feat!: add server discovery and negotiation (SEP-2575)

crates/rmcp/src/handler/server.rs

@@ -1,6 +1,6 @@
 // Sampling/Roots/Logging are SEP-2577-deprecated; internal references are expected.
 #![expect(deprecated)]
-use std::sync::Arc;
+use std::{borrow::Cow, sync::Arc};
 
 use crate::{
     error::ErrorData as McpError,
@@ -30,11 +30,46 @@ impl<H: ServerHandler> Service<RoleServer> for H {
         let mrtr_supported = protocol_version
             .as_ref()
             .is_some_and(|v| v.as_str() >= ProtocolVersion::V_2026_07_28.as_str());
+        let requested_version = context.meta.protocol_version();
+        let uses_inline_negotiation = !matches!(&request, ClientRequest::InitializeRequest(_));
+        if uses_inline_negotiation && let Some(requested_version) = requested_version.as_ref() {
+            let supported_versions = self.supported_protocol_versions();
+            if !supported_versions.contains(requested_version) {
+                return Err(McpError::unsupported_protocol_version(
+                    requested_version.clone(),
+                    &supported_versions,
+                ));
+            }
+        }
+        if matches!(&request, ClientRequest::DiscoverRequest(_)) {
+            if requested_version.is_none() {
+                return Err(McpError::invalid_params(
+                    "server/discover requires protocolVersion in request _meta",
+                    None,
+                ));
+            }
+            if context.meta.client_info().is_none() {
+                return Err(McpError::invalid_params(
+                    "server/discover requires clientInfo in request _meta",
+                    None,
+                ));
+            }
+            if context.meta.client_capabilities().is_none() {
+                return Err(McpError::invalid_params(
+                    "server/discover requires clientCapabilities in request _meta",
+                    None,
+                ));
+            }
+        }
         let result = match request {
             ClientRequest::InitializeRequest(request) => self
                 .initialize(request.params, context)
                 .await
                 .map(ServerResult::InitializeResult),
+            ClientRequest::DiscoverRequest(_request) => self
+                .discover(context)
+                .await
+                .map(ServerResult::DiscoverResult),
             ClientRequest::PingRequest(_request) => {
                 self.ping(context).await.map(ServerResult::empty)
             }
@@ -225,6 +260,20 @@ macro_rules! server_handler_methods {
             );
             std::future::ready(Ok(info))
         }
+        /// Return the protocol versions supported by this server.
+        fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+            Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
+        }
+        /// Return this server's discovery information.
+        fn discover(
+            &self,
+            context: RequestContext<RoleServer>,
+        ) -> impl Future<Output = Result<DiscoverResult, McpError>> + MaybeSendFuture + '_ {
+            std::future::ready(Ok(DiscoverResult::from_server_info(
+                self.supported_protocol_versions().into_owned(),
+                self.get_info(),
+            )))
+        }
         fn complete(
             &self,
             request: CompleteRequestParams,
@@ -479,6 +528,17 @@ macro_rules! impl_server_handler_for_wrapper {
                 (**self).initialize(request, context)
             }
 
+            fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+                (**self).supported_protocol_versions()
+            }
+
+            fn discover(
+                &self,
+                context: RequestContext<RoleServer>,
+            ) -> impl Future<Output = Result<DiscoverResult, McpError>> + MaybeSendFuture + '_ {
+                (**self).discover(context)
+            }
+
             fn complete(
                 &self,
                 request: CompleteRequestParams,

crates/rmcp/src/lib.rs

@@ -19,7 +19,7 @@ pub use handler::server::wrapper::Json;
 #[cfg(any(feature = "client", feature = "server"))]
 pub use service::{Peer, Service, ServiceError, ServiceExt};
 #[cfg(feature = "client")]
-pub use service::{RoleClient, serve_client};
+pub use service::{RoleClient, select_protocol_version, serve_client};
 #[cfg(feature = "server")]
 pub use service::{RoleServer, serve_server};
 

