Cannot pass both 'metadata' and individual parameters to fro

Error message

Cannot pass both 'metadata' and individual parameters to from_function(). Use metadata alone or individual parameters alone.

What it means

FunctionTool.from_function() accepts either a bundled `metadata` object (ToolMeta) or individual keyword parameters (name, description, tags, etc.), but not both. The library raises this TypeError to prevent ambiguity about which source of configuration wins when both are supplied.

Source

Thrown at fastmcp_slim/fastmcp/tools/function_tool.py:269

                    name,
                    version,
                    title,
                    description,
                    icons,
                    tags,
                    annotations,
                    meta,
                    task,
                    timeout,
                    auth,
                    run_in_thread,
                ]
            )
            or output_schema is not NotSet
        )

        if metadata is not None and individual_params_provided:
            raise TypeError(
                "Cannot pass both 'metadata' and individual parameters to from_function(). "
                "Use metadata alone or individual parameters alone."
            )

        if metadata is None and not individual_params_provided:
            fmeta = get_fastmcp_meta(fn)
            if isinstance(fmeta, ToolMeta):
                metadata = fmeta

        # Build metadata from kwargs if not provided
        if metadata is None:
            metadata = ToolMeta(
                name=name,
                version=version,
                title=title,
                description=description,
                icons=icons,
                tags=tags,

View on GitHub (pinned to 1f02114297)

Solutions

Remove the individual keyword arguments and put all configuration into the metadata object
Remove the metadata argument and configure the tool with individual parameters only
If metadata may or may not be present, pass individual params only when metadata is None

Example fix

// before
FunctionTool.from_function(fn, metadata=meta, name="my_tool", tags={"a"})
// after
FunctionTool.from_function(fn, metadata=meta)  # all config in meta
// or
FunctionTool.from_function(fn, name="my_tool", tags={"a"})

Defensive patterns

Strategy: validation

Validate before calling

def check_from_function_args(metadata=None, **kwargs):
    individual = [k for k, v in kwargs.items() if v is not None]
    if metadata is not None and individual:
        raise TypeError(f"Pass metadata OR individual params, not both: {individual}")

Try / catch

try:
    tool = FunctionTool.from_function(fn, metadata=meta, name="x")
except TypeError as e:
    if "both 'metadata' and individual" in str(e):
        tool = FunctionTool.from_function(fn, metadata=meta)

Prevention

Pick one configuration style (metadata or kwargs) per codebase and enforce it in review
Centralize tool creation in a factory function that chooses the style
When migrating to ToolMeta, delete old kwargs in the same commit

When it happens

Trigger: Calling FunctionTool.from_function(fn, metadata=meta, name='foo') or any combination where metadata is not None AND at least one individual parameter (name, description, tags, annotations, timeout, output_schema, etc.) is also provided.

Common situations: Migrating code from the individual-kwargs API to the metadata API and leaving old kwargs in place; copying an example that uses metadata while a template already passes name=; programmatic tool generation where metadata is conditionally provided.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/a5232f76aa184031. Report an issue: GitHub.