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fastmcp.client.client

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Classes

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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).

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CallToolResult

Parsed result from a tool call.

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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:

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session 

session(self) -> ClientSession
Get the current active session. Raises RuntimeError if not connected.

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initialize_result 

initialize_result(self) -> mcp.types.InitializeResult | None
Get the result of the initialization request.

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set_roots 

set_roots(self, roots: RootsList | RootsHandler) -> None
Set the roots for the client. This does not automatically call send_roots_list_changed.

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set_sampling_callback 

set_sampling_callback(self, sampling_callback: ClientSamplingHandler) -> None
Set the sampling callback for the client.

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set_elicitation_callback 

set_elicitation_callback(self, elicitation_callback: ElicitationHandler) -> None
Set the elicitation callback for the client.

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is_connected 

is_connected(self) -> bool
Check if the client is currently connected.

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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.

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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.

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close 

close(self)

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ping 

ping(self) -> bool
Send a ping request.

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cancel 

cancel(self, request_id: str | int, reason: str | None = None) -> None
Send a cancellation notification for an in-progress request.

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progress 

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

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set_logging_level 

set_logging_level(self, level: mcp.types.LoggingLevel) -> None
Send a logging/setLevel request.

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send_roots_list_changed 

send_roots_list_changed(self) -> None
Send a roots/list_changed notification.

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list_resources_mcp 

list_resources_mcp(self) -> mcp.types.ListResourcesResult
Send a resources/list request and return the complete MCP protocol result. Returns:
  • mcp.types.ListResourcesResult: The complete response object from the protocol, containing the list of resources and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_resources 

list_resources(self) -> list[mcp.types.Resource]
Retrieve a list of resources available on the server. Returns:
  • list[mcp.types.Resource]: A list of Resource objects.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_resource_templates_mcp 

list_resource_templates_mcp(self) -> mcp.types.ListResourceTemplatesResult
Send a resources/listResourceTemplates request and return the complete MCP protocol result. Returns:
  • mcp.types.ListResourceTemplatesResult: The complete response object from the protocol, containing the list of resource templates and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_resource_templates 

list_resource_templates(self) -> list[mcp.types.ResourceTemplate]
Retrieve a list of resource templates available on the server. Returns:
  • list[mcp.types.ResourceTemplate]: A list of ResourceTemplate objects.
Raises:
  • RuntimeError: If called while the client is not connected.

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read_resource_mcp 

read_resource_mcp(self, uri: AnyUrl | str, meta: dict[str, Any] | None = None) -> mcp.types.ReadResourceResult
Send a resources/read request and return the complete MCP protocol result. Args:
  • uri: The URI of the resource to read. Can be a string or an AnyUrl object.
  • meta: Request metadata (e.g., for SEP-1686 tasks). Defaults to None.
Returns:
  • mcp.types.ReadResourceResult: The complete response object from the protocol, containing the resource contents and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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read_resource 

read_resource(self, uri: AnyUrl | str) -> list[mcp.types.TextResourceContents | mcp.types.BlobResourceContents]

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read_resource 

read_resource(self, uri: AnyUrl | str) -> ResourceTask

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read_resource 

read_resource(self, uri: AnyUrl | str) -> list[mcp.types.TextResourceContents | mcp.types.BlobResourceContents] | ResourceTask
Read the contents of a resource or resolved template. Args:
  • uri: The URI of the resource to read. Can be a string or an AnyUrl object.
  • task: If True, execute as background task (SEP-1686). Defaults to False.
  • task_id: Optional client-provided task ID (auto-generated if not provided).
  • ttl: Time to keep results available in milliseconds (default 60s).
Returns:
  • list[mcp.types.TextResourceContents | mcp.types.BlobResourceContents] | ResourceTask: A list of content objects if task=False, or a ResourceTask object if task=True.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_prompts_mcp 

list_prompts_mcp(self) -> mcp.types.ListPromptsResult
Send a prompts/list request and return the complete MCP protocol result. Returns:
  • mcp.types.ListPromptsResult: The complete response object from the protocol, containing the list of prompts and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_prompts 

list_prompts(self) -> list[mcp.types.Prompt]
Retrieve a list of prompts available on the server. Returns:
  • list[mcp.types.Prompt]: A list of Prompt objects.
Raises:
  • RuntimeError: If called while the client is not connected.

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get_prompt_mcp 

get_prompt_mcp(self, name: str, arguments: dict[str, Any] | None = None, meta: dict[str, Any] | None = None) -> mcp.types.GetPromptResult
Send a prompts/get request and return the complete MCP protocol result. Args:
  • name: The name of the prompt to retrieve.
  • arguments: Arguments to pass to the prompt. Defaults to None.
  • meta: Request metadata (e.g., for SEP-1686 tasks). Defaults to None.
Returns:
  • mcp.types.GetPromptResult: The complete response object from the protocol, containing the prompt messages and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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get_prompt 

get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -> mcp.types.GetPromptResult

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get_prompt 

get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -> PromptTask

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get_prompt 

get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -> mcp.types.GetPromptResult | PromptTask
Retrieve a rendered prompt message list from the server. Args:
  • name: The name of the prompt to retrieve.
  • arguments: Arguments to pass to the prompt. Defaults to None.
  • task: If True, execute as background task (SEP-1686). Defaults to False.
  • task_id: Optional client-provided task ID (auto-generated if not provided).
  • ttl: Time to keep results available in milliseconds (default 60s).
Returns:
  • mcp.types.GetPromptResult | PromptTask: The complete response object if task=False, or a PromptTask object if task=True.
Raises:
  • RuntimeError: If called while the client is not connected.

