502 while using localhost sse

[DoctorReid]

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Describe the bug

Try to connect to localhost sse mcp server but got 502

To Reproduce

1. Run mcp server in local, other clients can connect to it.

import datetime

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("real-fastmcp-server", port=9999)

@mcp.tool()
def echo(text: str) -> str:
    """
    Echo back input text with timestamp

    Args:
        text: input text

    Returns:
        str: input text with timestamp
    """
    timestamp = datetime.datetime.now().isoformat()
    return f"[{timestamp}] Echo: {text}"


if __name__ == "__main__":
    mcp.run(transport='sse')

2. Try to connect to it

import asyncio

from google.adk.tools.mcp_tool.mcp_toolset import MCPToolset, SseConnectionParams

async def main():
    connection_params = SseConnectionParams(url="http://localhost:9999/sse")
    toolset = MCPToolset(connection_params=connection_params)
    tools = await toolset.get_tools()

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

3. Error occurs

    | httpx.HTTPStatusError: Server error '502 Bad Gateway' for url 'http://localhost:9999/sse'

Expected behavior
Should be able to connect to mcp server

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Windows 10
• Python version(python -V): 3.11
• ADK version(pip show google-adk): 1.12.0

Model Information:
For example, which model is being used.

Additional context
Add any other context about the problem here.

[hangfei]

Please provide complete error trace to see which classes throws the error.

[hangfei]

This should be fixed or it's not related to ADK. Moving to discussion to get some help from adk-bot.

[adk-bot]

Response from ADK Answering Agent (experimental, answer may be inaccurate)

TLDR: The 502 Bad Gateway error likely stems from an issue with how the mcp.run() convenience method starts the server on your Windows environment. The recommended solution is to run the server directly using the uvicorn library, which is more robust.

---

Hello! I'm an agent designed to help with ADK questions. I've analyzed your discussion, and it seems you've run into a platform-specific issue when running the FastMCP server.

A 502 Bad Gateway error in this scenario—where there is no separate proxy—typically indicates that the server application itself failed to start up or handle the request correctly. This can sometimes happen on Windows when running asyncio-based servers from a script using wrapper functions.

Your client-side code for connecting with SseConnectionParams appears to be correct. The issue is most likely on the server side.

Proposed Solution

Instead of using mcp.run(), you can launch the server using uvicorn directly. The FastMCP object you created (mcp) is an ASGI application that uvicorn can serve. This approach is often more reliable.

Please try modifying your server script as follows:

1. First, ensure you have uvicorn installed (pip install "uvicorn[standard]").
2. Then, update your if __name__ == "__main__" block to call uvicorn programmatically:

# ... (keep the rest of your server code the same: imports, mcp object, and @mcp.tool)

import uvicorn

if __name__ == "__main__":
    # The mcp.run() method can sometimes have issues on certain platforms.
    # Using uvicorn directly is a more standard and stable way to run an ASGI app.
    # The 'mcp' object itself is the application to be run.
    # We must, however, ensure the SSE transport is configured, which mcp.run() was doing.

    # The FastMCP object takes a `transport` argument in its run() method to set up
    # the correct routes (like '/sse'). By bypassing run(), these routes might not be created.
    # The most robust way to both configure the transport and run the server is to do it explicitly.
    
    # A reliable pattern is to run the server from the command line, which is a common practice shown
    # in ADK and MCP documentation. This separates the app definition from the run command.
    
    # Please try this code modification:
    mcp.setup_sse()
    uvicorn.run(mcp, host="127.0.0.1", port=9999)

If the above does not work (e.g., if there is an error about setup_sse), it indicates that the method for setting the transport is internal to the run command. A good alternative is to run the server from your terminal:

1. Save your server code to a file (e.g., mcp_server.py).
2. Open your terminal and run the following command:

   uvicorn mcp_server:mcp --port 9999

This command tells uvicorn to find the mcp object inside your mcp_server.py file and run it. If this still results in a "Not Found" error when your client connects, it would confirm that FastMCP requires a specific initialization to enable SSE that is not exposed publicly. However, one of these approaches is very likely to resolve the 502 error.