Struct ClientBuilder 

pub struct ClientBuilder { /* private fields */ }

Builder for configuring an MCP client.

Use this to configure timeout, retry, and spawn options before connecting to an MCP server.

Implementations

impl ClientBuilder

pub fn new() -> ClientBuilder

Creates a new client builder with default settings.

Default configuration:

• Client name: “fastmcp-client”
• Timeout: 30 seconds
• Max retries: 0 (no retries)
• Retry delay: 1 second
• Inherit environment: true
• Auto-initialize: false (initialize immediately on connect)

pub fn client_info( self, name: impl Into<String>, version: impl Into<String>, ) -> ClientBuilder

Sets the client name and version.

This information is sent to the server during initialization.

pub fn timeout_ms(self, timeout: u64) -> ClientBuilder

Sets the request timeout in milliseconds.

This affects how long the client waits for responses from the server. Default is 30,000ms (30 seconds).

pub fn max_retries(self, retries: u32) -> ClientBuilder

Sets the maximum number of connection retries.

When connecting to a server fails, the client will retry up to this many times before returning an error. Default is 0 (no retries).

pub fn retry_delay_ms(self, delay: u64) -> ClientBuilder

Sets the delay between connection retries in milliseconds.

Default is 1,000ms (1 second).

pub fn working_dir(self, path: impl Into<PathBuf>) -> ClientBuilder

Sets the working directory for the subprocess.

If not set, the subprocess inherits the current working directory.

pub fn env( self, key: impl Into<String>, value: impl Into<String>, ) -> ClientBuilder

Adds an environment variable for the subprocess.

Multiple calls to this method accumulate environment variables.

pub fn envs<I, K, V>(self, vars: I) -> ClientBuilder

where I: IntoIterator<Item = (K, V)>, K: Into<String>, V: Into<String>,

Adds multiple environment variables for the subprocess.

pub fn inherit_env(self, inherit: bool) -> ClientBuilder

Sets whether to inherit the parent process’s environment.

If true (default), the subprocess starts with the parent’s environment plus any variables added via env or envs.

If false, the subprocess starts with only the explicitly set variables.

pub fn capabilities(self, capabilities: ClientCapabilities) -> ClientBuilder

Sets the client capabilities to advertise to the server.

pub fn auto_initialize(self, enabled: bool) -> ClientBuilder

Enables auto-initialization mode.

When enabled, the client defers the MCP initialization handshake until the first method call (e.g., list_tools, call_tool). This allows the subprocess to start immediately without blocking on initialization.

Default is false (initialize immediately on connect).

Example

let client = ClientBuilder::new()
    .auto_initialize(true)
    .connect_stdio("uvx", &["my-server"])?;

// Subprocess is running but not yet initialized
// Initialization happens on first use:
let tools = client.list_tools()?; // Initializes here

pub fn connect_stdio( self, command: &str, args: &[&str], ) -> Result<Client, McpError>

Connects to a server via stdio subprocess.

Spawns the specified command as a subprocess and communicates via stdin/stdout using JSON-RPC over NDJSON framing.

Arguments

• command - The command to run (e.g., “uvx”, “npx”)
• args - Arguments to pass to the command

Errors

Returns an error if:

• The subprocess fails to spawn
• The initialization handshake fails
• All retry attempts are exhausted

pub fn connect_stdio_with_cx( self, command: &str, args: &[&str], cx: &Cx, ) -> Result<Client, McpError>

Connects to a server via stdio subprocess with a provided Cx.

Same as connect_stdio but allows providing a custom capability context for cancellation support.

Trait Implementations

impl Clone for ClientBuilder

fn clone(&self) -> ClientBuilder

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ClientBuilder

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Default for ClientBuilder

fn default() -> ClientBuilder

Returns the “default value” for a type.