PrefectHQ/fastmcp · error · RuntimeError

Unexpected CreateTaskResult: Context calls should not have t

Error message

Unexpected CreateTaskResult: Context calls should not have task metadata

What it means

Context.get_prompt renders a prompt internally and asserts the result is not a CreateTaskResult (a task-metadata wrapper used by task-augmented requests). Context-initiated calls are plain, synchronous-style requests and can never carry task metadata; if one appears it indicates an internal contract violation, so a RuntimeError is raised.

Source

Thrown at fastmcp_slim/fastmcp/server/context.py:588

            call_handler=self.fastmcp._on_list_prompts,
            extract_items=lambda result: result.prompts,
        )

    async def get_prompt(
        self, name: str, arguments: dict[str, Any] | None = None
    ) -> GetPromptResult:
        """Get a prompt by name with optional arguments.

        Args:
            name: The name of the prompt to get
            arguments: Optional arguments to pass to the prompt

        Returns:
            The prompt result
        """
        result = await self.fastmcp.render_prompt(name, arguments)
        if isinstance(result, mcp_types.CreateTaskResult):
            raise RuntimeError(
                "Unexpected CreateTaskResult: Context calls should not have task metadata"
            )
        return result.to_mcp_prompt_result()

    async def read_resource(self, uri: str | AnyUrl) -> ResourceResult:
        """Read a resource by URI.

        Args:
            uri: Resource URI to read

        Returns:
            ResourceResult with contents
        """
        result = await self.fastmcp.read_resource(str(uri))
        if isinstance(result, mcp_types.CreateTaskResult):
            raise RuntimeError(
                "Unexpected CreateTaskResult: Context calls should not have task metadata"
            )

View on GitHub (pinned to 1f02114297)

Solutions

Do not override render_prompt or the internal result path; let Context.get_prompt call the default implementation.
Upgrade/align fastmcp and the mcp SDK to matching versions so internal results are not task-wrapped.
If you need task-augmented behavior, use the public task APIs on the client/request layer, not Context.get_prompt.
Catch RuntimeError only as a last-resort guard and re-inspect the prompt registration.

Defensive patterns

Strategy: try-catch

Type guard

def is_task_result(r) -> bool:
    return isinstance(r, mcp_types.CreateTaskResult)

Try / catch

try:
    pr = await ctx.get_prompt("name", {"arg": "v"})
except RuntimeError as e:
    if "CreateTaskResult" in str(e):
        logging.error("internal task metadata leaked into prompt read: %s", e)
    raise

Prevention

Keep fastmcp and the mcp SDK versions aligned (uv lock/uv sync).
Do not override render_prompt or wrap its return value.
Use the public task APIs for task-augmented flows instead of Context.get_prompt.

When it happens

Trigger: Calling ctx.get_prompt(name, arguments) when the underlying fastmcp.render_prompt unexpectedly returns an mcp_types.CreateTaskResult — effectively only through internal misconfiguration or a version mismatch where the prompt was resolved through a task-augmented path.

Common situations: Running code written against a different FastMCP/MCP SDK version where task metadata started leaking into internal results; custom subclass overrides of render_prompt that wrap results in CreateTaskResult.

Related errors

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/6097d44d260012cc. Report an issue: GitHub.