Avoid loading MCP and CLI stacks during lightweight imports (#4763)

fastmcp_slim/fastmcp/__init__.py

@@ -6,15 +6,13 @@ from importlib.metadata import PackageNotFoundError, version as _version
 from typing import TYPE_CHECKING
 
 from fastmcp import _install_hints
+from fastmcp._warnings import FastMCPDeprecationWarning
 from fastmcp.settings import Settings
 from fastmcp.utilities.logging import configure_logging as _configure_logging
 
 if TYPE_CHECKING:
     from fastmcp.client import Client as Client
     from fastmcp.apps.app import FastMCPApp as FastMCPApp
-    from fastmcp.exceptions import (
-        FastMCPDeprecationWarning as FastMCPDeprecationWarning,
-    )
     from fastmcp.server.context import Context as Context
     from fastmcp.server.server import FastMCP as FastMCP
 
@@ -39,12 +37,7 @@ except PackageNotFoundError:
     __version__ = _version("fastmcp")
 
 if settings.deprecation_warnings:
-    try:
+    warnings.simplefilter("default", FastMCPDeprecationWarning)
-        from fastmcp.exceptions import FastMCPDeprecationWarning
-    except ImportError:
-        pass
-    else:
-        warnings.simplefilter("default", FastMCPDeprecationWarning)
 
 
 # --- Lazy imports for performance (see #3292) ---
@@ -81,10 +74,6 @@ def __getattr__(name: str) -> object:
             raise ImportError(_install_hints.APP_SUPPORT) from exc
 
         return FastMCPApp
-    if name == "FastMCPDeprecationWarning":
-        from fastmcp.exceptions import FastMCPDeprecationWarning
-
-        return FastMCPDeprecationWarning
     if name == "client":
         try:
             return importlib.import_module("fastmcp.client")

fastmcp_slim/fastmcp/_compat.py

@@ -33,7 +33,7 @@ import warnings
 
 import mcp_types
 
-from fastmcp.exceptions import FastMCPDeprecationWarning
+from fastmcp._warnings import FastMCPDeprecationWarning
 
 # Map each SDK model class to the camelCase -> snake_case field reads we bridge.
 # Limited to fields FastMCP users actually read (docs boundary inventory).

fastmcp_slim/fastmcp/_warnings.py

@@ -0,0 +1,10 @@
+"""Warning types that can be imported without loading FastMCP's exception stack."""
+
+
+class FastMCPDeprecationWarning(DeprecationWarning):
+    """Deprecation warning for FastMCP APIs.
+
+    Subclass of DeprecationWarning so that standard warning filters
+    still apply, but FastMCP can selectively enable its own warnings
+    without affecting other libraries in the process.
+    """

fastmcp_slim/fastmcp/exceptions.py

@@ -5,6 +5,8 @@ from typing import Any
 
 from mcp_types import INTERNAL_ERROR, INVALID_PARAMS, ErrorData
 
+from fastmcp import _warnings
+
 try:
     from mcp import MCPError
 except ImportError:
@@ -30,14 +32,7 @@ except ImportError:
 # see the migration notes.
 McpError = MCPError
 
-
+FastMCPDeprecationWarning = _warnings.FastMCPDeprecationWarning
-class FastMCPDeprecationWarning(DeprecationWarning):
-    """Deprecation warning for FastMCP APIs.
-
-    Subclass of DeprecationWarning so that standard warning filters
-    still apply, but FastMCP can selectively enable its own warnings
-    without affecting other libraries in the process.
-    """
 
 
 class FastMCPError(Exception):

fastmcp_slim/fastmcp/server/mixins/transport.py

@@ -28,7 +28,6 @@ from fastmcp.server.http import (
 from fastmcp.server.providers.base import Provider
 from fastmcp.server.providers.fastmcp_provider import FastMCPProvider
 from fastmcp.server.providers.wrapped_provider import _WrappedProvider
-from fastmcp.utilities.cli import log_server_banner
 from fastmcp.utilities.logging import get_logger, temporary_log_level
 
 if TYPE_CHECKING:
@@ -230,6 +229,8 @@ class TransportMixin:
 
         # Display server banner
         if show_banner:
+            from fastmcp.utilities.cli import log_server_banner
+
             log_server_banner(server=self)
 
         token = set_transport("stdio")
@@ -337,6 +338,8 @@ class TransportMixin:
 
         # Display server banner
         if show_banner:
+            from fastmcp.utilities.cli import log_server_banner
+
             log_server_banner(server=self)
         uvicorn_config_from_user = uvicorn_config or {}
 

fastmcp_slim/fastmcp/utilities/logging.py

@@ -1,7 +1,9 @@
 """Logging utilities for FastMCP."""
 
 import contextlib
+import importlib.util
 import logging
+from pathlib import Path
 from typing import Any, Literal, cast
 
 from rich.console import Console
@@ -11,6 +13,17 @@ from typing_extensions import override
 import fastmcp
 
 
+def _get_package_path(package: str) -> str | None:
+    """Return a package directory without importing the package."""
+    try:
+        spec = importlib.util.find_spec(package)
+    except ImportError:
+        return None
+    if spec is None or spec.origin is None:
+        return None
+    return str(Path(spec.origin).parent)
+
+
 def get_logger(name: str) -> logging.Logger:
     """Get a logger nested under FastMCP namespace.
 
