Configuration

Fast Agent uses a hierarchical configuration system with support for environment variables, YAML files, and programmatic configuration.

Settings

The main settings class for Fast Agent configuration.

from fast_agent.config import Settings

settings = Settings(
    default_model="anthropic.claude-sonnet-4-20250514",
    execution_engine="asyncio",
    session_history=True
)

Core Settings

execution_engine

'asyncio'

default:"'asyncio'"

Execution engine for the agent application

environment_dir

str | None

default:"None"

Base directory for runtime data. Defaults to .fast-agent

default_model

str | None

default:"None"

Default model for agents. Format: provider.model_name.reasoning_effort or provider.model?reasoning=value. Examples: "openai.o3-mini.low", "anthropic.claude-sonnet-4-20250514?reasoning=high". Falls back to FAST_AGENT_MODEL env var, then "gpt-5-mini.low".

model_aliases

dict[str, dict[str, str]]

default:"{}"

Model aliases grouped by namespace. Example: {"$system": {"default": "gpt-5-mini"}}

auto_sampling

bool

default:"True"

Enable automatic sampling model selection if not explicitly configured

session_history

bool

default:"True"

Persist session history in the environment sessions folder

session_history_window

int

default:"20"

Maximum number of sessions to keep in the rolling window

MCP Configuration

mcp

MCPSettings | None

default:"MCPSettings()"

MCP server configuration and settings

Provider Settings

anthropic

AnthropicSettings | None

default:"None"

Settings for Anthropic models

openai

OpenAISettings | None

default:"None"

Settings for OpenAI models

responses

OpenAISettings | None

default:"None"

Settings for OpenAI Responses models

openresponses

OpenResponsesSettings | None

default:"None"

Settings for Open Responses models

codexresponses

CodexResponsesSettings | None

default:"None"

Settings for Codex Responses models

deepseek

DeepSeekSettings | None

default:"None"

Settings for DeepSeek models

google

GoogleSettings | None

default:"None"

Settings for Google models

xai

XAISettings | None

default:"None"

Settings for xAI Grok models

generic

GenericSettings | None

default:"None"

Settings for generic OpenAI-compatible models (e.g., Ollama)

openrouter

OpenRouterSettings | None

default:"None"

Settings for OpenRouter models

azure

AzureSettings | None

default:"None"

Settings for Azure OpenAI Service

groq

GroqSettings | None

default:"None"

Settings for Groq models

tensorzero

TensorZeroSettings | None

default:"None"

Settings for TensorZero LLM gateway

bedrock

BedrockSettings | None

default:"None"

Settings for AWS Bedrock models

huggingface

HuggingFaceSettings | None

default:"None"

Settings for HuggingFace inference providers

Logging and Telemetry

logger

LoggerSettings | None

default:"None"

Logger configuration for the agent

otel

OpenTelemetrySettings | None

default:"OpenTelemetrySettings()"

OpenTelemetry tracing configuration

skills

SkillsSettings | None

default:"None"

Skills directory and marketplace configuration

cards

CardsSettings | None

default:"None"

Card pack registry configuration

shell

ShellSettings | None

default:"None"

Shell execution behavior configuration

Provider Settings

AnthropicSettings

Configuration for Anthropic models.

from fast_agent.config import AnthropicSettings

anthropic = AnthropicSettings(
    api_key="sk-ant-...",
    default_model="claude-sonnet-4-20250514",
    cache_mode="auto",
    reasoning="medium"
)

api_key

str | None

default:"None"

Anthropic API key

base_url

str | None

default:"None"

Override API endpoint

default_model

str | None

default:"None"

Default model when provider is selected without explicit model

default_headers

dict[str, str] | None

default:"None"

Custom headers for all requests

cache_mode

'off' | 'prompt' | 'auto'

default:"'auto'"

Caching mode: off (disabled), prompt (cache tools+system), auto (same as prompt)

cache_ttl

'5m' | '1h'

default:"'5m'"

Cache TTL: 5m (standard) or 1h (extended, additional cost)

reasoning

ReasoningEffortSetting | str | int | bool | None

default:"None"

Reasoning setting. Supports effort strings (adaptive models), budget tokens (int), or toggle (bool). Use 0 or false to disable.

