Add logging infrastructure with MCPD_LOG_LEVEL support

Implements SDK logging supporting MCPD_LOG_LEVEL environment variable to help users diagnose issues with server health and tool availability.

- Add LogLevel enum and Logger protocol in _logger.py
- Integrate logger into McpdClient with lazy evaluation pattern
- Log warnings for non-existent servers, unhealthy servers, and tool fetch failures
- Export Logger and LogLevel in public API
- Add comprehensive test coverage for logging functionality
- Document logging configuration and custom logger usage in README

Logging is disabled by default to avoid contaminating stdout/stderr, particularly important for MCP server contexts where stdout is used for JSON-RPC communication.

Author: peteski22

README.md

@@ -157,6 +157,7 @@ from mcpd import McpdClient
 # Initialize the client with your mcpd API endpoint.
 # api_key is optional and sends an 'MCPD-API-KEY' header.
 # server_health_cache_ttl is optional and sets the time in seconds to cache a server health response.
+# logger is optional and allows you to provide a custom logger implementation (see Logging section).
 client = McpdClient(api_endpoint="http://localhost:8090", api_key="optional-key", server_health_cache_ttl=10)
 ```
 
@@ -182,6 +183,94 @@ client = McpdClient(api_endpoint="http://localhost:8090", api_key="optional-key"
 
 * `client.is_server_healthy(server_name: str) -> bool` - Checks if the specified server is healthy and can handle requests.
 
+## Logging
+
+The SDK includes built-in logging infrastructure that can be enabled via the `MCPD_LOG_LEVEL` environment variable. Logging is disabled by default to avoid contaminating stdout/stderr.
+
+> [!IMPORTANT]
+> Only enable `MCPD_LOG_LEVEL` in non-MCP-server contexts. MCP servers can use stdout for JSON-RPC communication,
+> and any logging output will break the protocol.
+
+### Available Log Levels
+
+Set the `MCPD_LOG_LEVEL` environment variable to one of the following values (from most to least verbose):
+
+* `trace` - Most verbose logging (includes all levels below)
+* `debug` - Debug-level logging
+* `info` - Informational logging
+* `warn` - Warning-level logging (recommended for most use cases)
+* `error` - Error-level logging only
+* `off` - Disable all logging (default)
+
+### Example Usage
+
+```bash
+# Enable warning-level logging
+export MCPD_LOG_LEVEL=warn
+python your_script.py
+```
+
+```python
+from mcpd import McpdClient
+
+# Warnings will be logged to stderr when MCPD_LOG_LEVEL=warn
+client = McpdClient(api_endpoint="http://localhost:8090")
+
+# For example, the SDK will log warnings for:
+# - Non-existent servers when calling agent_tools()
+# - Unhealthy servers when calling agent_tools()
+# - Servers that become unavailable during tool fetching
+```
+
+### Custom Logger
+
+You can provide your own logger implementation that implements the `Logger` protocol:
+
+```python
+from mcpd import McpdClient, Logger
+
+class CustomLogger:
+    def trace(self, msg: str, *args: object) -> None:
+        print(f"TRACE: {msg % args}")
+
+    def debug(self, msg: str, *args: object) -> None:
+        print(f"DEBUG: {msg % args}")
+
+    def info(self, msg: str, *args: object) -> None:
+        print(f"INFO: {msg % args}")
+
+    def warn(self, msg: str, *args: object) -> None:
+        print(f"WARN: {msg % args}")
+
+    def error(self, msg: str, *args: object) -> None:
+        print(f"ERROR: {msg % args}")
+
+# Use custom logger
+client = McpdClient(
+    api_endpoint="http://localhost:8090",
+    logger=CustomLogger()
+)
+```
+
+You can also provide a partial logger implementation. Any omitted methods will fall back to the default logger (which respects `MCPD_LOG_LEVEL`):
+
+```python
+class PartialLogger:
+    def warn(self, msg: str, *args: object) -> None:
+        # Custom warning handler
+        print(f"CUSTOM WARN: {msg % args}")
+
+    def error(self, msg: str, *args: object) -> None:
+        # Custom error handler
+        print(f"CUSTOM ERROR: {msg % args}")
+    # trace, debug, info use default logger (respects MCPD_LOG_LEVEL)
+
+client = McpdClient(
+    api_endpoint="http://localhost:8090",
+    logger=PartialLogger()
+)
+```
+
 ## Error Handling
 
 All SDK-level errors, including HTTP and connection errors, will raise a `McpdError` exception.

src/mcpd/__init__.py

@@ -12,6 +12,7 @@
 - Comprehensive error handling: Detailed exceptions for different failure modes
 """
 
+from ._logger import Logger, LogLevel
 from .exceptions import (
     AuthenticationError,
     ConnectionError,
@@ -28,6 +29,8 @@
 __all__ = [
     "McpdClient",
     "HealthStatus",
+    "Logger",
+    "LogLevel",
     "McpdError",
     "AuthenticationError",
     "ConnectionError",

src/mcpd/_logger.py

@@ -0,0 +1,268 @@
+"""Internal logging infrastructure for the mcpd SDK.
