Struct McpError 

pub struct McpError {
    pub code: McpErrorCode,
    pub message: String,
    pub data: Option<Value>,
}

An MCP error response.

This maps directly to the JSON-RPC error object and can be serialized for transport.

Fields

code: McpErrorCode

The error code.

message: String

A short description of the error.

data: Option<Value>

Additional error data (optional).

Implementations

impl McpError

pub fn new(code: McpErrorCode, message: impl Into<String>) -> Self

Creates a new MCP error.

pub fn with_data( code: McpErrorCode, message: impl Into<String>, data: Value, ) -> Self

Creates a new MCP error with additional data.

pub fn parse_error(message: impl Into<String>) -> Self

Creates a parse error.

pub fn invalid_request(message: impl Into<String>) -> Self

Creates an invalid request error.

pub fn method_not_found(method: &str) -> Self

Creates a method not found error.

pub fn invalid_params(message: impl Into<String>) -> Self

Creates an invalid params error.

pub fn internal_error(message: impl Into<String>) -> Self

Creates an internal error.

pub fn tool_error(message: impl Into<String>) -> Self

Creates a tool execution error.

pub fn resource_not_found(uri: &str) -> Self

Creates a resource not found error.

pub fn request_cancelled() -> Self

Creates a request cancelled error.

pub fn masked(&self, mask_enabled: bool) -> McpError

Returns a masked version of this error for client responses.

When masking is enabled, internal error details are hidden to prevent leaking sensitive information (file paths, stack traces, internal state).

What gets masked

• InternalError, ToolExecutionError, Custom codes: message replaced with “Internal server error” and data removed

What’s preserved

• Error code (for programmatic handling)
• Client errors (ParseError, InvalidRequest, MethodNotFound, InvalidParams, ResourceNotFound, ResourceForbidden, PromptNotFound, RequestCancelled): preserved as-is since they don’t contain internal details

Example

use fastmcp_core::{McpError, McpErrorCode};

let internal = McpError::internal_error("Connection failed at /etc/secrets/db.conf");
let masked = internal.masked(true);
assert_eq!(masked.message, "Internal server error");
assert!(masked.data.is_none());

// Client errors are preserved
let client = McpError::invalid_params("Missing field 'name'");
let masked_client = client.masked(true);
assert!(masked_client.message.contains("name"));

pub fn is_internal(&self) -> bool

Returns whether this error contains internal details that should be masked.

Trait Implementations

impl Clone for McpError

fn clone(&self) -> McpError

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for McpError

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

Formats the value using the given formatter.

impl Default for McpError

fn default() -> Self

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for McpError

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Display for McpError

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

Formats the value using the given formatter.

impl Error for McpError

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn description(&self) -> &str

👎Deprecated since 1.42.0: use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

impl From<CancelledError> for McpError

fn from(_: CancelledError) -> Self

Converts to this type from the input type.

impl From<Error> for McpError

fn from(err: Error) -> Self

Converts to this type from the input type.

impl<T> ResultExt<T, McpError> for Result<T, CancelledError>

fn into_outcome(self) -> Outcome<T, McpError>

Convert a Result to an Outcome.

impl Serialize for McpError

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.