structured_output_mode

'auto' | 'json' | 'tool_use'

default:"'auto'"

Structured output mode

web_search

AnthropicWebSearchSettings

default:"AnthropicWebSearchSettings()"

Built-in web search tool configuration

web_fetch

AnthropicWebFetchSettings

default:"AnthropicWebFetchSettings()"

Built-in web fetch tool configuration

OpenAISettings

Configuration for OpenAI models.

from fast_agent.config import OpenAISettings

openai = OpenAISettings(
    api_key="sk-...",
    default_model="gpt-5-mini",
    reasoning_effort="medium"
)

api_key

str | None

default:"None"

OpenAI API key

base_url

str | None

default:"None"

Override API endpoint

default_model

str | None

default:"None"

Default model when provider is selected

default_headers

dict[str, str] | None

default:"None"

Custom headers for all requests

text_verbosity

'low' | 'medium' | 'high'

default:"'medium'"

Text verbosity level for Responses models

transport

'sse' | 'websocket' | 'auto' | None

default:"None"

Responses transport mode. Defaults to websocket with SSE fallback.

service_tier

'fast' | 'flex' | None

default:"None"

Responses service tier: fast (priority) or flex

reasoning

ReasoningEffortSetting | str | int | bool | None

default:"None"

Unified reasoning setting (effort level or budget)

reasoning_effort

'minimal' | 'low' | 'medium' | 'high'

default:"'medium'"

Default reasoning effort

web_search

OpenAIWebSearchSettings

default:"OpenAIWebSearchSettings()"

Web search tool configuration

MCP Settings

MCPSettings

Configuration for MCP servers.

from fast_agent.config import MCPSettings, MCPServerSettings

mcp = MCPSettings(
    servers={
        "filesystem": MCPServerSettings(
            command="npx",
            args=["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
        )
    }
)

servers

dict[str, MCPServerSettings]

default:"{}"

Dictionary mapping server names to their configurations

MCPServerSettings

Configuration for an individual MCP server.

name

str | None

default:"None"

Server name

description

str | None

default:"None"

Server description

transport

'stdio' | 'sse' | 'http'

default:"'stdio'"

Transport mechanism. Auto-inferred from url or command presence.

command

str | None

default:"None"

Command to execute the server (e.g., npx)

args

list[str] | None

default:"None"

Arguments for the server command

url

str | None

default:"None"

URL for SSE/HTTP transport

headers

dict[str, str] | None

default:"None"

HTTP headers for connections

auth

MCPServerAuthSettings | None

default:"None"

Authentication configuration

roots

list[MCPRootSettings] | None

default:"None"

Root directories the server has access to

env

dict[str, str] | None

default:"None"

Environment variables for the server process

cwd

str | None

default:"None"

Working directory for the server command

load_on_start

bool

default:"True"

Whether to connect automatically when agent starts

include_instructions

bool

default:"True"

Whether to include server instructions in system prompt

reconnect_on_disconnect

bool

default:"True"

Whether to automatically reconnect on session termination

read_timeout_seconds

int | None

default:"None"

Timeout in seconds for the session

ping_interval_seconds

int

default:"30"

Interval for MCP ping requests. Set ≤0 to disable.

max_missed_pings

int

default:"3"

Consecutive missed pings before treating connection as failed

Shell Settings

ShellSettings

Configuration for shell execution behavior.

from fast_agent.config import ShellSettings

shell = ShellSettings(
    timeout_seconds=120,
    show_bash=True,
    enable_read_text_file=True
)

timeout_seconds

int

default:"90"

Maximum seconds to wait for command output before terminating. Supports duration strings like "90s", "2m", "1h".

warning_interval_seconds

int

default:"30"

Show timeout warnings every N seconds

interactive_use_pty

bool

default:"True"

Use a PTY for interactive prompt shell commands

output_display_lines

int | None

default:"5"

Maximum shell output lines to display. Set to None for no limit.

show_bash

bool

default:"True"

Show shell command output on the console

output_byte_limit

int | None

default:"None"

Override model-based output byte limit. None = auto.

missing_cwd_policy

'ask' | 'create' | 'warn' | 'error'

default:"'warn'"

