feat: expose protocol version negotiation to server handlers

crates/rmcp/src/handler/server.rs

@@ -321,16 +321,59 @@ macro_rules! server_handler_methods {
             context: RequestContext<RoleServer>,
         ) -> impl Future<Output = Result<InitializeResult, McpError>> + MaybeSendFuture + '_ {
             context.peer.set_peer_info(request.clone());
+            std::future::ready(self.negotiate_initialize(&request))
+        }
+        /// Build the `initialize` response for `request`, negotiating the
+        /// protocol version against [`Self::supported_protocol_versions`].
+        ///
+        /// This is the whole body of the default [`Self::initialize`] minus its
+        /// `set_peer_info` side effect, so a server that overrides `initialize`
+        /// to add its own can call this instead of restating the negotiation
+        /// rule:
+        ///
+        /// ```
+        /// use rmcp::{
+        ///     ErrorData as McpError, RoleServer, ServerHandler,
+        ///     model::{InitializeRequestParams, InitializeResult, ServerInfo},
+        ///     service::RequestContext,
+        /// };
+        ///
+        /// struct MyServer;
+        ///
+        /// impl ServerHandler for MyServer {
+        ///     fn get_info(&self) -> ServerInfo {
+        ///         ServerInfo::default()
+        ///     }
+        ///
+        ///     async fn initialize(
+        ///         &self,
+        ///         request: InitializeRequestParams,
+        ///         context: RequestContext<RoleServer>,
+        ///     ) -> Result<InitializeResult, McpError> {
+        ///         // ... record telemetry, register the peer, etc.
+        ///         context.peer.set_peer_info(request.clone());
+        ///         self.negotiate_initialize(&request)
+        ///     }
+        /// }
+        /// ```
+        ///
+        /// # Errors
+        ///
+        /// Returns [`ErrorCode::UNSUPPORTED_PROTOCOL_VERSION`] when this server
+        /// supports no version that still has an `initialize` handshake.
+        ///
+        /// [`ErrorCode::UNSUPPORTED_PROTOCOL_VERSION`]: crate::model::ErrorCode::UNSUPPORTED_PROTOCOL_VERSION
+        fn negotiate_initialize(
+            &self,
+            request: &InitializeRequestParams,
+        ) -> Result<InitializeResult, McpError> {
             let mut info = self.get_info();
-            let negotiated = negotiate_protocol_version(
+            info.protocol_version = negotiate_protocol_version(
                 &request.protocol_version,
                 std::mem::take(&mut info.protocol_version),
                 &self.supported_protocol_versions(),
-            );
-            std::future::ready(negotiated.map(|version| {
-                info.protocol_version = version;
-                info
-            }))
+            )?;
+            Ok(info)
         }
         /// Return the protocol versions supported by this server.
         ///
@@ -339,6 +382,10 @@ macro_rules! server_handler_methods {
         /// list is advertised by [`Self::discover`], bounds what `initialize`
         /// negotiation may agree to, and is what per-request versions are
         /// validated against.
+        ///
+        /// To support everything up to some ceiling, use
+        /// [`ProtocolVersion::known_up_to`] rather than filtering
+        /// [`ProtocolVersion::KNOWN_VERSIONS`] by hand.
         fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
             Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
         }
@@ -621,6 +668,13 @@ macro_rules! impl_server_handler_for_wrapper {
                 (**self).initialize(request, context)
             }
 
+            fn negotiate_initialize(
+                &self,
+                request: &InitializeRequestParams,
+            ) -> Result<InitializeResult, McpError> {
+                (**self).negotiate_initialize(request)
+            }
+
             fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
                 (**self).supported_protocol_versions()
             }

crates/rmcp/src/model.rs

@@ -177,7 +177,7 @@ impl ProtocolVersion {
     /// First protocol version that requires SEP-2243 standard HTTP headers.
     pub const STANDARD_HEADERS: Self = Self::V_2026_07_28;
 
