Enum McpError 

pub enum McpError {
    Parse {
        message: String,
        source: Option<BoxError>,
    },
    InvalidRequest {
        message: String,
        source: Option<BoxError>,
    },
    MethodNotFound {
        method: String,
        available: Box<[String]>,
    },
    InvalidParams(Box<InvalidParamsDetails>),
    Internal {
        message: String,
        source: Option<BoxError>,
    },
    Transport(Box<TransportDetails>),
    ToolExecution(Box<ToolExecutionDetails>),
    ResourceNotFound {
        uri: String,
    },
    ResourceAccessDenied {
        uri: String,
        reason: Option<String>,
    },
    ConnectionFailed {
        message: String,
        source: Option<BoxError>,
    },
    SessionExpired {
        session_id: String,
    },
    HandshakeFailed(Box<HandshakeDetails>),
    CapabilityNotSupported {
        capability: String,
        available: Box<[String]>,
    },
    UserRejected {
        message: String,
        operation: String,
    },
    Timeout {
        operation: String,
        duration: Duration,
    },
    Cancelled {
        operation: String,
        reason: Option<String>,
    },
    WithContext {
        context: String,
        source: Box<McpError>,
    },
    InternalMessage {
        message: String,
    },
}

The primary error type for the MCP SDK.

This unified error type replaces nested error hierarchies with a single, context-rich type that preserves error chains and provides excellent diagnostic output.

Large error variants are boxed to keep Result<T, McpError> small (approximately 24 bytes on 64-bit systems).

Variants

Parse

Invalid JSON was received by the server.

Fields

message: String

Human-readable error message.

source: Option<BoxError>

The underlying parse error, if available.

InvalidRequest

The JSON sent is not a valid Request object.

Fields

message: String

Human-readable error message.

source: Option<BoxError>

The underlying error, if available.

MethodNotFound

The method does not exist or is not available.

Fields

method: String

The method that was requested.

available: Box<[String]>

List of available methods for suggestions (boxed to reduce size).

InvalidParams(Box<InvalidParamsDetails>)

Invalid method parameter(s) (details boxed to reduce enum size).

Internal

Internal JSON-RPC error.

Fields

message: String

Human-readable error message.

source: Option<BoxError>

The underlying error, if available.

Transport(Box<TransportDetails>)

Transport-level error (details boxed to reduce enum size).

ToolExecution(Box<ToolExecutionDetails>)

A tool execution failed (details boxed to reduce enum size).

ResourceNotFound

A requested resource was not found.

Fields

uri: String

The URI of the resource that was not found.

ResourceAccessDenied

Access to a resource was denied.

Fields

uri: String

The URI of the resource.

reason: Option<String>

The reason for denial, if available.

ConnectionFailed

Connection establishment failed.

Fields

message: String

Human-readable error message.

source: Option<BoxError>

The underlying error, if available.

SessionExpired

Session has expired.

Fields

session_id: String

The expired session ID.

HandshakeFailed(Box<HandshakeDetails>)

Protocol handshake failed (details boxed to reduce enum size).

CapabilityNotSupported

A requested capability is not supported.

Fields

capability: String

The capability that was requested.

available: Box<[String]>

List of available capabilities (boxed to reduce size).

UserRejected

User rejected an operation.

Fields

message: String

Human-readable message about what was rejected.

operation: String

The operation that was rejected.

Timeout

An operation timed out.

Fields

operation: String

The operation that timed out.

duration: Duration

How long we waited before timing out.

Cancelled

An operation was cancelled.

Fields

operation: String

The operation that was cancelled.

reason: Option<String>

Reason for cancellation, if provided.

WithContext

An error with additional context.

Fields

context: String

The context message.

source: Box<McpError>

The underlying error.

InternalMessage

A simple internal error with just a message.

Fields

message: String

Human-readable error message.

Implementations

impl McpError

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

Create a parse error.

pub fn parse_with_source<E: Error + Send + Sync + 'static>( message: impl Into<String>, source: E, ) -> Self

Create a parse error with a source.

