Elicitation

The direction of communication flips here

Every primitive so far flows one way: the client asks a server for something. Elicitation is the opposite: a server, mid-tool-call, asks the user something and waits for an answer before continuing. Think "are you sure you want to delete this?", except the confirmation dialog is defined by the server, not hardcoded into your client.

Declaring what the server needs

from pydantic import BaseModel, Field
from mcp.server.fastmcp import Context, FastMCP


class ConfirmDeletion(BaseModel):
    confirm: bool = Field(description="Type true to confirm deletion.")


@mcp.tool()
async def delete_file(filename: str, ctx: Context) -> str:
    """Delete a file, after asking the user to confirm."""
    result = await ctx.elicit(
        message=f"Really delete '{filename}'?",
        schema=ConfirmDeletion,
    )
    if result.action != "accept" or not result.data.confirm:
        return "Deletion cancelled."
    return f"Deleted '{filename}'."

schema is a Pydantic model, the same idea as a tool's inputSchema (Lesson 3), just describing what shape of answer the server expects back from the user this time, not from the AI. ctx.elicit(...) sends the question and suspends the tool call until a response arrives.

Answering it on the client

A client that wants to support elicitation supplies an elicitation_callback.

import mcp.types as types


async def elicitation_callback(context, params):
    print("Server is asking:", params.message)
    # A real client would show a UI dialog here. This one auto-confirms.
    return types.ElicitResult(action="accept", content={"confirm": True})


async with ClientSession(read, write, elicitation_callback=elicitation_callback) as session:
    ...

action is one of "accept", "decline", or "cancel", giving the server a clear three-way answer rather than just true/false: declining answers "no", cancelling means "stop asking me things entirely for this call." A client that doesn't provide a callback rejects every elicitation automatically, the server always gets a definite answer, never hangs waiting.