Struct A2aError 

#[non_exhaustive]
pub struct A2aError {
    pub code: ErrorCode,
    pub message: String,
    pub data: Option<Value>,
}

The canonical error type for A2A protocol operations.

Carries an ErrorCode, a human-readable message, and an optional data payload (arbitrary JSON) for additional diagnostics.

Fields (Non-exhaustive)

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

code: ErrorCode

Machine-readable error code.

message: String

Human-readable error message.

data: Option<Value>

Optional structured error details.

Implementations

impl A2aError

pub fn new(code: ErrorCode, message: impl Into<String>) -> A2aError

Creates a new A2aError with the given code and message.

pub fn with_data( code: ErrorCode, message: impl Into<String>, data: Value, ) -> A2aError

Creates a new A2aError with the given code, message, and data.

pub fn task_not_found(task_id: impl Display) -> A2aError

Creates a “Task not found” error for the given task ID string.

pub fn task_not_cancelable(task_id: impl Display) -> A2aError

Creates a “Task not cancelable” error.

pub fn internal(msg: impl Into<String>) -> A2aError

Creates an internal error with the provided message.

pub fn invalid_params(msg: impl Into<String>) -> A2aError

Creates an “Invalid params” error.

pub fn is_stream_lagged(&self) -> bool

Returns true when this error is the recoverable consumer-lag signal a server emits on a streaming subscription.

This is not a terminal error. The server surfaces it to say “you read too slowly and n events were dropped”; the stream continues, and a consumer that keeps polling still receives every later event, including the terminal status. A consumer that treats it like any other error and stops reading will silently miss the end of the task.

It is detected by the STREAM_LAGGED_MARKER key in A2aError::data rather than by code or message, because the code is a generic InternalError and the message is human-facing prose.

use a2a_protocol_types::error::{A2aError, ErrorCode};

let lag = A2aError::stream_lagged(7);
assert!(lag.is_stream_lagged());
assert_eq!(lag.dropped_event_count(), Some(7));

// An ordinary internal error is not a lag signal.
assert!(!A2aError::internal("boom").is_stream_lagged());
assert!(!A2aError::new(ErrorCode::TaskNotFound, "gone").is_stream_lagged());

pub fn dropped_event_count(&self) -> Option<u64>

Returns how many events were dropped, when this is a consumer-lag signal (see A2aError::is_stream_lagged); None otherwise.

pub fn stream_lagged(dropped: u64) -> A2aError

Builds the consumer-lag signal carrying dropped.

Servers emit this; consumers identify it with A2aError::is_stream_lagged. It lives here rather than in the server crate so that both sides — and any third-party client — agree on one definition of the marker.

pub fn unsupported_operation(msg: impl Into<String>) -> A2aError

Creates an “Unsupported operation” error.

pub fn parse_error(msg: impl Into<String>) -> A2aError

Creates a “Parse error” error.

pub fn invalid_agent_response(msg: impl Into<String>) -> A2aError

Creates an “Invalid agent response” error.

pub fn extended_card_not_configured(msg: impl Into<String>) -> A2aError

Creates an “Extended agent card not configured” error.

pub fn push_not_supported(msg: impl Into<String>) -> A2aError

Creates a “Push notification not supported” error.

pub fn content_type_not_supported(msg: impl Into<String>) -> A2aError

Creates a “Content type not supported” error.

pub fn extension_support_required(msg: impl Into<String>) -> A2aError

Creates an “Extension support required” error.

pub fn version_not_supported(msg: impl Into<String>) -> A2aError

Creates a “Version not supported” error.

pub fn error_info_data(&self, metadata: Option<Value>) -> Value

Builds the google.rpc.ErrorInfo data array as required by the spec.

Per Section 9.5, 10.6, and 11.6, A2A-specific errors MUST include an ErrorInfo entry with @type, reason, domain, and optional metadata.

Trait Implementations

impl Clone for A2aError

fn clone(&self) -> A2aError

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for A2aError

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for A2aError

fn deserialize<__D>( __deserializer: __D, ) -> Result<A2aError, <__D as Deserializer<'de>>::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Display for A2aError

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

Formats the value using the given formatter.

impl Error for A2aError

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<A2aError> for ClientError

fn from(e: A2aError) -> ClientError

Converts to this type from the input type.

impl From<A2aError> for ServerError

fn from(e: A2aError) -> ServerError

Converts to this type from the input type.

impl Serialize for A2aError

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

where __S: Serializer,

Serialize this value into the given Serde serializer.