crates/rmcp/src/model.rs

@@ -519,6 +519,10 @@ pub struct JsonRpcNotification<N = Notification> {
 pub struct ErrorCode(pub i32);
 
 impl ErrorCode {
+    /// The request used a protocol version the server does not support.
+    pub const UNSUPPORTED_PROTOCOL_VERSION: Self = Self(-32022);
+    /// Processing the request requires a client capability that was not declared.
+    pub const MISSING_REQUIRED_CLIENT_CAPABILITY: Self = Self(-32021);
     pub const HEADER_MISMATCH: Self = Self(-32020);
     pub const RESOURCE_NOT_FOUND: Self = Self(-32002);
     pub const INVALID_REQUEST: Self = Self(-32600);
@@ -568,6 +572,30 @@ impl ErrorData {
     pub fn header_mismatch(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
         Self::new(ErrorCode::HEADER_MISMATCH, message, data)
     }
+    /// Create an unsupported-protocol-version error.
+    pub fn unsupported_protocol_version(
+        requested: ProtocolVersion,
+        supported: &[ProtocolVersion],
+    ) -> Self {
+        Self::new(
+            ErrorCode::UNSUPPORTED_PROTOCOL_VERSION,
+            "Unsupported protocol version",
+            Some(serde_json::json!({
+                "requested": requested,
+                "supported": supported,
+            })),
+        )
+    }
+    /// Create a missing-required-capability error.
+    pub fn missing_required_client_capability(required: ClientCapabilities) -> Self {
+        Self::new(
+            ErrorCode::MISSING_REQUIRED_CLIENT_CAPABILITY,
+            "Missing required client capability",
+            Some(serde_json::json!({
+                "requiredCapabilities": required,
+            })),
+        )
+    }
     pub fn parse_error(message: impl Into<Cow<'static, str>>, data: Option<Value>) -> Self {
         Self::new(ErrorCode::PARSE_ERROR, message, data)
     }
@@ -993,6 +1021,126 @@ impl InitializeResult {
 pub type ServerInfo = InitializeResult;
 pub type ClientInfo = InitializeRequestParams;
 
