New in version: 2.0.0

MCP servers can request LLM completions from clients. The client handles these requests through a sampling handler callback.

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Sampling Handler

Provide a sampling_handler function when creating the client:

from fastmcp import Client
from fastmcp.client.sampling import (
    SamplingMessage,
    SamplingParams,
    RequestContext,
)

async def sampling_handler(
    messages: list[SamplingMessage],
    params: SamplingParams,
    context: RequestContext
) -> str:
    # Your LLM integration logic here
    # Extract text from messages and generate a response
    return "Generated response based on the messages"

client = Client(
    "my_mcp_server.py",
    sampling_handler=sampling_handler,
)

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Handler Parameters

The sampling handler receives three parameters:

Sampling Handler Parameters

​
SamplingMessage
Sampling Message Object
​
SamplingParams
Sampling Parameters Object
​
RequestContext
Request Context Object

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Basic Example

from fastmcp import Client
from fastmcp.client.sampling import SamplingMessage, SamplingParams, RequestContext

async def basic_sampling_handler(
    messages: list[SamplingMessage],
    params: SamplingParams,
    context: RequestContext
) -> str:
    # Extract message content
    conversation = []
    for message in messages:
        content = message.content.text if hasattr(message.content, 'text') else str(message.content)
        conversation.append(f"{message.role}: {content}")

    # Use the system prompt if provided
    system_prompt = params.systemPrompt or "You are a helpful assistant."

    # Here you would integrate with your preferred LLM service
    # This is just a placeholder response
    return f"Response based on conversation: {' | '.join(conversation)}"

client = Client(
    "my_mcp_server.py",
    sampling_handler=basic_sampling_handler
)