+
+This module provides a logging shim controlled by the MCPD_LOG_LEVEL environment
+variable. Logging is disabled by default to avoid contaminating stdout/stderr in
+MCP server contexts.
+
+CRITICAL: Only enable MCPD_LOG_LEVEL in non-MCP-server contexts. MCP servers use
+stdout for JSON-RPC communication, and any logging output will break the protocol.
+"""
+
+import logging
+import os
+from enum import Enum
+from typing import Protocol
+
+
+class LogLevel(str, Enum):
+    """Valid log level values for MCPD_LOG_LEVEL environment variable.
+
+    Aligns with mcpd server binary log levels for consistency across the mcpd ecosystem.
+    """
+
+    TRACE = "trace"
+    DEBUG = "debug"
+    INFO = "info"
+    WARN = "warn"
+    ERROR = "error"
+    OFF = "off"
+
+
+class Logger(Protocol):
+    """Logger protocol defining the SDK's logging interface.
+
+    This protocol matches standard logging levels and allows custom logger injection.
+    All methods accept a message and optional formatting arguments.
+    """
+
+    def trace(self, msg: str, *args: object) -> None:
+        """Log a trace-level message (most verbose)."""
+        ...
+
+    def debug(self, msg: str, *args: object) -> None:
+        """Log a debug-level message."""
+        ...
+
+    def info(self, msg: str, *args: object) -> None:
+        """Log an info-level message."""
+        ...
+
+    def warn(self, msg: str, *args: object) -> None:
+        """Log a warning-level message."""
+        ...
+
+    def error(self, msg: str, *args: object) -> None:
+        """Log an error-level message."""
+        ...
+
+
+# Custom TRACE level (below DEBUG=10).
+_TRACE = 5
+logging.addLevelName(_TRACE, "TRACE")
+
+_RANKS: dict[str, int] = {
+    LogLevel.TRACE.value: _TRACE,
+    LogLevel.DEBUG.value: logging.DEBUG,
+    LogLevel.INFO.value: logging.INFO,
+    LogLevel.WARN.value: logging.WARNING,
+    "warning": logging.WARNING,  # Alias for backwards compatibility.
+    LogLevel.ERROR.value: logging.ERROR,
+    LogLevel.OFF.value: 1000,  # Higher than any standard level.
+}
+
+
+def _resolve_log_level(raw: str | None) -> str:
+    """Resolve the log level from environment variable value.
+
+    Args:
+        raw: Raw value from MCPD_LOG_LEVEL environment variable.
+
+    Returns:
+        Valid log level string matching LogLevel enum values.
+        Returns LogLevel.OFF.value if raw is None, empty, or not a valid level.
+    """
+    candidate = raw.lower() if raw else None
+    return candidate if candidate and candidate in _RANKS else LogLevel.OFF.value
+
+
+def _get_level() -> str:
+    """Get the current log level from environment variable (lazy evaluation).
+
+    This function is called on each log statement to support dynamic level changes.
+
+    Note:
+        Dynamic level changes can facilitate testing.
+
+    Returns:
+        The resolved log level string.
+    """
+    return _resolve_log_level(os.getenv("MCPD_LOG_LEVEL"))
+
+
+def _create_default_logger() -> Logger:
+    """Create the default logger with lazy level evaluation.
+
+    Returns:
+        A Logger instance that checks MCPD_LOG_LEVEL on each log call,
+        enabling dynamic level changes without module reloading.
+    """
+    # Create logger and handler once (not per-call).
+    _logger = logging.getLogger(__name__)
+
+    if not _logger.handlers:
+        # Add stderr handler (default for StreamHandler).
+        handler = logging.StreamHandler()
+        handler.setFormatter(logging.Formatter("%(levelname)s: %(message)s"))
+        _logger.addHandler(handler)
+        _logger.propagate = False
+
+    class _DefaultLogger:
+        """Default logger that checks level on each call (lazy evaluation)."""
+
+        def trace(self, msg: str, *args: object) -> None:
+            """Log trace-level message."""
+            lvl = _get_level()
+            if lvl != LogLevel.OFF.value and _RANKS[lvl] <= _RANKS[LogLevel.TRACE.value]:
+                _logger.setLevel(_TRACE)
+                _logger.log(_TRACE, msg, *args)
+
+        def debug(self, msg: str, *args: object) -> None:
+            """Log debug-level message."""
+            lvl = _get_level()
+            if lvl != LogLevel.OFF.value and _RANKS[lvl] <= _RANKS[LogLevel.DEBUG.value]:
+                _logger.setLevel(logging.DEBUG)
+                _logger.debug(msg, *args)
+
+        def info(self, msg: str, *args: object) -> None:
+            """Log info-level message."""
+            lvl = _get_level()
+            if lvl != LogLevel.OFF.value and _RANKS[lvl] <= _RANKS[LogLevel.INFO.value]:
+                _logger.setLevel(logging.INFO)
+                _logger.info(msg, *args)
+
+        def warn(self, msg: str, *args: object) -> None:
+            """Log warning-level message."""
+            lvl = _get_level()
