Use elicitation to select one or more items from a dynamically-generated list

[arpieb]

Hi there! I'm trying to build out a FastMCP server to expose some data analysis tools. I need to be able to handle a tool request like, "Pull up the data from last Monday," and if there is more than one file I would like to be able to leverage elicitiation to ask which file(s) to "load" into the session context.

At present it seems that only pydantic models containing scalars or nested pedantic models are allowed; trying to pass a list or even an Enum as an elicitation schema fails on the FastMCP side as being an invalid schema type since it doesn't subclass pydantic's BaseModel:

class Priority(enum.Enum):
    LOW = "low"
    MEDIUM = "medium"
    HIGH = "high"

@mcp.tool()
async def set_priority_enum(ctx: Context) -> str:
    """Set task priority level."""
    result = await ctx.elicit(
        "What priority level?",
        schema=Priority,
    )

@mcp.tool()
async def set_priority_literal(ctx: Context) -> str:
    """Set task priority level."""
    result = await ctx.elicit(
        "What priority level?",
        schema=Literal["low", "medium", "high"],
    )

@mcp.tool()
async def set_priority_list(ctx: Context) -> str:
    """Set task priority level."""
    result = await ctx.elicit(
        "What priority level?",
        schema=["low", "medium", "high"],
    )

It feels like a finite list of scalars could be handled by the EnumSchema defined in MCP...?

Running on Python 3.12.11, fastmcp==2.13.1, macOS.

Any suggestions on how to achieve this in FastMCP? Many thanks in advance!