What causes "Cannot specify name both as first argument and keyword" in PrefectHQ/fastmcp?

Remove the name keyword and keep only the positional string: @prompt('my_prompt')

PrefectHQ/fastmcp · error · TypeError

Cannot specify name both as first argument and keyword

Error message

Cannot specify name both as first argument and keyword

What it means

The @prompt decorator accepts the prompt name either as the first positional argument (@prompt('my_name')) or as the name keyword (@prompt(name='my_name')), but not both. Supplying a string as name_or_fn while name is also non-None is ambiguous, so prompt() raises TypeError immediately.

Source

Thrown at fastmcp_slim/fastmcp/prompts/function_prompt.py:440

            title=title,
            description=description,
            icons=icons,
            tags=tags,
            meta=meta,
            auth=auth,
        )
        target = fn.__func__ if isinstance(fn, staticmethod | MethodType) else fn
        cast(Any, target).__fastmcp__ = metadata
        return fn

    def decorator(fn: F, prompt_name: str | None) -> F:
        return attach_metadata(fn, prompt_name)

    if inspect.isroutine(name_or_fn):
        return decorator(name_or_fn, name)
    elif isinstance(name_or_fn, str):
        if name is not None:
            raise TypeError("Cannot specify name both as first argument and keyword")
        prompt_name = name_or_fn
    elif name_or_fn is None:
        prompt_name = name
    else:
        raise TypeError(f"Invalid first argument: {type(name_or_fn)}")

    def wrapper(fn: F) -> F:
        return decorator(fn, prompt_name)

    return wrapper

View on GitHub (pinned to 1f02114297)

Solutions

Remove the name keyword and keep only the positional string: @prompt('my_prompt')
Or remove the positional string and use only the keyword: @prompt(name='my_prompt')
If you meant to decorate a function directly, pass the function (a routine, not a string) first with an optional name keyword

Example fix

// before
@prompt('weather_prompt', name='weather')
def weather(): ...
// after
@prompt('weather')
def weather(): ...

Defensive patterns

Strategy: validation

Validate before calling

def safe_prompt(first=None, name=None):
    if isinstance(first, str) and name is not None:
        raise TypeError('specify name positionally or as keyword, not both')
    return prompt(first, name=name)

Try / catch

try:
    p = prompt('name_a', name='name_b')
except TypeError as e:
    logger.error('decorator misused: %s', e)
    p = prompt(name='name_b')  # pick one canonical form

Prevention

Pick one style per codebase (positional string or name= keyword) and enforce it in review
Never pass both a positional string and name= keyword
Watch for IDE autocompletes that append name= to an existing positional name

When it happens

Trigger: Calling @prompt('my_prompt', name='other_name') or @prompt('my_prompt', name='other_name')(fn) — a string first argument plus a non-None name keyword.

Common situations: Merging two call styles during a refactor; IDE autocomplete adding name= alongside an existing positional name; copy-paste where one call form was partially converted to the other.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/0483bfb80195266a. Report an issue: GitHub.