+const_string!(DiscoverRequestMethod = "server/discover");
+
+/// Parameters for [`DiscoverRequest`].
+#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, Default)]
+#[serde(deny_unknown_fields)]
+#[expect(clippy::exhaustive_structs, reason = "intentionally exhaustive")]
+pub struct DiscoverRequestParams {}
+
+#[cfg(feature = "schemars")]
+#[derive(schemars::JsonSchema)]
+#[expect(dead_code, reason = "schema-only representation of request metadata")]
+struct DiscoverRequestMetaSchema {
+    #[schemars(rename = "io.modelcontextprotocol/protocolVersion")]
+    protocol_version: ProtocolVersion,
+    #[schemars(rename = "io.modelcontextprotocol/clientInfo")]
+    client_info: Implementation,
+    #[schemars(rename = "io.modelcontextprotocol/clientCapabilities")]
+    client_capabilities: ClientCapabilities,
+    #[schemars(rename = "io.modelcontextprotocol/logLevel")]
+    log_level: Option<LoggingLevel>,
+}
+
+#[cfg(feature = "schemars")]
+#[derive(schemars::JsonSchema)]
+#[expect(dead_code, reason = "schema-only representation of request parameters")]
+struct DiscoverRequestParamsSchema {
+    #[schemars(rename = "_meta")]
+    meta: DiscoverRequestMetaSchema,
+}
+
+#[cfg(feature = "schemars")]
+impl schemars::JsonSchema for DiscoverRequestParams {
+    fn schema_name() -> Cow<'static, str> {
+        Cow::Borrowed("DiscoverRequestParams")
+    }
+
+    fn json_schema(generator: &mut schemars::SchemaGenerator) -> schemars::Schema {
+        DiscoverRequestParamsSchema::json_schema(generator)
+    }
+}
+
+/// A request for the server's supported protocol versions and capabilities.
+pub type DiscoverRequest = Request<DiscoverRequestMethod, DiscoverRequestParams>;
+
+/// The server's response to a [`DiscoverRequest`].
+#[derive(Debug, Serialize, Deserialize, Clone, PartialEq)]
+#[serde(rename_all = "camelCase")]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[non_exhaustive]
+pub struct DiscoverResult {
+    /// Identifies how the result should be parsed.
+    pub result_type: ResultType,
+    /// Protocol versions implemented by this server.
+    pub supported_versions: Vec<ProtocolVersion>,
+    /// Capabilities provided by this server.
+    pub capabilities: ServerCapabilities,
+    /// Information about the server implementation.
+    pub server_info: Implementation,
+    /// Optional guidance for using the server.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub instructions: Option<String>,
+    /// How long clients may consider this response fresh, in milliseconds.
+    pub ttl_ms: u64,
+    /// Whether the cached result may be shared across authorization contexts.
+    pub cache_scope: CacheScope,
+    /// Protocol-level response metadata.
+    #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+    pub meta: Option<Meta>,
+}
+
+impl DiscoverResult {
+    /// Create a non-cacheable private discovery result.
+    pub fn new(
+        supported_versions: Vec<ProtocolVersion>,
+        capabilities: ServerCapabilities,
+        server_info: Implementation,
+    ) -> Self {
+        Self {
+            result_type: ResultType::COMPLETE,
+            supported_versions,
+            capabilities,
+            server_info,
+            instructions: None,
+            ttl_ms: 0,
+            cache_scope: CacheScope::Private,
+            meta: None,
+        }
+    }
+
+    /// Create a discovery result from the server's initialization information.
+    pub fn from_server_info(
+        supported_versions: Vec<ProtocolVersion>,
+        server_info: ServerInfo,
+    ) -> Self {
+        let ServerInfo {
+            capabilities,
+            server_info,
+            instructions,
+            meta,
+            ..
+        } = server_info;
+        let mut result = Self::new(supported_versions, capabilities, server_info);
+        result.instructions = instructions;
+        result.meta = meta;
+        result
+    }
+
+    /// Set the cache lifetime hint in milliseconds.
+    pub fn with_ttl_ms(mut self, ttl_ms: u64) -> Self {
+        self.ttl_ms = ttl_ms;
+        self
+    }
+
+    /// Set the cache scope.
+    pub fn with_cache_scope(mut self, cache_scope: CacheScope) -> Self {
+        self.cache_scope = cache_scope;
+        self
+    }
+}
+
 #[allow(clippy::derivable_impls)]
 impl Default for ServerInfo {
     fn default() -> Self {
@@ -3788,6 +3936,7 @@ ts_union!(
     export type ClientRequest =
     | PingRequest
     | InitializeRequest
+    | DiscoverRequest
     | CompleteRequest
     | SetLevelRequest
     | GetPromptRequest
@@ -3811,6 +3960,7 @@ impl ClientRequest {
         match &self {
             ClientRequest::PingRequest(r) => r.method.as_str(),
             ClientRequest::InitializeRequest(r) => r.method.as_str(),
+            ClientRequest::DiscoverRequest(r) => r.method.as_str(),
             ClientRequest::CompleteRequest(r) => r.method.as_str(),
             ClientRequest::SetLevelRequest(r) => r.method.as_str(),
             ClientRequest::GetPromptRequest(r) => r.method.as_str(),
@@ -3882,6 +4032,7 @@ ts_union!(
 
 ts_union!(
     export type ServerResult =
+    | DiscoverResult
     | InitializeResult
     | CompleteResult
     | GetPromptResult

crates/rmcp/src/model/meta.rs

@@ -169,6 +169,7 @@ variant_extension! {
     ClientRequest {
         PingRequest
         InitializeRequest
+        DiscoverRequest
         CompleteRequest
         SetLevelRequest
         GetPromptRequest

