientTransport, pipe back up here and I'll help.

[badrinathvm]

Hi @stallent , I did a try a sample example Here's the myserver.py where it is using streamable-http

from mcp.server.fastmcp import FastMCP

mcp = FastMCP(name="my_server")

@mcp.tool()
def my_tool(message: str) -> str:
    """Echo a message"""
    return f"Echo: {message}"

if __name__ == "__main__":
    mcp.run(transport="streamable-http", host="127.0.0.1", port=8001)

Swift Code

func connectToMCP() async {
        let client = Client(name: "my_server", version: "0.1")
        
        // Create a streaming HTTP transport
        let transport = HTTPClientTransport(
            endpoint: URL(string: "http://127.0.0.1:8001")!,
            streaming: true  // Enable Server-Sent Events for real-time updates
        )
        
        do {
            try await client.connect(transport: transport)
      
            
            // List of available Tools
            let tools = try await client.listTools()
            print("Available tools: \(tools.tools.map { $0.name })")
        } catch let error {
            print(error.localizedDescription)
        }
    }

I still see the same issue like below in Xcode.

When i try the same thing on MCP Inspector with streamable HTTP conenction fails

Well, how can i connect to STDIO in swift code by pointing to local server http://127.0.0.1:8001 ?

If i try on MCP inspector on STDIO , this tool works great, I am trying to get it working from Swift SDK. Any help in narrowing down is appreciated.

If i use below Swift Code to StdioTransport ? How does it refer to my server connection, I don't see the expected output.

func connectToMCP() async {
        let client = Client(name: "my_server", version: "0.1")
        do {
            let transport = StdioTransport()
            try await client.connect(transport: transport)
      
            // List of available Tools
            let tools = try await client.listTools()
            print("Available tools: \(tools.tools.map { $0.name })")
        } catch let error {
            print(error.localizedDescription)
        }
    }

[stallent]

alright, i cannot help you on the stdio stuff. i only use http so thats the code path i'm most familiar with. apologies! I suspect you are not pointing the client at the server. Would assume it would be something similar to what you put in the command field in the MCP Inspector

[badrinathvm]

@stallent I got the MCP inspector working for the StreambleHttp, Below is the screen shot. I was able to get the output if i do via Python client like below

import asyncio
from fastmcp import Client
from fastmcp.client.transports import StreamableHttpTransport

async def test_tools():
    transport = StreamableHttpTransport("http://127.0.0.1:8000/mcp/")

    async with Client(transport=transport) as client:
        try:
            await client.ping()
            print("Ping successful")

            # list available tools
            tools = await client.list_tools()
            print(f"Available tools: {tools}")

            # Test add
            result = await client.call_tool("add", {"a": 5, "b": 3})
            print(f"Add result: {result}")  # Should print 8
           
        except Exception as e:
            print(f"Error occurred: {e}")

if __name__ == "__main__":
    asyncio.run(test_tools()) 

---

However when i try to test the client via Swift SDK i seeInternal error: Bad Request

---

Swift Code

func connectToMCP() async {
        do {
            // Initialize the client
            let client = Client(name: "MyApp", version: "1.0.0")
            
            // Create a streaming HTTP transport
            let transport = HTTPClientTransport(
                endpoint: URL(string: "http://127.0.0.1:8000/mcp")!,
                streaming: false
            )
            
            try await client.connect(transport: transport)
            
            let tools = try await client.listTools()
            print("Available tools: \(tools.tools.map { $0.name }.joined(separator: ", "))")
        } catch let error {
            print(error)
        }
        
    }

It's failign at highlighted section

Any pointer is appreciated!

[mattt]

@badrinathvm Can you share the MCP server logs? It would be helpful to get the HTTP request sent by the Swift MCP client and the 500 response sent from the Python MCP server.

[badrinathvm]

@mattt This is what i see in the Server Logs.

[badrinathvm]

Hi @mattt i was able to fix the error,

1. Missing "/" at the end URL(string: "http://127.0.0.1:8000/mcp/")!
2. Need to do let result = try await client.initialize() after try await client.connect(t

func connectToMCP() async {
        do {
            // Initialize the client
            let client = Client(name: "MyApp", version: "1.0.0")
            
            // Create a streaming HTTP transport
            let transport = HTTPClientTransport(
                endpoint: URL(string: "http://127.0.0.1:8000/mcp/")!
            )
            
            try await client.connect(transport: transport)
            let result = try await client.initialize()
            print(result)
            
            let tools = try await client.listTools()
            print("Available tools: \(tools.tools.map { $0.name }.joined(separator: ", "))")
        } catch let error {
            print(error)
        }
    }

Server logs

Thank you @mattt @stallent for being responsive on this issue.

[mattt]

@badrinathvm So glad you were able to get this working!

2. Need to do let result = try await client.initialize() after try await client.connect

This is changing in the next release. See #100