Enable raise errors in tool_manager._load_tools

[huiwy]

Enhancement

There may be errors when listing tools, but i cannot know the existance of errors besides the warning.

from fastmcp import Client

async def main():
    client = Client({
        "a": {
            "command": "uvx",
            "args": ["--version"],
        },
        "b": {
            "command": "uvx",
            "args": ["--version"],
        },
    })

    await client._connect()
    print(await client.list_tools())

if __name__ == "__main__":
    import asyncio
    asyncio.run(main())

output:

[08/18/25 14:06:00] WARNING  Failed to get tools from server: 'FastMCP', mounted at: 'a': Client failed to connect: Connection closed                          tool_manager.py:86
                    WARNING  Failed to get tools from server: 'FastMCP', mounted at: 'b': Client failed to connect: Connection closed                          tool_manager.py:86
[]

Is it possible to add an env variable or an argument to explicitly raise an error to inform me?

For example

tools/tool_manager

    async def _load_tools(self, *, via_server: bool = False) -> dict[str, Tool]:
        try:
        ...
        except Exception as e:
                # Skip failed mounts silently, matches existing behavior
+                if os.envron.get("RAISE_ERROR_GET_TOOLS", "false") == "true": raise e
                logger.warning(
                    f"Failed to get tools from server: {mounted.server.name!r}, mounted at: {mounted.prefix!r}: {e}"
                )
                continue

[jlowin]

Yes, that's a good idea. I think it should default to the current behavior to avoid interruption but we can definitely add a global setting for this behavior. I'd suggest FASTMCP_MOUNTED_COMPONENTS_RAISE_ON_LOAD_ERROR:bool=False or fastmcp.settings.mounted_components_raise_on_load_error. That might be a little too verbose.

/marvin open a PR to add and document this setting; also add respect for this global setting in the 4 relevant locations in the 3 manager files. Do not add this setting to the FastMCP instance itself; it is for global debugging.

[marvin-context-protocol]

Claude finished @jlowin's task —— View job • claude/issue-1531-20250818-1153 • Create PR ➔

---

Implementation Complete ✅

Added FASTMCP_MOUNTED_COMPONENTS_RAISE_ON_LOAD_ERROR setting as requested.

Tasks:

• Set up development environment
• Explore codebase to find manager files and current error handling
• Add global setting to FastMCP settings system
• Implement setting respect in all manager files (4 locations across 3 files)
• Update documentation for new setting
• Run pre-commit hooks and tests
• Commit and push changes
• Provide PR creation link

Summary

Successfully implemented the global setting mounted_components_raise_on_load_error that controls error handling when loading mounted components (tools, resources, prompts). When set to true, errors are raised instead of logged as warnings, enabling better debugging of mount failures.

Changes Made:

• src/fastmcp/settings.py:323-335 - Added new global setting with default False
• src/fastmcp/tools/tool_manager.py:86-87 - Tool loading error handling
• src/fastmcp/resources/resource_manager.py:114-115 - Resource loading error handling
• src/fastmcp/resources/resource_manager.py:167-168 - Resource template loading error handling
• src/fastmcp/prompts/prompt_manager.py:80-81 - Prompt loading error handling
• tests/tools/test_tool_manager.py:1001-1046 - Added comprehensive tests

Usage:

export FASTMCP_MOUNTED_COMPONENTS_RAISE_ON_LOAD_ERROR=true

Or programmatically:

import fastmcp
fastmcp.settings.mounted_components_raise_on_load_error = True

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