PrefectHQ/fastmcp · error · TypeError

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

FunctionResource.from_function supports two mutually exclusive configuration styles: a single metadata=ResourceMeta(...) object, or the individual keyword parameters (uri, name, title, description, etc.). Passing metadata together with any individual parameter is ambiguous, so it raises this TypeError.

Source

Thrown at fastmcp_slim/fastmcp/resources/function_resource.py:133

                x is not None
                for x in [
                    name,
                    version,
                    title,
                    description,
                    icons,
                    mime_type,
                    tags,
                    annotations,
                    meta,
                    auth,
                ]
            )
            or uri is not None
        )

        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."
            )

        # Build metadata from kwargs if not provided
        if metadata is None:
            if uri is None:
                raise TypeError("uri is required when metadata is not provided")
            metadata = ResourceMeta(
                uri=str(uri),
                name=name,
                version=version,
                title=title,
                description=description,
                icons=icons,
                tags=tags,
                mime_type=mime_type,
                annotations=annotations,

View on GitHub (pinned to 1f02114297)

Solutions

1. Move all parameters into a single ResourceMeta(...) and pass only metadata=
2. Or delete the metadata argument and keep the individual keyword arguments
3. Check for falsy-but-set kwargs like name=None defaulting patterns in wrapper code

Example fix

// before
from_function(fn, metadata=ResourceMeta(uri="data://x"), description="d")
// after
from_function(fn, metadata=ResourceMeta(uri="data://x", description="d"))

def check_from_function_kwargs(**kw):
    individual = [k for k in ("uri", "name", "title", "description", "version") if kw.get(k) is not None]
    if kw.get("metadata") is not None and individual:
        raise TypeError("metadata and individual params are mutually exclusive")

def is_valid_from_function_call(metadata, **kwargs) -> bool:
    individual = any(v is not None for v in kwargs.values())
    return not (metadata is not None and individual)

try:
    res = FunctionResource.from_function(fn, metadata=meta, uri=uri)
except TypeError as e:
    res = FunctionResource.from_function(fn, metadata=meta)  # drop stray kwargs

When it happens

Trigger: from_function(fn, metadata=ResourceMeta(uri=...), uri="data://x"); from_function(fn, metadata=m, name="n"); any combination where metadata is not None AND (name or title or description or version or uri ...) is not None.

Common situations: Refactoring calls from individual params to metadata but leaving one legacy kwarg behind; copying example code that mixes both styles.

Related errors

• contents[{i}] must be ResourceContent, got {type(item).__nam
• contents must be str, bytes, or list[ResourceContent], got {
• Either name or uri must be provided
• Subclasses must implement read()
• uri is required when metadata is not provided

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/6c3d5fe1ac018aac. Report an issue: GitHub.