ResourcesAsTools requires a FastMCP server instance, not a {

Error message

ResourcesAsTools requires a FastMCP server instance, not a {type(provider).__name__}. The generated tools route through the server's middleware chain at runtime for auth and visibility. Pass your FastMCP server: ResourcesAsTools(mcp)

What it means

ResourcesAsTools exposes a server's resources as tools and, like PromptsAsTools, must route generated tools through the server's middleware chain for auth and visibility — so it only accepts a FastMCP instance. Any other Provider raises TypeError.

Source

Thrown at fastmcp_slim/fastmcp/server/transforms/resources_as_tools.py:67

    middleware, and visibility apply automatically.

    This transform should be applied to a FastMCP server instance, not
    a raw Provider, because the generated tools need the server's
    middleware chain for auth and visibility filtering.

    Example:
        ```python
        mcp = FastMCP("Server")
        mcp.add_transform(ResourcesAsTools(mcp))
        # Now has list_resources and read_resource tools
        ```
    """

    def __init__(self, provider: Provider) -> None:
        from fastmcp.server.server import FastMCP

        if not isinstance(provider, FastMCP):
            raise TypeError(
                "ResourcesAsTools requires a FastMCP server instance, not a"
                f" {type(provider).__name__}. The generated tools route through"
                " the server's middleware chain at runtime for auth and"
                " visibility. Pass your FastMCP server: ResourcesAsTools(mcp)"
            )
        self._provider = provider

    def __repr__(self) -> str:
        return f"ResourcesAsTools({self._provider!r})"

    async def list_tools(self, tools: Sequence[Tool]) -> Sequence[Tool]:
        """Add resource tools to the tool list."""
        return [
            *tools,
            self._make_list_resources_tool(),
            self._make_read_resource_tool(),
        ]

View on GitHub (pinned to 1f02114297)

Solutions

Pass your FastMCP server: ResourcesAsTools(mcp)
Mount the provider into a FastMCP server first, then wrap that server
Check argument order in your transform pipeline

Example fix

// before
ResourcesAsTools(resource_provider)
// after
ResourcesAsTools(mcp)

Defensive patterns

Strategy: type-guard

Validate before calling

from fastmcp.server.server import FastMCP
assert isinstance(mcp, FastMCP), "ResourcesAsTools needs a FastMCP server"

Type guard

def is_fastmcp(obj) -> bool:
    from fastmcp.server.server import FastMCP
    return isinstance(obj, FastMCP)

Try / catch

try:
    t = ResourcesAsTools(mcp)
except TypeError as e:
    logger.error("bad transform arg: %s", e)

Prevention

Pass the FastMCP server instance, not a resource provider
Review transform docs for accepted types
Wire transforms after the server is constructed

When it happens

Trigger: ResourcesAsTools(provider) where provider is a raw Provider/transform rather than a FastMCP server.

Common situations: Passing a resource provider directly instead of the server that owns it; misreading the API as accepting any Provider.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/652e3cfaff4e012a. Report an issue: GitHub.