+            if lvl != LogLevel.OFF.value and _RANKS[lvl] <= _RANKS[LogLevel.WARN.value]:
+                _logger.setLevel(logging.WARNING)
+                _logger.warning(msg, *args)
+
+        def error(self, msg: str, *args: object) -> None:
+            """Log error-level message."""
+            lvl = _get_level()
+            if lvl != LogLevel.OFF.value and _RANKS[lvl] <= _RANKS[LogLevel.ERROR.value]:
+                _logger.setLevel(logging.ERROR)
+                _logger.error(msg, *args)
+
+    return _DefaultLogger()
+
+
+class _PartialLoggerWrapper:
+    """Wrapper that combines partial custom logger with default logger fallback.
+
+    This enables partial logger implementations where users can override specific
+    methods while keeping defaults for others.
+    """
+
+    def __init__(self, custom: object, default: Logger):
+        """Initialize the wrapper.
+
+        Args:
+            custom: Partial logger implementation (may not have all methods).
+            default: Default logger to use for missing methods.
+        """
+        self._custom = custom
+        self._default = default
+
+    def trace(self, msg: str, *args: object) -> None:
+        """Log trace-level message."""
+        if hasattr(self._custom, LogLevel.TRACE.value):
+            self._custom.trace(msg, *args)
+        else:
+            self._default.trace(msg, *args)
+
+    def debug(self, msg: str, *args: object) -> None:
+        """Log debug-level message."""
+        if hasattr(self._custom, LogLevel.DEBUG.value):
+            self._custom.debug(msg, *args)
+        else:
+            self._default.debug(msg, *args)
+
+    def info(self, msg: str, *args: object) -> None:
+        """Log info-level message."""
+        if hasattr(self._custom, LogLevel.INFO.value):
+            self._custom.info(msg, *args)
+        else:
+            self._default.info(msg, *args)
+
+    def warn(self, msg: str, *args: object) -> None:
+        """Log warning-level message."""
+        if hasattr(self._custom, LogLevel.WARN.value):
+            self._custom.warn(msg, *args)
+        else:
+            self._default.warn(msg, *args)
+
+    def error(self, msg: str, *args: object) -> None:
+        """Log error-level message."""
+        if hasattr(self._custom, LogLevel.ERROR.value):
+            self._custom.error(msg, *args)
+        else:
+            self._default.error(msg, *args)
+
+
+def create_logger(impl: Logger | object | None = None) -> Logger:
+    """Create a logger, optionally using a custom implementation.
+
+    This function allows SDK users to inject their own logger implementation.
+    Supports partial implementations - any omitted methods will fall back to the
+    default logger, which respects the MCPD_LOG_LEVEL environment variable.
+
+    Args:
+        impl: Optional custom Logger implementation or partial implementation.
+              If None, uses the default logger controlled by MCPD_LOG_LEVEL.
+              If partially provided, custom methods are used and omitted methods
+              fall back to default logger (which respects MCPD_LOG_LEVEL).
+
+    Returns:
+        A Logger instance with all methods implemented.
+
+    Example:
+        >>> # Use default logger (controlled by MCPD_LOG_LEVEL).
+        >>> logger = create_logger()
+        >>>
+        >>> # Full custom logger.
+        >>> class MyLogger:
+        ...     def trace(self, msg, *args): pass
+        ...     def debug(self, msg, *args): pass
+        ...     def info(self, msg, *args): pass
+        ...     def warn(self, msg, *args): print(f"WARN: {msg % args}")
+        ...     def error(self, msg, *args): print(f"ERROR: {msg % args}")
+        >>> logger = create_logger(MyLogger())
+        >>>
+        >>> # Partial logger: custom warn/error, default (MCPD_LOG_LEVEL-aware) for others.
+        >>> class PartialLogger:
+        ...     def warn(self, msg, *args): print(f"WARN: {msg % args}")
+        ...     def error(self, msg, *args): print(f"ERROR: {msg % args}")
+        ...     # trace, debug, info use default logger (respects MCPD_LOG_LEVEL)
+        >>> logger = create_logger(PartialLogger())
+    """
+    if impl is None:
+        return _default_logger
+
+    # Check if it's a full Logger implementation (has all required methods).
+    required_methods = [
+        LogLevel.TRACE.value,
+        LogLevel.DEBUG.value,
+        LogLevel.INFO.value,
+        LogLevel.WARN.value,
+        LogLevel.ERROR.value,
+    ]
+    if all(hasattr(impl, method) for method in required_methods):
+        return impl
+
+    # Partial implementation - wrap with fallback to default logger.
+    return _PartialLoggerWrapper(impl, _default_logger)
+
+
+# Module-level default logger (created at import time).
+_default_logger: Logger = _create_default_logger()