Add configure_logging opt-out to MCPServer (#1656)

Author: gee-46

src/mcp/server/mcpserver/server.py

@@ -82,7 +82,8 @@
 )
 from mcp.server.mcpserver.tools import Tool, ToolManager
 from mcp.server.mcpserver.utilities.context_injection import find_context_parameter
-from mcp.server.mcpserver.utilities.logging import configure_logging, get_logger
+from mcp.server.mcpserver.utilities.logging import configure_logging as _configure_logging
+from mcp.server.mcpserver.utilities.logging import get_logger
 from mcp.server.request_state import RequestStateBoundary, RequestStateSecurity
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
@@ -174,6 +175,7 @@ def __init__(
         extensions: Sequence[Extension] | None = None,
         debug: bool = False,
         log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO",
+        configure_logging: bool = True,
         warn_on_duplicate_resources: bool = True,
         warn_on_duplicate_tools: bool = True,
         warn_on_duplicate_prompts: bool = True,
@@ -256,8 +258,10 @@ def __init__(
             self._token_verifier = ProviderTokenVerifier(auth_server_provider)
         self._custom_starlette_routes: list[Route] = []
 
-        # Configure logging
-        configure_logging(self.settings.log_level)
+        # Configure logging (opt-out via configure_logging=False for applications
+        # that embed MCPServer and manage their own logging setup; see #1656)
+        if configure_logging:
+            _configure_logging(self.settings.log_level)
 
         self._extensions: list[Extension] = []
         for extension in extensions or ():

tests/server/mcpserver/test_configure_logging.py

@@ -0,0 +1,62 @@
+import logging
+
+import pytest
+
+from mcp.server.mcpserver import MCPServer
+
+
+@pytest.fixture
+def clean_root_logger():
+    """Save and restore the root logger's handlers/level around each test.
+
+    `MCPServer` can mutate the root logger via `logging.basicConfig()`, so
+    tests that exercise this behavior must not leak handlers into other
+    tests (or into pytest's own log capturing, which itself attaches a
+    handler to the root logger -- hence comparing against a baseline
+    snapshot below, rather than assuming an empty handler list).
+    """
+    root = logging.getLogger()
+    original_handlers = list(root.handlers)
+    original_level = root.level
+    yield root
+    root.handlers.clear()
+    root.handlers.extend(original_handlers)
+    root.setLevel(original_level)
+
+
+class TestConfigureLogging:
+    def test_default_does_not_override_existing_handlers(self, clean_root_logger):
+        """If an application has already configured the root logger before
+        creating an `MCPServer`, the default behavior must not replace it.
+
+        This matches the standard library's own `logging.basicConfig()`
+        contract: it is a no-op if the root logger already has handlers.
+        """
+        baseline = list(clean_root_logger.handlers)
+        app_handler = logging.StreamHandler()
+        clean_root_logger.addHandler(app_handler)
+
+        MCPServer("test-server")
+
+        assert clean_root_logger.handlers == [*baseline, app_handler]
+
+    def test_configure_logging_false_leaves_root_logger_untouched(self, clean_root_logger):
+        """`configure_logging=False` opts a server out of touching the root
+        logger entirely, so an application can safely configure its own
+        logging before or after constructing the server.
+
+        See: https://github.com/modelcontextprotocol/python-sdk/issues/1656
+        """
+        baseline = list(clean_root_logger.handlers)
+
+        MCPServer("test-server", configure_logging=False)
+        assert clean_root_logger.handlers == baseline
+
+        # The application's own logging setup, performed afterwards, must
+        # take effect (i.e. must not have been pre-empted by the server).
+        # Clear first so `basicConfig` (a no-op if handlers already exist)
+        # is guaranteed to actually apply the new format.
+        clean_root_logger.handlers.clear()
+        logging.basicConfig(format="MYAPP: %(message)s")
+        assert len(clean_root_logger.handlers) == 1
+        assert clean_root_logger.handlers[0].formatter._fmt == "MYAPP: %(message)s"