MCP cannot process fastapi JSONResponse

[Chenxan]

Description

I found that when the tool returns fastapi JSONResponse, it cannot be parsed.

[Kludex]

Please create an MRE so we can help.

[Chenxan]

Summary

When I use fastmcp to mount to FastAPI, and when I call a tool, it returns a text response:"<starlette.responses.JSONResponse object at 0x000001481E9FC3E0>". This means my MCP Client can only receive "<starlette.responses.JSONResponse object at 0x000001481E9FC3E0>", and the LLM cannot parse it, preventing the actual invocation of the tool. How can I properly parse the returned result?

Enviroment

• Python: 3.12
• FastAPI: 0.116.1
• FastMCP: 2.11.3
• MCP Python SDK: 1.12.4

Minimal Example

from fastapi import APIRouter, FastAPI, Query
from fastapi.responses import JSONResponse
from fastmcp import FastMCP

test_router = APIRouter(prefix='/test', tags=['test'])
mcp = FastMCP("test")
@mcp.tool()
@test_router.get('/test', summary='test')
def get_test_data(
    test_id: str = Query(..., title='test_id', description='test_id'),
    test_str: str = Query(..., title='test_str', description='test_str')
):
    
    return JSONResponse(content={"test_id": test_id, "test_str": test_str})

mcp_app = mcp.http_app(path='/mcp')

app = FastAPI(
    lifespan=mcp_app.lifespan
)
app.mount("/mcp_server", mcp_app)

app.include_router(test_router)

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app="main:app", host="127.0.0.1", port=8000)

The MCP Inspector result

[Kludex]

The MRE seems to be mixing two concepts: MCP tool and the endpoint function itself.