-    /// All protocol versions known to this SDK.
+    /// All protocol versions known to this SDK, oldest first.
     pub const KNOWN_VERSIONS: &[Self] = &[
         Self::V_2024_11_05,
         Self::V_2025_03_26,
@@ -190,6 +190,43 @@ impl ProtocolVersion {
     pub fn as_str(&self) -> &str {
         &self.0
     }
+
+    /// The known versions up to and including `max`, oldest first.
+    ///
+    /// Servers that implement every revision up to some ceiling can return
+    /// this from `supported_protocol_versions` instead of filtering
+    /// [`Self::KNOWN_VERSIONS`] by hand. `max` itself need not be a known
+    /// version; the result is empty when it predates all of them.
+    ///
+    /// The result borrows from [`Self::KNOWN_VERSIONS`], so call it directly
+    /// in the method body — it needs no `static` and no `LazyLock`:
+    ///
+    /// ```rust,ignore
+    /// const MAX_SUPPORTED: ProtocolVersion = ProtocolVersion::V_2025_11_25;
+    ///
+    /// fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+    ///     Cow::Borrowed(ProtocolVersion::known_up_to(&MAX_SUPPORTED))
+    /// }
+    /// ```
+    ///
+    /// ```
+    /// # use rmcp::model::ProtocolVersion;
+    /// assert_eq!(
+    ///     ProtocolVersion::known_up_to(&ProtocolVersion::V_2025_06_18),
+    ///     &[
+    ///         ProtocolVersion::V_2024_11_05,
+    ///         ProtocolVersion::V_2025_03_26,
+    ///         ProtocolVersion::V_2025_06_18,
+    ///     ],
+    /// );
+    /// ```
+    pub fn known_up_to(max: &Self) -> &'static [Self] {
+        let count = Self::KNOWN_VERSIONS
+            .iter()
+            .take_while(|version| version.as_str() <= max.as_str())
+            .count();
+        &Self::KNOWN_VERSIONS[..count]
+    }
 }
 
 impl Serialize for ProtocolVersion {
@@ -4643,6 +4680,51 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn known_versions_are_ordered_oldest_first() {
+        // `known_up_to` walks the list as a sorted prefix.
+        assert!(
+            ProtocolVersion::KNOWN_VERSIONS
+                .windows(2)
+                .all(|pair| pair[0].as_str() < pair[1].as_str())
+        );
+    }
+
+    #[test]
+    fn known_up_to_includes_the_ceiling_itself() {
+        assert_eq!(
+            ProtocolVersion::known_up_to(&ProtocolVersion::V_2024_11_05),
+            &[ProtocolVersion::V_2024_11_05]
+        );
+    }
+
+    #[test]
+    fn known_up_to_the_newest_version_yields_every_known_version() {
+        assert_eq!(
+            ProtocolVersion::known_up_to(&ProtocolVersion::V_2026_07_28),
+            ProtocolVersion::KNOWN_VERSIONS
+        );
+    }
+
+    #[test]
+    fn known_up_to_an_unknown_ceiling_stops_at_the_versions_below_it() {
+        let unknown = ProtocolVersion(Cow::Borrowed("2025-07-01"));
+        assert_eq!(
+            ProtocolVersion::known_up_to(&unknown),
+            &[
+                ProtocolVersion::V_2024_11_05,
+                ProtocolVersion::V_2025_03_26,
+                ProtocolVersion::V_2025_06_18,
+            ]
+        );
+    }
+
+    #[test]
+    fn known_up_to_a_ceiling_below_every_known_version_is_empty() {
+        let ancient = ProtocolVersion(Cow::Borrowed("1999-01-01"));
+        assert!(ProtocolVersion::known_up_to(&ancient).is_empty());
+    }
+
     #[cfg(feature = "transport-streamable-http-client")]
     #[test]
     fn transport_closed_marker_accepts_only_the_process_local_token() {

crates/rmcp/src/service/server.rs

@@ -499,7 +499,9 @@ pub(crate) fn negotiate_protocol_version(
             server_supported,
         ));
     };
-    tracing::warn!(
+    // Falling back is the designed answer for a pinned client, and stateless
+    // HTTP re-runs it on every request, so this is not a warning.
+    tracing::debug!(
         client_requested = %client_requested,
         server_fallback = %legacy_fallback,
         "client requested a protocol version unavailable over initialize; falling back to server default"

crates/rmcp/tests/test_protocol_version_negotiation.rs

@@ -5,13 +5,19 @@
 #![cfg(not(feature = "local"))]
 #![cfg(feature = "client")]
 