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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.

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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.

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list_tools_mcp 

list_tools_mcp(self) -> mcp.types.ListToolsResult
Send a tools/list request and return the complete MCP protocol result. Returns:
  • mcp.types.ListToolsResult: The complete response object from the protocol, containing the list of tools and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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list_tools 

list_tools(self) -> list[mcp.types.Tool]
Retrieve a list of tools available on the server. Returns:
  • list[mcp.types.Tool]: A list of Tool objects.
Raises:
  • RuntimeError: If called while the client is not connected.

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call_tool_mcp 

call_tool_mcp(self, name: str, arguments: dict[str, Any], progress_handler: ProgressHandler | None = None, timeout: datetime.timedelta | float | int | None = None, meta: dict[str, Any] | None = None) -> mcp.types.CallToolResult
Send a tools/call request and return the complete MCP protocol result. This method returns the raw CallToolResult object, which includes an isError flag and other metadata. It does not raise an exception if the tool call results in an error. Args:
  • name: The name of the tool to call.
  • arguments: Arguments to pass to the tool.
  • timeout: The timeout for the tool call. Defaults to None.
  • progress_handler: The progress handler to use for the tool call. Defaults to None.
  • meta: Additional metadata to include with the request. This is useful for passing contextual information (like user IDs, trace IDs, or preferences) that shouldn’t be tool arguments but may influence server-side processing. The server can access this via context.request_context.meta. Defaults to None.
Returns:
  • mcp.types.CallToolResult: The complete response object from the protocol, containing the tool result and any additional metadata.
Raises:
  • RuntimeError: If called while the client is not connected.

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call_tool 

call_tool(self, name: str, arguments: dict[str, Any] | None = None) -> CallToolResult

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call_tool 

call_tool(self, name: str, arguments: dict[str, Any] | None = None) -> ToolTask

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call_tool 

call_tool(self, name: str, arguments: dict[str, Any] | None = None) -> CallToolResult | ToolTask
Call a tool on the server. Unlike call_tool_mcp, this method raises a ToolError if the tool call results in an error. Args:
  • name: The name of the tool to call.
  • arguments: Arguments to pass to the tool. Defaults to None.
  • timeout: The timeout for the tool call. Defaults to None.
  • progress_handler: The progress handler to use for the tool call. Defaults to None.
  • raise_on_error: Whether to raise an exception if the tool call results in an error. Defaults to True.
  • meta: Additional metadata to include with the request. This is useful for passing contextual information (like user IDs, trace IDs, or preferences) that shouldn’t be tool arguments but may influence server-side processing. The server can access this via context.request_context.meta. Defaults to None.
  • task: If True, execute as background task (SEP-1686). Defaults to False.
  • task_id: Optional client-provided task ID (auto-generated if not provided).
  • ttl: Time to keep results available in milliseconds (default 60s).
Returns:
  • CallToolResult | ToolTask: The content returned by the tool if task=False, or a ToolTask object if task=True. If the tool returns structured outputs, they are returned as a dataclass (if an output schema is available) or a dictionary; otherwise, a list of content blocks is returned. Note: to receive both structured and unstructured outputs, use call_tool_mcp instead and access the raw result object.
Raises:
  • ToolError: If the tool call results in an error.
  • RuntimeError: If called while the client is not connected.

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get_task_status 

get_task_status(self, task_id: str) -> GetTaskResult
Query the status of a background task. Sends a ‘tasks/get’ MCP protocol request over the existing transport. Args:
  • task_id: The task ID returned from call_tool_as_task
Returns:
  • Status information including taskId, status, pollInterval, etc.
Raises:
  • RuntimeError: If client not connected

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get_task_result 

get_task_result(self, task_id: str) -> Any
Retrieve the raw result of a completed background task. Sends a ‘tasks/result’ MCP protocol request over the existing transport. Returns the raw result - callers should parse it appropriately. Args:
  • task_id: The task ID returned from call_tool_as_task
Returns:
  • The raw result (could be tool, prompt, or resource result)
Raises:
  • RuntimeError: If client not connected, task not found, or task failed

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list_tasks 

list_tasks(self, cursor: str | None = None, limit: int = 50) -> dict[str, Any]
List background tasks. Sends a ‘tasks/list’ MCP protocol request to the server. If the server returns an empty list (indicating client-side tracking), falls back to querying status for locally tracked task IDs. Args:
  • cursor: Optional pagination cursor
  • limit: Maximum number of tasks to return (default 50)
Returns:
  • Response with structure:
  • tasks: List of task status dicts with taskId, status, etc.
  • nextCursor: Optional cursor for next page
Raises:
  • RuntimeError: If client not connected

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cancel_task 

cancel_task(self, task_id: str) -> mcp.types.CancelTaskResult
Cancel a task, transitioning it to cancelled state. Sends a ‘tasks/cancel’ MCP protocol request. Task will halt execution and transition to cancelled state. Args:
  • task_id: The task ID to cancel
Returns:
  • The task status showing cancelled state
Raises:
  • RuntimeError: If task doesn’t exist

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generate_name 

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