Enum NegotiationError 

pub enum NegotiationError {
    UnsupportedServerVersion {
        answered: ProtocolVersion,
        supported: String,
    },
    HeaderMismatch {
        negotiated: ProtocolVersion,
        header: String,
    },
    HeaderNotAVersion {
        header: String,
    },
    MissingStatelessVersion {
        version: ProtocolVersion,
    },
}

Why negotiation could not produce a session.

Every variant is a case where answering with a fallback would be a guess about what the peer meant. Cases where a fallback is specified — an unknown version, a missing header — are not errors and do not appear here.

Variants

UnsupportedServerVersion

The server answered with a revision this client cannot speak, and the client has no revision in common with it.

Fields

answered: ProtocolVersion

What the server answered.

supported: String

What this client can speak, ascending.

HeaderMismatch

A present MCP-Protocol-Version header disagrees with the negotiated version.

A rejection rather than a reconciliation: a header that says one thing while the session says another is a client that has lost track, and picking either would be guessing.

Fields

negotiated: ProtocolVersion

The negotiated version.

header: String

The version the header named, verbatim.

HeaderNotAVersion

The MCP-Protocol-Version header was not a version this build knows.

Fields

header: String

The header value, verbatim.

MissingStatelessVersion

A stateless request carried no version in _meta, which 2026-07-28 requires.

This one has no fallback on purpose. The whole point of the stateless lifecycle is that every request is self-describing, so a request that is not has left the only signal the protocol provides.

Fields

version: ProtocolVersion

The lifecycle that demanded it.

Trait Implementations

impl Clone for NegotiationError

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for NegotiationError

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

Formats the value using the given formatter.

impl Display for NegotiationError

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

Formats the value using the given formatter.

impl Eq for NegotiationError

impl Error for NegotiationError

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<NegotiationError> for ConnectError

fn from(source: NegotiationError) -> Self

Converts to this type from the input type.

impl From<NegotiationError> for HandshakeError

fn from(source: NegotiationError) -> Self

Converts to this type from the input type.

impl PartialEq for NegotiationError

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for NegotiationError