-use std::borrow::Cow;
+use std::{
+    borrow::Cow,
+    sync::{
+        Arc,
+        atomic::{AtomicUsize, Ordering},
+    },
+};
 
 use rmcp::{
     ClientHandler, ErrorData, RoleServer, ServerHandler, ServiceExt,
     model::{
-        ClientInfo, ErrorCode, InitializeRequestParams, InitializeResult, ProtocolVersion,
-        ServerInfo,
+        ClientCapabilities, ClientInfo, ErrorCode, Implementation, InitializeRequestParams,
+        InitializeResult, ProtocolVersion, ServerInfo,
     },
     service::{ClientInitializeError, RequestContext},
 };
@@ -232,3 +238,91 @@ async fn narrowed_server_caps_even_when_it_overrides_initialize() {
         "the handshake layer should not raise the version above what the server supports"
     );
 }
+
+/// Overrides `initialize` to run a side effect, then delegates the version
+/// answer back to the SDK with [`ServerHandler::negotiate_initialize`].
+#[derive(Debug, Clone, Default)]
+struct DelegatingServer {
+    initializations: Arc<AtomicUsize>,
+}
+
+impl ServerHandler for DelegatingServer {
+    fn get_info(&self) -> ServerInfo {
+        ServerInfo::default()
+    }
+
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(HANDSHAKE_VERSIONS)
+    }
+
+    async fn initialize(
+        &self,
+        request: InitializeRequestParams,
+        context: RequestContext<RoleServer>,
+    ) -> Result<InitializeResult, ErrorData> {
+        self.initializations.fetch_add(1, Ordering::Relaxed);
+        context.peer.set_peer_info(request.clone());
+        self.negotiate_initialize(&request)
+    }
+}
+
+fn initialize_params(protocol_version: ProtocolVersion) -> InitializeRequestParams {
+    let mut params = InitializeRequestParams::new(
+        ClientCapabilities::default(),
+        Implementation::new("test-client", "0.0.0"),
+    );
+    params.protocol_version = protocol_version;
+    params
+}
+
+#[test]
+fn negotiate_initialize_echoes_a_supported_version() {
+    let result = NarrowedServer
+        .negotiate_initialize(&initialize_params(ProtocolVersion::V_2025_06_18))
+        .expect("a supported handshake version should negotiate");
+    assert_eq!(result.protocol_version, ProtocolVersion::V_2025_06_18);
+}
+
+#[test]
+fn negotiate_initialize_caps_at_supported_versions() {
+    let result = NarrowedServer
+        .negotiate_initialize(&initialize_params(ProtocolVersion::V_2026_07_28))
+        .expect("an unsupported version should fall back rather than fail");
+    assert_eq!(result.protocol_version, ProtocolVersion::V_2025_11_25);
+}
+
+#[test]
+fn negotiate_initialize_keeps_the_rest_of_get_info() {
+    let server = NarrowedServer;
+    let result = server
+        .negotiate_initialize(&initialize_params(ProtocolVersion::V_2026_07_28))
+        .expect("an unsupported version should fall back rather than fail");
+    assert_eq!(result.capabilities, server.get_info().capabilities);
+}
+
+#[test]
+fn negotiate_initialize_rejects_when_no_handshake_version_is_supported() {
+    let error = ModernOnlyServer
+        .negotiate_initialize(&initialize_params(ProtocolVersion::V_2026_07_28))
+        .expect_err("a server with no handshake version cannot answer initialize");
+    assert_eq!(error.code, ErrorCode::UNSUPPORTED_PROTOCOL_VERSION);
+}
+
+#[tokio::test]
+async fn delegating_server_negotiates_like_the_default_initialize() {
+    let negotiated =
+        negotiated_version_with(DelegatingServer::default(), ProtocolVersion::V_2026_07_28).await;
+    assert_eq!(
+        negotiated,
+        ProtocolVersion::V_2025_11_25,
+        "an override that delegates should answer what the default initialize would"
+    );
+}
+
+#[tokio::test]
+async fn delegating_server_still_runs_its_own_side_effect() {
+    let server = DelegatingServer::default();
+    let initializations = Arc::clone(&server.initializations);
+    negotiated_version_with(server, ProtocolVersion::V_2025_06_18).await;
+    assert_eq!(initializations.load(Ordering::Relaxed), 1);
+}

examples/servers/src/common/counter.rs

@@ -270,15 +270,16 @@ impl ServerHandler for Counter {
 
     async fn initialize(
         &self,
-        _request: InitializeRequestParams,
+        request: InitializeRequestParams,
         context: RequestContext<RoleServer>,
     ) -> Result<InitializeResult, McpError> {
         if let Some(http_request_part) = context.extensions.get::<axum::http::request::Parts>() {
             let initialize_headers = &http_request_part.headers;
             let initialize_uri = &http_request_part.uri;
             tracing::info!(?initialize_headers, %initialize_uri, "initialize from http server");
         }
-        Ok(self.get_info())
+        context.peer.set_peer_info(request.clone());
+        self.negotiate_initialize(&request)
     }
 }