Enum LowerError 

pub enum LowerError {
    FeatureUnavailable {
        feature: Feature,
        negotiated: ProtocolVersion,
    },
    NoRuleForFeature {
        feature: Feature,
    },
    FieldNotRepresentable {
        field: String,
        expected: &'static str,
        detail: String,
        target: ProtocolVersion,
    },
}

Why a value could not be expressed at the target revision.

Variants

FeatureUnavailable

The value needs a feature the negotiated revision cannot express, and no policy authorises a downgrade.

Fields

feature: Feature

The feature the value needs.

negotiated: ProtocolVersion

The revision it could not be expressed at.

NoRuleForFeature

The policy names a feature it can degrade, but names no rule for it.

Distinct from LowerError::FeatureUnavailable because the fix is different: the caller asked for a downgrade and did not say how. Failing here rather than picking a rule is what keeps a policy from quietly choosing the destructive option.

Fields

feature: Feature

The feature with no rule.

FieldNotRepresentable

A field the target revision types as a struct does not fit it.

This is the 2026-07-28 meta case, and it is deliberately not a LowerError::FeatureUnavailable. That variant says a revision cannot express a shape at all, so the fix is to negotiate something else or authorise a downgrade. This one says a value does not fit a shape: the same value lowers cleanly at 2025-06-18, where meta is an untyped serde_json::Value, and fails at every revision that types it. Reporting it as a missing feature would put a false claim in the error — that the field is unavailable at the older revision — and would refuse messages that lower perfectly well.

Why no DegradeRule applies

Every other variant here can be talked out of by a policy, and three options were considered for this one. All three are worse than refusing:

• Synthesise the missing member. A fabricated version reads as authoritative and was never sent by anyone.
• Drop the field, keep the rest of meta. Silent data loss, and meta is exactly where a sender puts something it meant to be understood.
• Move the value into the struct’s extra bag. This one is a trap. ResultMetaObject really does carry #[serde(flatten)] extra, so putting io.modelcontextprotocol/serverInfo in there serialises straight back out and looks lossless. It is not: the reader treats that key as a known member and tries to parse it as an Implementation, which is the parse that just failed here. So the document is unreadable by its intended reader, and building it anyway launders the rejection out of this layer and onto the peer — where the bridge looks healthy and the error lands a long way from its cause.

Refusing is the only option that leaves the failure where it can be diagnosed. Note this leaves nothing for a DegradePolicy to do, and that is the intent rather than an omission: the only rule available would discard the sender’s data.

The blast radius is narrower than it looks

The IR holds meta as a raw value, so a meta lifted from a 2026-07-28 peer has already been through that peer’s deserialiser and will always re-parse. Only a value from a revision that treats meta as untyped, or one built by hand, can reach this.

Fields

field: String

The JSON key that could not be represented.

expected: &'static str

The type the target revision requires there.

detail: String

Why the value did not fit it.

target: ProtocolVersion

The revision that types the field.

Trait Implementations

impl Clone for LowerError

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 LowerError

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

Formats the value using the given formatter.

impl Display for LowerError

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

Formats the value using the given formatter.

impl Eq for LowerError

impl Error for LowerError

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<LowerError> for BridgeError

fn from(source: LowerError) -> Self

Converts to this type from the input type.

impl PartialEq for LowerError

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for LowerError