prompting

Prompting Agents

fast-agent provides a flexible MCP based API for sending messages to agents, with convenience methods for handling Files, Prompts and Resources.

Read more about the use of MCP types in fast-agent in the MCP overview.

Sending Messages

The simplest way of sending a message to an agent is the send method:

response: str = await agent.send("how are you?")

This returns the text of the agent's response as a string, making it ideal for simple interactions.

You can attach files by using Prompt.user() method to construct your message:

from fast_agent import Prompt
from pathlib import Path

plans: str = await agent.send(Prompt.user("Summarise this PDF", Path("secret-plans.pdf")))

Prompt.user() automatically converts content to the appropriate MCP Type. For example, image/png becomes ImageContent and application/pdf becomes an EmbeddedResource.

You can also use MCP Types directly - for example:

from mcp_types import ImageContent, TextContent

mcp_text: TextContent = TextContent(type="text", text="Analyse this image.")
mcp_image: ImageContent = ImageContent(type="image", mime_type="image/png", data=base_64_encoded)

response: str = await agent.send(Prompt.user(mcp_text, mcp_image))

Note: use Prompt.assistant() to produce messages for the assistant role.

Using generate() and multipart content

The generate() method allows you to access multimodal content from an agent, or its Tool Calls as well as send conversational pairs.

from fast_agent import FastAgent, Prompt, PromptMessageExtended

message = Prompt.user("Describe an image of a sunset")

response: PromptMessageExtended = await agent.generate([message])

print(response.last_text())  # Main text response

The key difference between send() and generate() is that generate() returns a PromptMessageExtended object, giving you access to the complete response structure: