PrefectHQ/fastmcp · error · NotImplementedError

Subclasses must implement create_resource(). Use FunctionRes

Error message

Subclasses must implement create_resource(). Use FunctionResourceTemplate for task support.

What it means

ResourceTemplate.create_resource() materializes a concrete Resource from a matched URI and parameters; the base implementation deliberately does not support it (background-task-capable reads require create_resource, which only FunctionResourceTemplate provides). Subclasses must either implement it or not rely on the code path that calls it.

Source

Thrown at fastmcp_slim/fastmcp/resources/template.py:302

    async def _read(self, uri: str, params: dict[str, Any]) -> ResourceResult:
        """Server entry point for template reads.

        The server calls this instead of create_resource()/read() directly so
        subclasses can customize dispatch (e.g. FastMCPProviderResourceTemplate
        delegates to child-server middleware).
        """
        resource = await self.create_resource(uri, params)
        result = await resource.read()
        return self.convert_result(result)

    async def create_resource(self, uri: str, params: dict[str, Any]) -> Resource:
        """Create a resource from the template with the given parameters.

        The base implementation does not support background tasks.
        Use FunctionResourceTemplate for task support.
        """
        raise NotImplementedError(
            "Subclasses must implement create_resource(). "
            "Use FunctionResourceTemplate for task support."
        )

    def to_mcp_template(
        self,
        **overrides: Any,
    ) -> SDKResourceTemplate:
        """Convert the resource template to an SDKResourceTemplate."""

        return SDKResourceTemplate(
            name=overrides.get("name", self.name),
            uri_template=overrides.get("uriTemplate", self.uri_template),
            description=overrides.get("description", self.description),
            mime_type=overrides.get("mimeType", self.mime_type),
            title=overrides.get("title", self.title),
            icons=overrides.get("icons", self.icons),
            annotations=overrides.get("annotations", self.annotations),

View on GitHub (pinned to 1f02114297)

Solutions

1. Implement async def create_resource(self, uri, params) returning a configured Resource instance
2. Or subclass FunctionResourceTemplate, which provides a working create_resource for functions
3. If you only need simple reads and no tasks, ensure your code path calls read() directly, not create_resource()

Example fix

// before
class T(ResourceTemplate):
    async def read(self, args): return "data"
// after
class T(ResourceTemplate):
    async def read(self, args): return "data"
    async def create_resource(self, uri, params):
        return FunctionResource(fn=lambda: self.read(params), uri=uri)

import inspect
assert inspect.iscoroutinefunction(MyTemplate.create_resource), "must implement create_resource()"

def implements_create_resource(cls) -> bool:
    return getattr(cls.create_resource, "__qualname__", "") != "ResourceTemplate.create_resource"

try:
    resource = await template.create_resource(uri, params)
except NotImplementedError:
    resource = FunctionResourceTemplate(fn, uri_template=template.uri_template)

When it happens

Trigger: Custom ResourceTemplate subclass where _read reaches create_resource() without an override; using a bare ResourceTemplate subclass in a context requiring task-capable resource creation.

Common situations: Authors of custom templates who only implemented read() but the caller path (task-enabled reads) invokes create_resource; forgetting the docstring's guidance to use FunctionResourceTemplate.

Related errors

• Subclasses must implement read() or override create_resource
• Subclasses must implement render()
• Subclasses must implement read()
• Invalid boolean value for {param_name}: {param_value!r}
• Subclasses must implement verify_token

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/cd8622339454e641. Report an issue: GitHub.