fix: avoid configuring root logger in mcpserver

Author: Henry Lee

src/mcp/server/mcpserver/utilities/logging.py

@@ -24,16 +24,18 @@ def configure_logging(
     Args:
         level: The log level to use.
     """
-    handlers: list[logging.Handler] = []
-    try:
-        from rich.console import Console
-        from rich.logging import RichHandler
+    logger = logging.getLogger("mcp")
+    if not logger.handlers:
+        try:
+            from rich.console import Console
+            from rich.logging import RichHandler
 
-        handlers.append(RichHandler(console=Console(stderr=True), rich_tracebacks=True))
-    except ImportError:  # pragma: no cover
-        pass
+            handler: logging.Handler = RichHandler(console=Console(stderr=True), rich_tracebacks=True)
+        except ImportError:  # pragma: no cover
+            handler = logging.StreamHandler()
 
-    if not handlers:  # pragma: no cover
-        handlers.append(logging.StreamHandler())
+        handler.setFormatter(logging.Formatter("%(message)s"))
+        logger.addHandler(handler)
 
-    logging.basicConfig(level=level, format="%(message)s", handlers=handlers)
+    logger.setLevel(level)
+    logger.propagate = True

tests/server/mcpserver/test_logging.py

@@ -0,0 +1,54 @@
+import logging
+from collections.abc import Iterator
+
+import pytest
+
+from mcp.server.mcpserver.utilities.logging import configure_logging
+
+LoggingState = tuple[logging.Logger, logging.Logger]
+
+
+@pytest.fixture
+def restore_logging_state() -> Iterator[LoggingState]:
+    root_logger = logging.getLogger()
+    mcp_logger = logging.getLogger("mcp")
+    mcp_handlers = list(mcp_logger.handlers)
+    root_level = root_logger.level
+    mcp_level = mcp_logger.level
+    mcp_propagate = mcp_logger.propagate
+
+    mcp_logger.handlers.clear()
+
+    try:
+        yield root_logger, mcp_logger
+    finally:
+        mcp_logger.handlers[:] = mcp_handlers
+        root_logger.setLevel(root_level)
+        mcp_logger.setLevel(mcp_level)
+        mcp_logger.propagate = mcp_propagate
+
+
+def test_configure_logging_does_not_install_root_handler(restore_logging_state: LoggingState):
+    root_logger, mcp_logger = restore_logging_state
+    root_handlers = list(root_logger.handlers)
+    root_logger.setLevel(logging.WARNING)
+
+    configure_logging("INFO")
+
+    assert root_logger.handlers == root_handlers
+    assert root_logger.level == logging.WARNING
+    assert len(mcp_logger.handlers) == 1
+    assert mcp_logger.level == logging.INFO
+    assert mcp_logger.propagate is True
+
+
+def test_configure_logging_reuses_mcp_handler(restore_logging_state: LoggingState):
+    _, mcp_logger = restore_logging_state
+
+    configure_logging("INFO")
+    handlers: list[logging.Handler] = list(mcp_logger.handlers)
+    configure_logging("DEBUG")
+
+    assert mcp_logger.handlers == handlers
+    assert mcp_logger.level == logging.DEBUG
+    assert mcp_logger.propagate is True