prefecthq--fastmcp/docs/v2/clients/sampling.mdx

---
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>

    <ResponseField name="tools" type="list[Tool] | None">
      Optional list of tools the LLM can use during sampling. See [Using the OpenAI Handler](#using-the-openai-handler).
    </ResponseField>

    <ResponseField name="toolChoice" type="ToolChoice | None">
      Optional control over tool usage behavior (`auto`, `required`, or `none`).
    </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
)
```

<Note>
If the client doesn't provide a sampling handler, servers can optionally configure a fallback handler. See [Server Sampling](/v2/servers/sampling#sampling-fallback-handler) for details.
</Note>

## Sampling Capabilities

When you provide a `sampling_handler`, FastMCP automatically advertises full sampling capabilities to the server, including tool support. To disable tool support (for simpler handlers that don't support tools), pass `sampling_capabilities` explicitly:

```python
from mcp.types import SamplingCapability

client = Client(
    "my_mcp_server.py",
    sampling_handler=basic_handler,
    sampling_capabilities=SamplingCapability(),  # No tool support
)
```

## Built-in Handlers

FastMCP provides built-in sampling handlers for OpenAI and Anthropic APIs. These handlers support the full sampling API including tool use, handling message conversion and response formatting automatically.

### OpenAI Handler

<VersionBadge version="2.11.0" />

The OpenAI handler works with OpenAI's API and any OpenAI-compatible provider:

```python
from fastmcp import Client
from fastmcp.client.sampling.handlers.openai import OpenAISamplingHandler

client = Client(
    "my_mcp_server.py",
    sampling_handler=OpenAISamplingHandler(default_model="gpt-4o"),
)
```

For OpenAI-compatible APIs (like local models), pass a custom client:

```python
from openai import AsyncOpenAI

client = Client(
    "my_mcp_server.py",
    sampling_handler=OpenAISamplingHandler(
        default_model="llama-3.1-70b",
        client=AsyncOpenAI(base_url="http://localhost:8000/v1"),
    ),
)
```

<Note>
Install the OpenAI handler with `pip install fastmcp[openai]`.
</Note>

### Anthropic Handler

<VersionBadge version="2.14.1" />

The Anthropic handler uses Claude models via the Anthropic API:

```python
from fastmcp import Client
from fastmcp.client.sampling.handlers.anthropic import AnthropicSamplingHandler

client = Client(
    "my_mcp_server.py",
    sampling_handler=AnthropicSamplingHandler(default_model="claude-sonnet-4-5"),
)
```

You can pass a custom client for advanced configuration:

```python
from anthropic import AsyncAnthropic

client = Client(
    "my_mcp_server.py",
    sampling_handler=AnthropicSamplingHandler(
        default_model="claude-sonnet-4-5",
        client=AsyncAnthropic(),  # Uses ANTHROPIC_API_KEY env var
    ),
)
```

<Note>
Install the Anthropic handler with `pip install fastmcp[anthropic]`.
</Note>

### Tool Execution

Tool execution happens on the server side. The client's role is to pass tools to the LLM and return the LLM's response (which may include tool use requests). The server then executes the tools and may send follow-up sampling requests with tool results.

<Tip>
To implement a custom sampling handler, see the [handler source code](https://github.com/PrefectHQ/fastmcp/tree/main/fastmcp_slim/fastmcp/client/sampling/handlers) as a reference.
</Tip>