Add mounted_components_raise_on_load_error setting for debugging (#1534)

Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>
Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>

Author: jlowin

src/fastmcp/prompts/prompt_manager.py

@@ -80,6 +80,8 @@ async def _load_prompts(self, *, via_server: bool = False) -> dict[str, Prompt]:
                 logger.warning(
                     f"Failed to get prompts from server: {mounted.server.name!r}, mounted at: {mounted.prefix!r}: {e}"
                 )
+                if settings.mounted_components_raise_on_load_error:
+                    raise
                 continue
 
         # Finally, add local prompts, which always take precedence

src/fastmcp/resources/resource_manager.py

@@ -114,6 +114,8 @@ async def _load_resources(self, *, via_server: bool = False) -> dict[str, Resour
                 logger.warning(
                     f"Failed to get resources from server: {mounted.server.name!r}, mounted at: {mounted.prefix!r}: {e}"
                 )
+                if settings.mounted_components_raise_on_load_error:
+                    raise
                 continue
 
         # Finally, add local resources, which always take precedence
@@ -165,6 +167,8 @@ async def _load_resource_templates(
                 logger.warning(
                     f"Failed to get templates from server: {mounted.server.name!r}, mounted at: {mounted.prefix!r}: {e}"
                 )
+                if settings.mounted_components_raise_on_load_error:
+                    raise
                 continue
 
         # Finally, add local templates, which always take precedence

src/fastmcp/settings.py

@@ -320,6 +320,20 @@ def normalize_log_level(cls, v):
         ),
     ] = True
 
+    mounted_components_raise_on_load_error: Annotated[
+        bool,
+        Field(
+            default=False,
+            description=inspect.cleandoc(
+                """
+                If True, errors encountered when loading mounted components (tools, resources, prompts)
+                will be raised instead of logged as warnings. This is useful for debugging
+                but will interrupt normal operation.
+                """
+            ),
+        ),
+    ] = False
+
 
 def __getattr__(name: str):
     """

src/fastmcp/tools/tool_manager.py

@@ -86,6 +86,8 @@ async def _load_tools(self, *, via_server: bool = False) -> dict[str, Tool]:
                 logger.warning(
                     f"Failed to get tools from server: {mounted.server.name!r}, mounted at: {mounted.prefix!r}: {e}"
                 )
+                if settings.mounted_components_raise_on_load_error:
+                    raise
                 continue
 
         # Finally, add local tools, which always take precedence

tests/tools/test_tool_manager.py

@@ -13,7 +13,7 @@
 from fastmcp.tools import FunctionTool, ToolManager
 from fastmcp.tools.tool import Tool
 from fastmcp.tools.tool_transform import ArgTransformConfig, ToolTransformConfig
-from fastmcp.utilities.tests import caplog_for_fastmcp
+from fastmcp.utilities.tests import caplog_for_fastmcp, temporary_settings
 from fastmcp.utilities.types import Image
 
 
@@ -996,3 +996,42 @@ async def async_buggy_tool(x: int) -> int:
         # Exception message should contain the tool name but not the internal details
         assert "Error calling tool 'async_buggy_tool'" in str(excinfo.value)
         assert "Internal async error details" not in str(excinfo.value)
+
+
+class TestMountedComponentsRaiseOnLoadError:
+    """Test the mounted_components_raise_on_load_error setting."""
+
+    async def test_mounted_components_raise_on_load_error_default_false(self):
+        """Test that by default, mounted component load errors are warned and not raised."""
+        import fastmcp
+
+        # Ensure default setting is False
+        assert fastmcp.settings.mounted_components_raise_on_load_error is False
+
+        parent_mcp = FastMCP("ParentServer")
+        child_mcp = FastMCP("FailingChildServer")
+
+        # Create a failing mounted server by corrupting it
+        parent_mcp.mount(child_mcp, prefix="child")
+        # Corrupt the child server to make it fail during tool loading
+        child_mcp._tool_manager._mounted_servers.append("invalid")  # type: ignore
+
+        # Should not raise, just warn
+        tools = await parent_mcp._tool_manager.list_tools()
+        assert isinstance(tools, list)  # Should return empty list, not raise
+
+    async def test_mounted_components_raise_on_load_error_true(self):
+        """Test that when enabled, mounted component load errors are raised."""
+        parent_mcp = FastMCP("ParentServer")
+        child_mcp = FastMCP("FailingChildServer")
+
+        # Create a failing mounted server
+        parent_mcp.mount(child_mcp, prefix="child")
+        # Corrupt the child server to make it fail during tool loading
+        child_mcp._tool_manager._mounted_servers.append("invalid")  # type: ignore
+
+        # Use temporary settings context manager
+        with temporary_settings(mounted_components_raise_on_load_error=True):
+            # Should raise the exception
+            with pytest.raises(AttributeError, match=""):
+                await parent_mcp._tool_manager.list_tools()