Prompts are reusable message templates that help LLMs generate structured, purposeful responses. FastMCP simplifies defining these templates, primarily using the @mcp.prompt decorator.

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What Are Prompts?

Prompts provide parameterized message templates for LLMs. When a client requests a prompt:

  1. FastMCP finds the corresponding prompt definition.
  2. If it has parameters, they are validated against your function signature.
  3. Your function executes with the validated inputs.
  4. The generated message(s) are returned to the LLM to guide its response.

This allows you to define consistent, reusable templates that LLMs can use across different clients and contexts.

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Defining Prompts

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The @prompt Decorator

The most common way to define a prompt is by decorating a Python function. The decorator uses the function name as the prompt’s identifier.

from fastmcp import FastMCP
from fastmcp.prompts.prompt import UserMessage, AssistantMessage, Message

mcp = FastMCP(name="PromptServer")

# Basic prompt returning a string (converted to UserMessage)
@mcp.prompt()
def ask_about_topic(topic: str) -> str:
    """Generates a user message asking for an explanation of a topic."""
    return f"Can you please explain the concept of '{topic}'?"

# Prompt returning a specific message type
@mcp.prompt()
def generate_code_request(language: str, task_description: str) -> UserMessage:
    """Generates a user message requesting code generation."""
    content = f"Write a {language} function that performs the following task: {task_description}"
    return UserMessage(content=content)

Key Concepts:

  • Name: By default, the prompt name is taken from the function name.
  • Parameters: The function parameters define the inputs needed to generate the prompt.
  • Inferred Metadata: By default:
    • Prompt Name: Taken from the function name (ask_about_topic).
    • Prompt Description: Taken from the function’s docstring.

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Return Values

FastMCP intelligently handles different return types from your prompt function:

  • str: Automatically converted to a single UserMessage.
  • Message (e.g., UserMessage, AssistantMessage): Used directly as provided.
  • dict: Parsed as a Message object if it has the correct structure.
  • list[Message]: Used as a sequence of messages (a conversation).
@mcp.prompt()
def roleplay_scenario(character: str, situation: str) -> list[Message]:
    """Sets up a roleplaying scenario with initial messages."""
    return [
        UserMessage(f"Let's roleplay. You are {character}. The situation is: {situation}"),
        AssistantMessage("Okay, I understand. I am ready. What happens next?")
    ]

@mcp.prompt()
def ask_for_feedback() -> dict:
    """Generates a user message asking for feedback."""
    return {"role": "user", "content": "What did you think of my previous response?"}

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Type Annotations

Type annotations are important for prompts. They:

  1. Inform FastMCP about the expected types for each parameter.
  2. Allow validation of parameters received from clients.
  3. Are used to generate the prompt’s schema for the MCP protocol.
from pydantic import Field
from typing import Literal, Optional

@mcp.prompt()
def generate_content_request(
    topic: str = Field(description="The main subject to cover"),
    format: Literal["blog", "email", "social"] = "blog",
    tone: str = "professional",
    word_count: Optional[int] = None
) -> str:
    """Create a request for generating content in a specific format."""
    prompt = f"Please write a {format} post about {topic} in a {tone} tone."
    
    if word_count:
        prompt += f" It should be approximately {word_count} words long."
        
    return prompt

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Required vs. Optional Parameters

Parameters in your function signature are considered required unless they have a default value.

@mcp.prompt()
def data_analysis_prompt(
    data_uri: str,                        # Required - no default value
    analysis_type: str = "summary",       # Optional - has default value
    include_charts: bool = False          # Optional - has default value
) -> str:
    """Creates a request to analyze data with specific parameters."""
    prompt = f"Please perform a '{analysis_type}' analysis on the data found at {data_uri}."
    if include_charts:
        prompt += " Include relevant charts and visualizations."
    return prompt

In this example, the client must provide data_uri. If analysis_type or include_charts are omitted, their default values will be used.

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Prompt Metadata

While FastMCP infers the name and description from your function, you can override these and add tags using arguments to the @mcp.prompt decorator:

@mcp.prompt(
    name="analyze_data_request",          # Custom prompt name
    description="Creates a request to analyze data with specific parameters",  # Custom description
    tags={"analysis", "data"}             # Optional categorization tags
)
def data_analysis_prompt(
    data_uri: str = Field(description="The URI of the resource containing the data."),
    analysis_type: str = Field(default="summary", description="Type of analysis.")
) -> str:
    """This docstring is ignored when description is provided."""
    return f"Please perform a '{analysis_type}' analysis on the data found at {data_uri}."
  • name: Sets the explicit prompt name exposed via MCP.
  • description: Provides the description exposed via MCP. If set, the function’s docstring is ignored for this purpose.
  • tags: A set of strings used to categorize the prompt. Clients might use tags to filter or group available prompts.

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Asynchronous Prompts

FastMCP seamlessly supports both standard (def) and asynchronous (async def) functions as prompts.

# Synchronous prompt
@mcp.prompt()
def simple_question(question: str) -> str:
    """Generates a simple question to ask the LLM."""
    return f"Question: {question}"

# Asynchronous prompt
@mcp.prompt()
async def data_based_prompt(data_id: str) -> str:
    """Generates a prompt based on data that needs to be fetched."""
    # In a real scenario, you might fetch data from a database or API
    async with aiohttp.ClientSession() as session:
        async with session.get(f"https://api.example.com/data/{data_id}") as response:
            data = await response.json()
            return f"Analyze this data: {data['content']}"

Use async def when your prompt function performs I/O operations like network requests, database queries, file I/O, or external service calls.

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The MCP Session

Prompts can access the MCP features via the Context object, just like tools.

from fastmcp import Context

@mcp.prompt()
async def generate_report_request(report_type: str, ctx: Context) -> str:
    """Generates a request for a report based on available data."""
    # Log the request
    await ctx.info(f"Generating prompt for report type: {report_type}")
    
    # Could potentially use ctx.read_resource to fetch data
    # Or ctx.sample to get additional input from the LLM
    
    return f"Please create a {report_type} report based on the available data."

Using the ctx parameter (based on its Context type hint), you can access:

  • Logging: ctx.debug(), ctx.info(), etc.
  • Resource Access: ctx.read_resource(uri)
  • LLM Sampling: ctx.sample(...)
  • Request Info: ctx.request_id, ctx.client_id

Refer to the Context documentation for more details on these capabilities.

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Server Behavior

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Duplicate Prompts

You can configure how the FastMCP server handles attempts to register multiple prompts with the same name. Use the on_duplicate_prompts setting during FastMCP initialization.

from fastmcp import FastMCP

mcp = FastMCP(
    name="PromptServer",
    on_duplicate_prompts="error"  # Raise an error if a prompt name is duplicated
)

@mcp.prompt()
def greeting(): return "Hello, how can I help you today?"

# This registration attempt will raise a ValueError because
# "greeting" is already registered and the behavior is "error".
# @mcp.prompt()
# def greeting(): return "Hi there! What can I do for you?"

The duplicate behavior options are:

  • "warn" (default): Logs a warning, and the new prompt replaces the old one.
  • "error": Raises a ValueError, preventing the duplicate registration.
  • "replace": Silently replaces the existing prompt with the new one.
  • "ignore": Keeps the original prompt and ignores the new registration attempt.