PrefectHQ/fastmcp · error · TypeError

uri is required when metadata is not provided

Error message

uri is required when metadata is not provided

What it means

When from_function() is called without a metadata object, it builds one itself, and uri is the one mandatory field of ResourceMeta. Omitting both metadata and uri leaves the resource without an identity, so a TypeError is raised.

Source

Thrown at fastmcp_slim/fastmcp/resources/function_resource.py:141

                    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,
                meta=meta,
                auth=auth,
            )

        uri_obj = AnyUrl(metadata.uri)

        # Get function name - use class name for callable objects
        func_name = (

View on GitHub (pinned to 1f02114297)

Solutions

1. Pass uri="scheme://path" to from_function
2. Or construct a ResourceMeta with uri included and pass metadata=
3. Ensure wrapper functions don't swallow the uri kwarg before calling from_function

Example fix

// before
from_function(read_fn, name="config")
// after
from_function(read_fn, uri="config://app", name="config")

def require_uri(kwargs):
    if kwargs.get("metadata") is None and kwargs.get("uri") is None:
        raise TypeError("from_function requires uri or metadata")

def from_function_args_ok(metadata, uri) -> bool:
    return metadata is not None or uri is not None

try:
    res = FunctionResource.from_function(fn, **kw)
except TypeError:
    res = FunctionResource.from_function(fn, uri=derive_uri(fn.__name__))

When it happens

Trigger: from_function(fn); from_function(fn, name="cfg") with no uri= and no metadata=.

Common situations: Convenience helpers wrapping from_function that forward optional kwargs, dropping the uri; tests constructing FunctionResource without a URI.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

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()
• Cannot pass both 'metadata' and individual parameters to fro

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/d76d0aebbbdc1e0e. Report an issue: GitHub.