Struct RequestMetaObject 

pub struct RequestMetaObject {
    pub progress_token: Option<ProgressToken>,
    pub protocol_version: String,
    pub client_info: Option<Implementation>,
    pub client_capabilities: ClientCapabilities,
    pub log_level: Option<LoggingLevel>,
    pub extra: HashMap<String, Value>,
}

Core MCP protocol types and error handling Strictly-typed request _meta carrying the per-request capability negotiation. See also MetaObject for key naming rules and reserved prefixes, and General fields: _meta for the full spec section.

Replaces the 2025-11-25 initialize handshake: every 2026-07-28 request’s params._meta is REQUIRED and carries the per-request protocol version, client info, and client capabilities. The server cannot infer these from prior requests in the stateless model (SEP-2567, SEP-2575).

Required fields (schema: not marked ?):

• io.modelcontextprotocol/protocolVersion: string
• io.modelcontextprotocol/clientCapabilities: ClientCapabilities

Optional fields:

• progressToken?: ProgressToken — caller opts in to notifications/progress
• io.modelcontextprotocol/clientInfo?: Implementation — clients SHOULD send it unless configured otherwise. It is self-reported and unverified: servers MUST NOT key behavior on it and MUST NOT treat it as a security identity. A request that omits it is well-formed and MUST be served.
• io.modelcontextprotocol/logLevel?: LoggingLevel — replaces the removed logging/setLevel RPC; client opts in to log notifications per-request

Extra keys per MetaObject rules are preserved in Self::extra.

Implementation and ClientCapabilities are imported from crate::initialize.

Serialize is hand-written rather than #[derive] + #[serde(flatten)]: extra is public and caller-writable, so a caller could otherwise insert one of the reserved typed keys (progressToken or any io.modelcontextprotocol/* field) into it and produce the same key twice on the wire. The typed field always wins; a colliding extra entry is dropped rather than emitted. Mirrors the same guard on crate::subscriptions::SubscriptionsListenResultMetaObject.

Fields

progress_token: Option<ProgressToken>

Caller-supplied progress token. Optional.

protocol_version: String

Protocol version this request is encoded against. Required.

For HTTP transport, MUST match the MCP-Protocol-Version header. If unsupported by the server, the server returns McpError::UnsupportedProtocolVersion.

client_info: Option<Implementation>

Identifies the client. Optional — absent is well-formed. Present but not a valid Implementation is still a parse failure.

client_capabilities: ClientCapabilities

Client capabilities for this specific request. Required.

Per spec: “Capabilities are declared per-request rather than once at initialization; an empty object means the client supports no optional capabilities. Servers MUST NOT infer capabilities from prior requests.”

log_level: Option<LoggingLevel>

👎Deprecated since 0.4.0:

Deprecated per SEP-2577 (2026-07-28) along with the whole Logging surface. Earliest removal: first release on/after 2027-07-28.

Per-request log level opt-in. Optional.

Replaces the former logging/setLevel RPC. If absent, the server MUST NOT send notifications/message for this request. Deprecated per SEP-2577 along with the whole Logging surface; functional through the migration window.

extra: HashMap<String, Value>

Additional caller-supplied meta keys per the MetaObject extension rules.

Implementations

impl RequestMetaObject

pub fn new( protocol_version: impl Into<String>, client_info: Implementation, client_capabilities: ClientCapabilities, ) -> RequestMetaObject

Construct with the two required fields plus client_info, which clients SHOULD send. Use Self::without_client_info for the configured-off case. All other optionals start None, extra empty.

pub fn without_client_info(self) -> RequestMetaObject

Drop clientInfo, modelling a client configured not to report itself.

pub fn with_progress_token( self, token: impl Into<ProgressToken>, ) -> RequestMetaObject

Attach a progress token.

pub fn with_log_level(self, level: LoggingLevel) -> RequestMetaObject

👎Deprecated since 0.4.0:

Deprecated per SEP-2577 (2026-07-28) along with the whole Logging surface.

Opt in to log notifications at this level for this request. Deprecated per SEP-2577 along with the whole Logging surface.

pub fn with_extra( self, key: impl Into<String>, value: impl Into<Value>, ) -> RequestMetaObject

Add an extra meta key. Key must follow MetaObject naming rules (no validation performed here; keep keys spec-conformant).

Trait Implementations

impl Clone for RequestMetaObject

fn clone(&self) -> RequestMetaObject

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for RequestMetaObject

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for RequestMetaObject

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

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Serialize for RequestMetaObject

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.