PrefectHQ/fastmcp · error · NotImplementedError

Subclasses must implement render()

Error message

Subclasses must implement render()

What it means

The base Prompt class's render() is intentionally abstract: it raises NotImplementedError('Subclasses must implement render()'). Every concrete prompt subclass must supply its own render() returning str, list[Message|str], or PromptResult; calling render on a subclass that forgot to override it produces this error.

Source

Thrown at fastmcp_slim/fastmcp/prompts/base.py:307

            description=description,
            icons=icons,
            tags=tags,
            meta=meta,
            auth=auth,
        )

    async def render(
        self,
        arguments: dict[str, Any] | None = None,
    ) -> str | list[Message | str] | PromptResult:
        """Render the prompt with arguments.

        Subclasses must implement this method. Return one of:
        - str: Wrapped as single user Message
        - list[Message | str]: Converted to list[Message]
        - PromptResult: Used directly
        """
        raise NotImplementedError("Subclasses must implement render()")

    def convert_result(self, raw_value: Any) -> PromptResult:
        """Convert a raw return value to PromptResult.

        Accepts:
            - PromptResult: passed through
            - str: wrapped as single Message
            - list[Message | str]: converted to list[Message]

        Raises:
            TypeError: for unsupported types
        """
        if isinstance(raw_value, PromptResult):
            return raw_value

        if isinstance(raw_value, mcp_types.InputRequiredResult):
            # The prompt asked the client for input (SEP-2322). Wrap it so the
            # ask travels the middleware chain as an ordinary result; the wire

View on GitHub (pinned to 1f02114297)

Solutions

1. Define render() (or an async render) in your Prompt subclass returning str, list[Message|str], or PromptResult
2. Check the method name is exactly 'render' and the signature matches the base class
3. If you meant a static prompt, use from_message/factory helpers rather than a bare subclass

Example fix

// before
class MyPrompt(Prompt):
    pass
// after
class MyPrompt(Prompt):
    def render(self) -> str:
        return "Hello!"

def check_render_override(cls) -> None:
    if cls.render is Prompt.render:
        raise TypeError(f"{cls.__name__} must implement render()")

try:
    result = await prompt._render()
except NotImplementedError as e:
    raise RuntimeError(f"{type(prompt).__name__} forgot render(): {e}") from e

When it happens

Trigger: Subclassing Prompt (or using a custom prompt class) without defining render(); a rename/typo like renders() or _render() overriding the wrong method; instantiating a half-implemented abstract subclass and sending it to _render via a client prompt call.

Common situations: Writing custom dynamic prompts; upgrading fastmcp where a hook method was renamed; copy-pasted subclass missing the render body.

Related errors

• messages[{i}] must be Message, got {type(item).__name__}. Us
• messages must be str or list[Message], got {type(messages)._
• messages[{i}] must be Message or str, got {type(item).__name
• Prompt must return str, list[Message], or PromptResult, got
• Subclasses must implement read()

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/d9c5526cdf798fad. Report an issue: GitHub.