Python API

Basic Agents

Defining an agent is as simple as:

@fast.agent(
  instruction="Given an object, respond only with an estimate of its size."
)

We can then send messages to the Agent:

async with fast.run() as agent:
  moon_size = await agent("the moon")
  print(moon_size)

Or start an interactive chat with the Agent:

async with fast.run() as agent:
  await agent.interactive()

Here is the complete sizer.py Agent application, with boilerplate code:

sizer.py

import asyncio
from fast_agent.core.fastagent import FastAgent

# Create the application
fast = FastAgent("Agent Example")

@fast.agent(
  instruction="Given an object, respond only with an estimate of its size."
)
async def main():
  async with fast.run() as agent:
    await agent()

if __name__ == "__main__":
    asyncio.run(main())

The Agent can then be run with uv run sizer.py.

Specify a model with the --model switch - for example uv run sizer.py --model sonnet.

You can also pass a Path for the instruction - e.g.

from pathlib import Path

@fast.agent(
  instruction=Path("./sizing_prompt.md")
)

See Workflows for chaining, routing, parallelism, orchestrators, and MAKER.

Human Input

Agents can request Human Input to assist with a task or get additional context:

@fast.agent(
    instruction="An AI agent that assists with basic tasks. Request Human Input when needed.",
    human_input=True,
)

await agent("print the next number in the sequence")

In the example human_input.py, the agent will prompt the user for additional information to complete the task.

Agent and Workflow Reference

Calling Agents

All definitions allow omitting the name and instructions arguments for brevity:

@fast.agent("You are a helpful agent")          # Create an agent with a default name.
@fast.agent("greeter","Respond cheerfully!")    # Create an agent with the name "greeter"

moon_size = await agent("the moon")             # Call the default (first defined agent) with a message

result = await agent.greeter("Good morning!")   # Send a message to an agent by name using dot notation
result = await agent.greeter.send("Hello!")     # You can call 'send' explicitly

await agent["greeter"].send("Good Evening!")    # Dictionary access to agents is also supported

Configuring Agent Request Parameters

You can customize how an agent interacts with the LLM by passing request_params=RequestParams(...) when defining it.

Example

from fast_agent.types import RequestParams

@fast.agent(
  name="CustomAgent",                              # name of the agent
  instruction="You have my custom configurations", # base instruction for the agent
  request_params=RequestParams(
    maxTokens=8192,
    use_history=False,
    max_iterations=20
  )
)

Available `RequestParams` Fields (Generated)

Field	Type	Default
`task`	`mcp.types.TaskMetadata \| None`	`None`
`meta`	`mcp.types.RequestParams.Meta \| None`	`None`
`messages`	`list[mcp.types.SamplingMessage]`	`[]`
`modelPreferences`	`mcp.types.ModelPreferences \| None`	`None`
`systemPrompt`	`str \| None`	`None`
`includeContext`	`Literal['none', 'thisServer', 'allServers'] \| None`	`None`
`temperature`	`float \| None`	`None`
`maxTokens`	`int \| None`	`None`
`stopSequences`	`list[str] \| None`	`None`
`metadata`	`dict[str, Any] \| None`	`None`
`tools`	`list[mcp.types.Tool] \| None`	`None`
`toolChoice`	`mcp.types.ToolChoice \| None`	`None`
`model`	`str \| None`	`None`
`use_history`	`bool`	`True`
`max_iterations`	`int`	`199`
`parallel_tool_calls`	`bool`	`True`
`response_format`	`Any \| None`	`None`
`structured_schema`	`dict[str, Any] \| None`	`None`
`structured_tool_policy`	`Literal['auto', 'always', 'defer', 'no_tools']`	`'auto'`
`template_vars`	`dict[str, Any]`	`{}`
`mcp_metadata`	`dict[str, Any] \| None`	`None`
`tool_execution_handler`	`Any \| None`	`None`
`emit_loop_progress`	`bool`	`False`
`tool_result_mode`	`Literal['postprocess', 'passthrough', 'selectable']`	`'postprocess'`
`batch_context`	`fast_agent.llm.request_params.BatchRequestContext \| None`	`None`
`streaming_timeout`	`float \| None`	`120.0`
`top_p`	`float \| None`	`None`
`top_k`	`int \| None`	`None`
`min_p`	`float \| None`	`None`
`presence_penalty`	`float \| None`	`None`
`frequency_penalty`	`float \| None`	`None`
`repetition_penalty`	`float \| None`	`None`
`service_tier`	`Literal['fast', 'flex'] \| None`	`None`

Defining Agents

Basic Agent

@fast.agent(
  name="agent",                          # name of the agent
  instruction="You are a helpful Agent", # base instruction for the agent
  servers=["filesystem"],                # list of MCP Servers for the agent
  #tools={"filesystem": ["tool_1", "tool_2"]  # Filter the tools available to the agent. Defaults to all
  #resources={"filesystem": ["resource_1", "resource_2"]} # Filter the resources available to the agent. Defaults to all
  #prompts={"filesystem": ["prompt_1", "prompt_2"]}  # Filter the prompts available to the agent. Defaults to all.
  model="gpt-5.4-mini?reasoning=high",   # specify a model for the agent
  use_history=True,                      # agent maintains chat history
  request_params=RequestParams(temperature= 0.7), # additional parameters for the LLM (or RequestParams())
  human_input=True,                      # agent can request human input
  elicitation_handler=ElicitationFnT,    # custom elicitation handler (from mcp.client.session)
  api_key="programmatic-api-key",        # specify the API KEY programmatically, it will override which provided in config file or env var
)

Workflow definitions (chain/parallel/router/orchestrator/maker) are documented on the Workflows page.

Custom

@fast.custom(
  cls=Custom,                            # agent class
  name="custom",                         # name of the custom agent
  instruction="instruction",             # base instruction for the orchestrator
  servers=["filesystem"],                # list of MCP Servers for the agent
  #tools={"filesystem": ["tool_1", "tool_2"]  # Filter the tools available to the agent. Defaults to all
  #resources={"filesystem": ["resource_1", "resource_2"]} # Filter the resources available to the agent. Defaults to all
  #prompts={"filesystem": ["prompt_1", "prompt_2"]}  # Filter the prompts available to the agent. Defaults to all
  model="gpt-5.4-mini?reasoning=high",   # specify a model for the agent
  use_history=True,                      # agent maintains chat history
  request_params=RequestParams(temperature= 0.7), # additional parameters for the LLM (or RequestParams())
  human_input=True,                      # agent can request human input
  elicitation_handler=ElicitationFnT,    # custom elicitation handler (from mcp.client.session)
  api_key="programmatic-api-key",        # specify the API KEY programmatically, it will override which provided in config file or env var
)