Policy when agent shell cwd is missing or invalid

enable_read_text_file

bool

default:"True"

Expose local read_text_file tool (ACP-compatible) when shell runtime is enabled

write_text_file_mode

'auto' | 'on' | 'off' | 'apply_patch' | None

default:"None"

Control which local file edit tool is exposed:

auto: Uses apply_patch for GPT-5/Codex models, write_text_file otherwise
on: Always expose write_text_file
apply_patch: Always expose apply_patch
off: Disable local file edit tools

Logger Settings

LoggerSettings

Configuration for logging and console output.

from fast_agent.config import LoggerSettings

logger = LoggerSettings(
    type="file",
    level="info",
    path="fastagent.jsonl",
    show_chat=True
)

type

'none' | 'console' | 'file' | 'http'

default:"'file'"

Logger type

level

'debug' | 'info' | 'warning' | 'error'

default:"'warning'"

Minimum logging level

progress_display

bool

default:"True"

Enable or disable progress display

path

str

default:"'fastagent.jsonl'"

Path to log file when type is 'file'

batch_size

int

default:"100"

Number of events to accumulate before processing

flush_interval

float

default:"2.0"

How often to flush events in seconds

max_queue_size

int

default:"2048"

Maximum queue size for event processing

show_chat

bool

default:"True"

Show User/Assistant chat on console

show_tools

bool

default:"True"

Show MCP server tool calls on console

truncate_tools

bool

default:"True"

Truncate display of long tool calls

enable_markup

bool

default:"True"

Enable markup in console output

enable_prompt_marks

bool

default:"True"

Emit OSC 133 prompt marks for terminal scrollbar markers

streaming

'markdown' | 'plain' | 'none'

default:"'markdown'"

Streaming renderer for assistant responses

message_style

'classic' | 'a3'

default:"'a3'"

Chat message layout style for console output

Configuration Files

Fast Agent supports layered YAML configuration:

Project config: fastagent.config.yaml in project root
Environment config: .fast-agent/fastagent.config.yaml (overrides project)
Secrets: fastagent.secrets.yaml (merged with config)

Environment Variable Substitution

Use ${VAR_NAME} or ${VAR_NAME:default} syntax:

anthropic:
  api_key: ${ANTHROPIC_API_KEY}
  default_model: ${DEFAULT_MODEL:claude-sonnet-4-20250514}

Example Configuration

default_model: anthropic.claude-sonnet-4-20250514

anthropic:
  api_key: ${ANTHROPIC_API_KEY}
  cache_mode: auto
  reasoning: medium

mcp:
  servers:
    filesystem:
      command: npx
      args:
        - -y
        - "@modelcontextprotocol/server-filesystem"
        - /tmp
    
    web-search:
      url: https://api.example.com/mcp
      transport: http
      headers:
        Authorization: Bearer ${MCP_TOKEN}

shell:
  timeout_seconds: 120
  show_bash: true

logger:
  type: file
  level: info
  show_chat: true
  streaming: markdown

Core API

Types & Settings

MCP Integration

Utilities

Settings

Core Settings

MCP Configuration

Provider Settings

Logging and Telemetry

Provider Settings

AnthropicSettings

OpenAISettings

MCP Settings

MCPSettings

MCPServerSettings

Shell Settings

ShellSettings

Logger Settings

LoggerSettings

Configuration Files

Environment Variable Substitution

Example Configuration

Build docs developers (and LLMs) love

Core API

Types & Settings

MCP Integration

Utilities

​Settings

​Core Settings

​MCP Configuration

​Provider Settings

​Logging and Telemetry

​Provider Settings

​AnthropicSettings

​OpenAISettings

​MCP Settings

​MCPSettings

​MCPServerSettings

​Shell Settings

​ShellSettings

​Logger Settings

​LoggerSettings

​Configuration Files

​Environment Variable Substitution

​Example Configuration

Build docs developers (and LLMs) love

Settings

Core Settings

MCP Configuration

Provider Settings

Logging and Telemetry

Provider Settings

AnthropicSettings

OpenAISettings

MCP Settings

MCPSettings

MCPServerSettings

Shell Settings

ShellSettings

Logger Settings

LoggerSettings

Configuration Files

Environment Variable Substitution

Example Configuration