PrefectHQ/fastmcp · error · TypeError

Expected Resource, ResourceTemplate, or @resource-decorated

Error message

Expected Resource, ResourceTemplate, or @resource-decorated function, got {type(resource).__name__}. Use @resource('uri') decorator or pass a Resource/ResourceTemplate instance.

What it means

FastMCP's add_resource() accepts only a Resource instance, a ResourceTemplate instance, or a function already decorated with @resource. The argument passed had some other type, so the server cannot turn it into a registerable resource component and aborts with a TypeError naming the offending type.

Source

Thrown at fastmcp_slim/fastmcp/server/providers/local_provider/decorators/resources.py:93

                        security=meta.security,
                    )
                else:
                    resource = Resource.from_function(
                        fn=resource,
                        uri=meta.uri,
                        name=meta.name,
                        version=meta.version,
                        title=meta.title,
                        description=meta.description,
                        icons=meta.icons,
                        mime_type=meta.mime_type,
                        tags=meta.tags,
                        annotations=meta.annotations,
                        meta=meta.meta,
                        auth=meta.auth,
                    )
            else:
                raise TypeError(
                    f"Expected Resource, ResourceTemplate, or @resource-decorated function, got {type(resource).__name__}. "
                    "Use @resource('uri') decorator or pass a Resource/ResourceTemplate instance."
                )
        self._add_component(resource)
        if not enabled:
            self.disable(keys={resource.key})
        return resource

    def add_template(
        self: LocalProvider, template: ResourceTemplate
    ) -> ResourceTemplate:
        """Add a resource template to this provider's storage."""
        return self._add_component(template)

    def resource(
        self: LocalProvider,
        uri: str,
        *,

View on GitHub (pinned to 1f02114297)

Solutions

Decorate the function with @resource('uri') and register the decorated result.
If you already have a function, construct an explicit FunctionResource and pass that.
For dynamic URI patterns, create a ResourceTemplate instead and add it via add_template/add_resource.

Example fix

// before
mcp.add_resource(get_config)

// after
from fastmcp.resources import resource

@resource('config://app')
def get_config() -> str:
    ...

mcp.add_resource(get_config)

Defensive patterns

Strategy: type-guard

Validate before calling

from fastmcp.resources import Resource, ResourceTemplate
import inspect

def can_register(obj) -> bool:
    return isinstance(obj, (Resource, ResourceTemplate)) or (
        inspect.isroutine(obj) and getattr(obj, '__fastmcp__', None) is not None
    )

assert can_register(candidate), 'decorate with @resource first'
mcp.add_resource(candidate)

Type guard

def is_registerable_resource(obj) -> bool:
    from fastmcp.resources import Resource, ResourceTemplate
    return isinstance(obj, (Resource, ResourceTemplate)) or (
        callable(obj) and hasattr(obj, '__fastmcp__')
    )

Try / catch

try:
    mcp.add_resource(obj)
except TypeError as e:
    if 'Expected Resource' in str(e):
        obj = resource('my-scheme://uri')(obj)
        mcp.add_resource(obj)
    else:
        raise

Prevention

Always decorate functions with @resource('uri') before passing to add_resource.
Never pass raw functions, strings, or dicts to add_resource.
Use add_template for URI templates rather than hand-built objects.

When it happens

Trigger: Calling mcp.add_resource() with a bare undecorated function, a method, a string URI, a dict, a functools.partial, or a class instead of a Resource/ResourceTemplate/@resource-decorated function.

Common situations: Migrating from older FastMCP APIs where add_resource(fn, uri=...) auto-wrapped functions; copying examples that show @resource but forgetting to apply the decorator; passing the result of a function call instead of the resource object itself.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/77e2861d85712fdc. Report an issue: GitHub.