Resources in FastMCP

Resources represent data or files that an MCP client can read. They provide read-only access to data for the LLM.

Basic Resource Definition

from fastmcp import FastMCP

mcp = FastMCP(name="DataServer")

@mcp.resource("resource://greeting")
def get_greeting() -> str:
    """Provides a simple greeting message."""
    return "Hello from FastMCP Resources!"

@mcp.resource("data://config")
def get_config() -> dict:
    """Provides application configuration as JSON."""
    return {
        "theme": "dark",
        "version": "1.2.0",
        "features": ["tools", "resources"],
    }

Key Concepts

• URI: Unique identifier (e.g., "resource://greeting")
• Lazy Loading: Function only executes when client requests the resource
• Auto-inference: Name from function name, description from docstring

Return Types

• str: Sent as text (text/plain)
• dict, list: Auto-serialized to JSON
• bytes: Base64 encoded as binary
• None: Empty content

Resource Templates

Dynamic resources with parameters:

@mcp.resource("users://{user_id}/profile")
def get_user_profile(user_id: int) -> dict:
    """Retrieves a user's profile by ID."""
    return {"id": user_id, "name": f"User {user_id}"}

Async Resources

import aiofiles

@mcp.resource("file:///app/log.txt", mime_type="text/plain")
async def read_log() -> str:
    """Reads log file asynchronously."""
    async with aiofiles.open("/app/log.txt", mode="r") as f:
        return await f.read()