Disable logs reconfigure on startup

src/mcp/server/fastmcp/server.py

@@ -88,7 +88,7 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
 
     # Server settings
     debug: bool
-    log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"]
+    log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] | None
 
     # HTTP settings
     host: str
@@ -152,7 +152,7 @@ def __init__(  # noqa: PLR0913
         *,
         tools: list[Tool] | None = None,
         debug: bool = False,
-        log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO",
+        log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] | None = "INFO",
         host: str = "127.0.0.1",
         port: int = 8000,
         mount_path: str = "/",
@@ -224,8 +224,9 @@ def __init__(  # noqa: PLR0913
         # Set up MCP protocol handlers
         self._setup_handlers()
 
-        # Configure logging
-        configure_logging(self.settings.log_level)
+        if self.settings.log_level is not None:
+            # Configure logging
+            configure_logging(self.settings.log_level)
 
     @property
     def name(self) -> str:
@@ -745,7 +746,7 @@ async def run_sse_async(self, mount_path: str | None = None) -> None:
             starlette_app,
             host=self.settings.host,
             port=self.settings.port,
-            log_level=self.settings.log_level.lower(),
+            **self._get_uvicorn_log_config(),
         )
         server = uvicorn.Server(config)
         await server.serve()
@@ -760,11 +761,17 @@ async def run_streamable_http_async(self) -> None:
             starlette_app,
             host=self.settings.host,
             port=self.settings.port,
-            log_level=self.settings.log_level.lower(),
+            **self._get_uvicorn_log_config(),
         )
         server = uvicorn.Server(config)
         await server.serve()
 
+    def _get_uvicorn_log_config(self) -> dict[str, Any]:
+        """Map FastMCP log level to Uvicorn log level."""
+        if self.settings.log_level is None:
+            return {"log_level": None, "log_config": None}
+        return {"log_level": self.settings.log_level.lower()}
+
     def _normalize_path(self, mount_path: str, endpoint: str) -> str:
         """
         Combine mount path and endpoint to return a normalized path.

tests/server/fastmcp/test_server.py

@@ -1,7 +1,7 @@
 import base64
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
-from unittest.mock import patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 from pydantic import AnyUrl, BaseModel
@@ -769,6 +769,37 @@ def get_data() -> str:
             assert resource.name == "test_get_data"
             assert resource.mimeType == "text/plain"
 
+    def test_server_no_log_reconfigure(self):
+        """Test that the server does not reconfigure logging if already configured."""
+        with patch("mcp.server.fastmcp.server.configure_logging") as configure_logging_mock:
+            # First instantiation should configure logging
+            FastMCP(log_level=None)
+            assert configure_logging_mock.call_count == 0
+
+    def test_server_uvicorn_no_logging(self):
+        """Test that the server does not reconfigure logging if uvicorn logging is set."""
+        with (
+            patch("mcp.server.fastmcp.server.configure_logging") as configure_logging_mock,
+            patch("uvicorn.Config") as uvicorn_config_mock,
+            patch("uvicorn.Server") as uvicorn_server_mock,
+        ):
+            uvicorn_server_mock.return_value.serve = AsyncMock()
+            # First instantiation should configure logging
+            # Launch mock server
+            mcp = FastMCP(log_level=None)
+            mcp.run(transport="streamable-http")
+            # check that logging was not configured
+            assert configure_logging_mock.call_count == 0
+            config_call_args = uvicorn_config_mock.call_args
+            # Verify that log_config and log_level are None on uvicorn Config
+            assert "log_config" in config_call_args.kwargs
+            assert config_call_args.kwargs["log_config"] is None
+            assert "log_level" in config_call_args.kwargs
+            assert config_call_args.kwargs["log_level"] is None
+            # Verify that the server was started
+            assert uvicorn_server_mock.call_count == 1
+            assert uvicorn_server_mock.return_value.serve.call_count == 1
+
 
 class TestServerResourceTemplates:
     @pytest.mark.anyio