---
title: LLM Sampling
sidebarTitle: Sampling
description: Handle server-initiated LLM sampling requests.
icon: robot
---

import { VersionBadge } from "/snippets/version-badge.mdx";

<VersionBadge version="2.0.0" />

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

## Sampling Handler

Provide a `sampling_handler` function when creating the client:

```python
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,
)
```

### Handler Parameters

The sampling handler receives three parameters:

<Card icon="code" title="Sampling Handler Parameters">
<ResponseField name="SamplingMessage" type="Sampling Message Object">
  <Expandable title="attributes">
    <ResponseField name="role" type='Literal["user", "assistant"]'>
      The role of the message.
    </ResponseField>

    <ResponseField name="content" type="TextContent | ImageContent | AudioContent">
      The content of the message.

      TextContent is most common, and has a `.text` attribute.
    </ResponseField>

  </Expandable>
</ResponseField>
<ResponseField name="SamplingParams" type="Sampling Parameters Object">
  <Expandable title="attributes">
    <ResponseField name="messages" type="list[SamplingMessage]">
      The messages to sample from
    </ResponseField>

    <ResponseField name="modelPreferences" type="ModelPreferences | None">
      The server's preferences for which model to select. The client MAY ignore
    these preferences.
    <Expandable title="attributes">
      <ResponseField name="hints" type="list[ModelHint] | None">
        The hints to use for model selection.
      </ResponseField>

      <ResponseField name="costPriority" type="float | None">
        The cost priority for model selection.
      </ResponseField>

      <ResponseField name="speedPriority" type="float | None">
        The speed priority for model selection.
      </ResponseField>

      <ResponseField name="intelligencePriority" type="float | None">
        The intelligence priority for model selection.
      </ResponseField>
    </Expandable>
    </ResponseField>

    <ResponseField name="systemPrompt" type="str | None">
      An optional system prompt the server wants to use for sampling.
    </ResponseField>

    <ResponseField name="includeContext" type="IncludeContext | None">
      A request to include context from one or more MCP servers (including the caller), to
      be attached to the prompt.
    </ResponseField>

    <ResponseField name="temperature" type="float | None">
      The sampling temperature.
    </ResponseField>

    <ResponseField name="maxTokens" type="int">
      The maximum number of tokens to sample.
    </ResponseField>

    <ResponseField name="stopSequences" type="list[str] | None">
      The stop sequences to use for sampling.
    </ResponseField>

    <ResponseField name="metadata" type="dict[str, Any] | None">
      Optional metadata to pass through to the LLM provider.
    </ResponseField>
    </Expandable>

</ResponseField>
<ResponseField name="RequestContext" type="Request Context Object">
  <Expandable title="attributes">
    <ResponseField name="request_id" type="RequestId">
      Unique identifier for the MCP request
    </ResponseField>
  </Expandable>
</ResponseField>
</Card>

## Basic Example

```python
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
)
```

## Sampling fallback

Client support for sampling is optional, if the client does not support sampling, the server will report an error indicating
that the client does not support sampling.

A `sampling_handler` can also be provided to the FastMCP server, which will be used to handle sampling requests if the client
does not support sampling. This sampling handler bypasses the client and sends sampling requests directly to the LLM provider.

Sampling handlers can be implemented using any LLM provider but a sample implementation for OpenAI is provided as a Contrib
module. Sampling lacks the full capabilities of typical LLM completions. For this reason, the OpenAI sampling handler, pointed at
a third-party provider's OpenAI-compatible API, is often sufficient to implement a sampling handler.

```python
import asyncio
import os

from mcp.types import ContentBlock
from openai import OpenAI

from fastmcp import FastMCP
from fastmcp.experimental.sampling.handlers.openai import OpenAISamplingHandler
from fastmcp.server.context import Context


async def async_main():
    server = FastMCP(
        name="OpenAI Sampling Fallback Example",
        sampling_handler=OpenAISamplingHandler(
            default_model="gpt-4o-mini",
            client=OpenAI(
                api_key=os.getenv("API_KEY"),
                base_url=os.getenv("BASE_URL"),
            ),
        ),
    )

    @server.tool
    async def test_sample_fallback(ctx: Context) -> ContentBlock:
        return await ctx.sample(
            messages=["hello world!"],
        )

    await server.run_http_async()


if __name__ == "__main__":
    asyncio.run(async_main())
```