client

fastmcp.client.client

Classes

ClientSessionState

Holds all session-related state for a Client instance.

This allows clean separation of configuration (which is copied) from session state (which should be fresh for each new client instance).

CallToolResult

Parsed result from a tool call.

Client

MCP client that delegates connection management to a Transport instance.

The Client class is responsible for MCP protocol logic, while the Transport handles connection establishment and management. Client provides methods for working with resources, prompts, tools and other MCP capabilities.

This client supports reentrant context managers (multiple concurrent async with client: blocks) using reference counting and background session management. This allows efficient session reuse in any scenario with nested or concurrent client usage.

MCP SDK 1.10 introduced automatic list_tools() calls during call_tool() execution. This created a race condition where events could be reset while other tasks were waiting on them, causing deadlocks. The issue was exposed in proxy scenarios but affects any reentrant usage.

The solution uses reference counting to track active context managers, a background task to manage the session lifecycle, events to coordinate between tasks, and ensures all session state changes happen within a lock. Events are only created when needed, never reset outside locks.

This design prevents race conditions where tasks wait on events that get replaced by other tasks, ensuring reliable coordination in concurrent scenarios.

Args:

• transport: Connection source specification, which can be:

  • ClientTransport: Direct transport instance
  • FastMCP: In-process FastMCP server
  • AnyUrl or str: URL to connect to
  • Path: File path for local socket
  • MCPConfig: MCP server configuration
  • dict: Transport configuration

• roots: Optional RootsList or RootsHandler for filesystem access

• sampling_handler: Optional handler for sampling requests

• log_handler: Optional handler for log messages

• message_handler: Optional handler for protocol messages

• progress_handler: Optional handler for progress notifications

• timeout: Optional timeout for requests (seconds or timedelta)

• init_timeout: Optional timeout for initial connection (seconds or timedelta). Set to 0 to disable. If None, uses the value in the FastMCP global settings.

Examples:

# Connect to FastMCP server
client = Client("http://localhost:8080")

async with client:
    # List available resources
    resources = await client.list_resources()

    # Call a tool
    result = await client.call_tool("my_tool", {"param": "value"})

Methods:

session

session(self) -> ClientSession

Get the current active session. Raises RuntimeError if not connected.

initialize_result

initialize_result(self) -> mcp.types.InitializeResult | None

Get the result of the initialization request.

set_roots

set_roots(self, roots: RootsList | RootsHandler) -> None

Set the roots for the client. This does not automatically call send_roots_list_changed.

set_sampling_callback

set_sampling_callback(self, sampling_callback: SamplingHandler, sampling_capabilities: mcp.types.SamplingCapability | None = None) -> None

Set the sampling callback for the client.

set_elicitation_callback

set_elicitation_callback(self, elicitation_callback: ElicitationHandler) -> None

Set the elicitation callback for the client.

is_connected

is_connected(self) -> bool

Check if the client is currently connected.

new

new(self) -> Client[ClientTransportT]

Create a new client instance with the same configuration but fresh session state.

This creates a new client with the same transport, handlers, and configuration, but with no active session. Useful for creating independent sessions that don’t share state with the original client.

Returns:

• A new Client instance with the same configuration but disconnected state.

initialize

initialize(self, timeout: datetime.timedelta | float | int | None = None) -> mcp.types.InitializeResult

Send an initialize request to the server.

This method performs the MCP initialization handshake with the server, exchanging capabilities and server information. It is idempotent - calling it multiple times returns the cached result from the first call.

The initialization happens automatically when entering the client context manager unless auto_initialize=False was set during client construction. Manual calls to this method are only needed when auto-initialization is disabled.

Args:

• timeout: Optional timeout for the initialization request (seconds or timedelta). If None, uses the client’s init_timeout setting.

Returns:

• The server’s initialization response containing server info, capabilities, protocol version, and optional instructions.

Raises:

• RuntimeError: If the client is not connected or initialization times out.

close

close(self)

ping

ping(self) -> bool

Send a ping request.

cancel

cancel(self, request_id: str | int, reason: str | None = None) -> None

Send a cancellation notification for an in-progress request.

progress

progress(self, progress_token: str | int, progress: float, total: float | None = None, message: str | None = None) -> None

Send a progress notification.

set_logging_level

set_logging_level(self, level: mcp.types.LoggingLevel) -> None

Send a logging/setLevel request.

send_roots_list_changed

send_roots_list_changed(self) -> None

Send a roots/list_changed notification.

complete_mcp

complete_mcp(self, ref: mcp.types.ResourceTemplateReference | mcp.types.PromptReference, argument: dict[str, str], context_arguments: dict[str, Any] | None = None) -> mcp.types.CompleteResult

Send a completion request and return the complete MCP protocol result.

Args:

• ref: The reference to complete.
• argument: Arguments to pass to the completion request.
• context_arguments: Optional context arguments to include with the completion request. Defaults to None.

Returns:

• mcp.types.CompleteResult: The complete response object from the protocol, containing the completion and any additional metadata.

Raises:

• RuntimeError: If called while the client is not connected.
• McpError: If the request results in a TimeoutError | JSONRPCError

complete

complete(self, ref: mcp.types.ResourceTemplateReference | mcp.types.PromptReference, argument: dict[str, str], context_arguments: dict[str, Any] | None = None) -> mcp.types.Completion

Send a completion request to the server.

Args:

• ref: The reference to complete.
• argument: Arguments to pass to the completion request.
• context_arguments: Optional context arguments to include with the completion request. Defaults to None.

Returns:

• mcp.types.Completion: The completion object.

Raises:

• RuntimeError: If called while the client is not connected.
• McpError: If the request results in a TimeoutError | JSONRPCError

generate_name

generate_name(cls, name: str | None = None) -> str