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

Create an invalid request error.

pub fn method_not_found(method: impl Into<String>) -> Self

Create a method not found error.

pub fn method_not_found_with_suggestions( method: impl Into<String>, available: Vec<String>, ) -> Self

Create a method not found error with suggestions.

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

Create an invalid params error.

pub fn invalid_params_detailed( method: impl Into<String>, message: impl Into<String>, param_path: Option<String>, expected: Option<String>, actual: Option<String>, ) -> Self

Create an invalid params error with full details.

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

Create an internal error.

pub fn internal_with_source<E: Error + Send + Sync + 'static>( message: impl Into<String>, source: E, ) -> Self

Create an internal error with a source.

pub fn transport(kind: TransportErrorKind, message: impl Into<String>) -> Self

Create a transport error.

pub fn transport_with_context( kind: TransportErrorKind, message: impl Into<String>, context: TransportContext, ) -> Self

Create a transport error with context.

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

Create a tool execution error.

pub fn tool_error_detailed( tool: impl Into<String>, message: impl Into<String>, is_recoverable: bool, data: Option<Value>, ) -> Self

Create a tool execution error with full details.

pub fn resource_not_found(uri: impl Into<String>) -> Self

Create a resource not found error.

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

Create a handshake failed error.

pub fn handshake_failed_with_versions( message: impl Into<String>, client_version: Option<String>, server_version: Option<String>, ) -> Self

Create a handshake failed error with version info.

pub fn capability_not_supported(capability: impl Into<String>) -> Self

Create a capability not supported error.

pub fn capability_not_supported_with_available( capability: impl Into<String>, available: Vec<String>, ) -> Self

Create a capability not supported error with available list.

pub fn timeout(operation: impl Into<String>, duration: Duration) -> Self

Create a timeout error.

pub fn cancelled(operation: impl Into<String>) -> Self

Create a cancelled error.

pub fn cancelled_with_reason( operation: impl Into<String>, reason: impl Into<String>, ) -> Self

Create a cancelled error with reason.

pub fn code(&self) -> i32

Get the JSON-RPC error code for this error.

pub fn is_recoverable(&self) -> bool

Check if this is a recoverable error (LLM can retry).

Trait Implementations

impl Debug for McpError

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

Formats the value using the given formatter.

impl Diagnostic for McpError

fn code(&self) -> Option<Box<dyn Display + '_>>

Unique diagnostic code that can be used to look up more information about this Diagnostic. Ideally also globally unique, and documented in the toplevel crate’s documentation for easy searching. Rust path format (foo::bar::baz) is recommended, but more classic codes like E0123 or enums will work just fine.

fn help(&self) -> Option<Box<dyn Display + '_>>

Additional help text related to this Diagnostic. Do you have any advice for the poor soul who’s just run into this issue?

fn severity(&self) -> Option<Severity>

Diagnostic severity. This may be used by ReportHandlers to change the display format of this diagnostic.

fn url<'a>(&'a self) -> Option<Box<dyn Display + 'a>>

URL to visit for a more detailed explanation/help about this Diagnostic.

fn source_code(&self) -> Option<&dyn SourceCode>

Source code to apply this Diagnostic’s Diagnostic::labels to.

fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>>

Labels to apply to this Diagnostic’s Diagnostic::source_code

fn related<'a>( &'a self, ) -> Option<Box<dyn Iterator<Item = &'a dyn Diagnostic> + 'a>>

Additional related Diagnostics.

fn diagnostic_source(&self) -> Option<&dyn Diagnostic>

The cause of the error.

impl Display for McpError

fn fmt(&self, __formatter: &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<&McpError> for JsonRpcError

fn from(err: &McpError) -> 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 From<Error> for McpError

fn from(err: Error) -> Self

Converts to this type from the input type.

impl From<McpError> for JsonRpcError

fn from(err: McpError) -> Self

Converts to this type from the input type.