@@ -83,14 +96,11 @@ def configure_logging(
     # no path or level name to maximize width available for the traceback
     # suppress framework frames and limit the number of frames to 3
 
-    import pydantic
+    tracebacks_suppress = [
-
+        package_path
-    try:
+        for package in ("fastmcp", "mcp", "pydantic")
-        import mcp
+        if (package_path := _get_package_path(package)) is not None
-    except ImportError:
+    ]
-        tracebacks_suppress = [fastmcp, pydantic]
-    else:
-        tracebacks_suppress = [fastmcp, mcp, pydantic]
 
     # Build traceback kwargs with defaults that can be overridden
     traceback_kwargs = {

tests/server/http/test_startup_imports.py

@@ -9,6 +9,51 @@ import textwrap
 import pytest
 
 
+@pytest.mark.subprocess_heavy
+def test_root_import_does_not_load_mcp_sdk() -> None:
+    script = textwrap.dedent(
+        """
+        import sys
+
+        import fastmcp
+
+        assert fastmcp.settings is not None
+        assert "mcp" not in sys.modules
+        assert "fastmcp.exceptions" not in sys.modules
+        """
+    )
+
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        capture_output=True,
+        text=True,
+    )
+
+    assert result.returncode == 0, result.stderr
+
+
+@pytest.mark.subprocess_heavy
+def test_server_import_does_not_load_cli() -> None:
+    script = textwrap.dedent(
+        """
+        import sys
+
+        from fastmcp import FastMCP
+
+        assert FastMCP is not None
+        assert "fastmcp.utilities.cli" not in sys.modules
+        """
+    )
+
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        capture_output=True,
+        text=True,
+    )
+
+    assert result.returncode == 0, result.stderr
+
+
 @pytest.mark.subprocess_heavy
 def test_default_http_app_does_not_load_opt_in_integrations() -> None:
     script = textwrap.dedent(

tests/test_compat.py

@@ -10,6 +10,7 @@ from mcp import MCPError as SDKMCPError
 import fastmcp
 import fastmcp._compat as _compat
 from fastmcp import Client, FastMCP
+from fastmcp import FastMCPDeprecationWarning as PublicWarning
 from fastmcp.client.transports import FastMCPTransport
 from fastmcp.exceptions import FastMCPDeprecationWarning, MCPError, McpError
 
@@ -30,6 +31,10 @@ def _reset_warn_once() -> None:
     _compat.install()
 
 
+def test_deprecation_warning_is_same_from_public_imports() -> None:
+    assert PublicWarning is FastMCPDeprecationWarning
+
+
 @pytest.fixture(autouse=True)
 def fresh_shims():
     _reset_warn_once()

tests/utilities/test_logging.py

@@ -1,4 +1,7 @@
 import logging
+from pathlib import Path
+
+from rich.logging import RichHandler
 
 import fastmcp
 from fastmcp.utilities.logging import configure_logging, get_logger
@@ -42,6 +45,19 @@ def test_configure_logging_with_traceback_kwargs():
     assert len(logger.handlers) == 2  # One for normal logs, one for tracebacks
 
 
+def test_configure_logging_suppresses_framework_package_paths():
+    configure_logging(enable_rich_tracebacks=True)
+
+    traceback_handler = logging.getLogger("fastmcp").handlers[-1]
+    assert isinstance(traceback_handler, RichHandler)
+    suppressed_packages = {
+        Path(path).name
+        for path in traceback_handler.tracebacks_suppress
+        if isinstance(path, str)
+    }
+    assert {"fastmcp", "mcp", "pydantic"} <= suppressed_packages
+
+
 def test_configure_logging_traceback_defaults_can_be_overridden():
     """Test that default traceback settings can be overridden by kwargs."""
     configure_logging(
@@ -91,8 +107,6 @@ def test_configure_logging_with_rich_enabled():
         # Should have two handlers when rich logging is enabled (normal + traceback)
         assert len(logger.handlers) == 2
         # Both should be RichHandler instances
-        from rich.logging import RichHandler
-
         assert all(isinstance(h, RichHandler) for h in logger.handlers)
     finally:
         fastmcp.settings.enable_rich_logging = original_enable_rich