PrefectHQ/fastmcp · error · ValueError

You must provide a name for lambda functions

Error message

You must provide a name for lambda functions

What it means

FastMCP's ResourceTemplate.from_function requires a name for every template. When no explicit name is passed, the library falls back to fn.__name__ (or the class name for callables). Anonymous lambdas report their __name__ as '<lambda>', which is not a valid, stable identifier, so the constructor raises immediately to prevent an unnamed/garbage-named component from being registered.

Source

Thrown at fastmcp_slim/fastmcp/resources/template.py:449

        fn: Callable[..., Any],
        uri_template: str,
        name: str | None = None,
        version: str | int | None = None,
        title: str | None = None,
        description: str | None = None,
        icons: list[Icon] | None = None,
        mime_type: str | None = None,
        tags: set[str] | None = None,
        annotations: Annotations | None = None,
        meta: dict[str, Any] | None = None,
        auth: AuthCheck | list[AuthCheck] | None = None,
        security: ResourceSecurity | None | InheritSecurity = INHERIT_SECURITY,
    ) -> FunctionResourceTemplate:
        """Create a template from a function."""

        func_name = name or getattr(fn, "__name__", None) or fn.__class__.__name__
        if func_name == "<lambda>":
            raise ValueError("You must provide a name for lambda functions")

        # Reject functions with *args
        # (**kwargs is allowed because the URI will define the parameter names)
        sig = inspect.signature(fn)
        for param in sig.parameters.values():
            if param.kind == inspect.Parameter.VAR_POSITIONAL:
                raise ValueError(
                    "Functions with *args are not supported as resource templates"
                )

        # Extract path and query parameters from URI template.
        # Allow hyphens in names and normalize to underscores so they
        # match Python function parameter names.
        raw_path_params = set(re.findall(r"{([\w-]+)(?:\*)?}", uri_template))
        raw_query_params = extract_query_params(uri_template)

        # Detect collisions: two raw param names that normalize to the
        # same Python identifier (e.g. {user-id} and {user_id}).

View on GitHub (pinned to 1f02114297)

Solutions

1. Pass an explicit name: FunctionResourceTemplate.from_function(lambda x: ..., uri_template="data://{x}", name="my-template")
2. Replace the lambda with a named def function so the fallback name works
3. Wrap the lambda with functools.partial-free named wrapper and use its __name__

Example fix

// before
FunctionResourceTemplate.from_function(lambda i: fetch(i), uri_template="data://{i}")
// after
def get_data(i: int) -> str: ...
FunctionResourceTemplate.from_function(get_data, uri_template="data://{i}")
// or
FunctionResourceTemplate.from_function(lambda i: fetch(i), uri_template="data://{i}", name="get-data")

def ensure_template_name(fn, name=None):
    if name is None and (getattr(fn, "__name__", None) in (None, "<lambda>")):
        raise ValueError("lambda passed without an explicit name=")

def has_name(fn) -> bool:
    n = getattr(fn, "__name__", None)
    return isinstance(n, str) and n != "<lambda>"

try:
    tpl = FunctionResourceTemplate.from_function(fn, uri_template=uri)
except ValueError as e:
    if "lambda" in str(e):
        tpl = FunctionResourceTemplate.from_function(fn, uri_template=uri, name=fallback_name)
    else:
        raise

When it happens

Trigger: Calling FunctionResourceTemplate.from_function(fn=uri_template, ...) (or FastMCP.add_template / resource_template) with a lambda as the function and no explicit name= argument.

Common situations: Registering a quick inline lambda template during prototyping; programmatically generating templates in a loop with lambdas and forgetting the name kwarg; converting a plain Resource from_function to a template while keeping lambda style.

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/a8e023b3a99316fd. Report an issue: GitHub.