crates/rmcp/src/service/client.rs

@@ -10,13 +10,14 @@ use crate::{
         ArgumentInfo, CallToolRequest, CallToolRequestParams, CallToolResponse, CallToolResult,
         CancelledNotification, CancelledNotificationParam, ClientInfo, ClientJsonRpcMessage,
         ClientNotification, ClientRequest, ClientResult, CompleteRequest, CompleteRequestParams,
-        CompleteResult, CompletionContext, CompletionInfo, DEFAULT_MRTR_MAX_ROUNDS, ErrorData,
-        GetExtensions, GetMeta, GetPromptRequest, GetPromptRequestParams, GetPromptResponse,
-        GetPromptResult, InitializeRequest, InitializedNotification, InputRequest,
-        InputRequiredResult, InputResponses, JsonRpcResponse, ListPromptsRequest,
-        ListPromptsResult, ListResourceTemplatesRequest, ListResourceTemplatesResult,
-        ListResourcesRequest, ListResourcesResult, ListToolsRequest, ListToolsResult,
-        NumberOrString, PaginatedRequestParams, ProgressNotification, ProgressNotificationParam,
+        CompleteResult, CompletionContext, CompletionInfo, DEFAULT_MRTR_MAX_ROUNDS,
+        DiscoverRequest, DiscoverRequestParams, DiscoverResult, ErrorData, GetExtensions, GetMeta,
+        GetPromptRequest, GetPromptRequestParams, GetPromptResponse, GetPromptResult,
+        InitializeRequest, InitializedNotification, InputRequest, InputRequiredResult,
+        InputResponses, JsonRpcResponse, ListPromptsRequest, ListPromptsResult,
+        ListResourceTemplatesRequest, ListResourceTemplatesResult, ListResourcesRequest,
+        ListResourcesResult, ListToolsRequest, ListToolsResult, Meta, NumberOrString,
+        PaginatedRequestParams, ProgressNotification, ProgressNotificationParam, ProtocolVersion,
         ReadResourceRequest, ReadResourceRequestParams, ReadResourceResponse, ReadResourceResult,
         Reference, RequestId, RootsListChangedNotification, ServerInfo, ServerJsonRpcMessage,
         ServerNotification, ServerRequest, ServerResult, SetLevelRequest, SetLevelRequestParams,
@@ -147,6 +148,19 @@ where
 #[expect(clippy::exhaustive_structs, reason = "intentionally exhaustive")]
 pub struct RoleClient;
 
+/// Select the first client-preferred protocol version supported by the server.
+///
+/// Returns `None` when no version is shared.
+pub fn select_protocol_version(
+    client_preference: &[ProtocolVersion],
+    server_supported: &[ProtocolVersion],
+) -> Option<ProtocolVersion> {
+    client_preference
+        .iter()
+        .find(|version| server_supported.contains(version))
+        .cloned()
+}
+
 impl ServiceRole for RoleClient {
     type Req = ClientRequest;
     type Resp = ClientResult;
@@ -363,6 +377,19 @@ macro_rules! method {
 }
 
 impl Peer<RoleClient> {
+    /// Discover the server's supported protocol versions and capabilities.
+    pub async fn discover(&self, meta: Meta) -> Result<DiscoverResult, ServiceError> {
+        let mut request = DiscoverRequest::new(DiscoverRequestParams {});
+        request.extensions.insert(meta);
+        let result = self
+            .send_request(ClientRequest::DiscoverRequest(request))
+            .await?;
+        match result {
+            ServerResult::DiscoverResult(result) => Ok(result),
+            _ => Err(ServiceError::UnexpectedResponse),
+        }
+    }
+
     /// Send one `tools/call` request and return either a final result or an MRTR
     /// `InputRequiredResult` without driving any follow-up rounds.
     pub async fn call_tool_once(