fix: honor supported_protocol_versions when negotiating initialize (#1093)

crates/rmcp/src/handler/server.rs

@@ -300,6 +300,10 @@ impl<H: ServerHandler> Service<RoleServer> for H {
     fn get_info(&self) -> <RoleServer as ServiceRole>::Info {
         self.get_info()
     }
+
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        ServerHandler::supported_protocol_versions(self)
+    }
 }
 
 macro_rules! server_handler_methods {
@@ -321,10 +325,17 @@ macro_rules! server_handler_methods {
             info.protocol_version = negotiate_protocol_version(
                 &request.protocol_version,
                 info.protocol_version,
+                &self.supported_protocol_versions(),
             );
             std::future::ready(Ok(info))
         }
         /// Return the protocol versions supported by this server.
+        ///
+        /// Defaults to every version this SDK knows. Override it to narrow the
+        /// set to the revisions the server actually implements: the returned
+        /// list is advertised by [`Self::discover`], bounds what `initialize`
+        /// negotiation may agree to, and is what per-request versions are
+        /// validated against.
         fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
             Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
         }

crates/rmcp/src/handler/server/router.rs

@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use std::{borrow::Cow, sync::Arc};
 
 use prompt::{IntoPromptRoute, PromptRoute};
 use tool::{IntoToolRoute, ToolRoute};
@@ -6,7 +6,10 @@ use tool::{IntoToolRoute, ToolRoute};
 use super::ServerHandler;
 use crate::{
     RoleServer, Service,
-    model::{ClientNotification, ClientRequest, ListPromptsResult, ListToolsResult, ServerResult},
+    model::{
+        ClientNotification, ClientRequest, ListPromptsResult, ListToolsResult, ProtocolVersion,
+        ServerResult,
+    },
     service::NotificationContext,
 };
 
@@ -155,6 +158,10 @@ where
             .list_changed = Some(true);
         info
     }
+
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        ServerHandler::supported_protocol_versions(&self.service)
+    }
 }
 
 #[cfg(test)]

crates/rmcp/src/service.rs

@@ -1,4 +1,4 @@
-use std::sync::OnceLock;
+use std::{borrow::Cow, sync::OnceLock};
 
 use futures::FutureExt;
 #[cfg(not(feature = "local"))]
@@ -284,6 +284,19 @@ pub trait Service<R: ServiceRole>: Send + Sync + 'static {
         context: NotificationContext<R>,
     ) -> impl Future<Output = Result<(), McpError>> + MaybeSendFuture + '_;
     fn get_info(&self) -> R::Info;
+    /// The protocol versions this service can speak, bounding what `initialize`
+    /// negotiation may agree to.
+    ///
+    /// Servers normally override
+    /// [`ServerHandler::supported_protocol_versions`] instead of this method;
+    /// the blanket `Service` impl forwards to it. This method exists so the
+    /// transport and handshake layers, which see only a `Service`, can read the
+    /// list and avoid agreeing to a version the server cannot serve.
+    ///
+    /// [`ServerHandler::supported_protocol_versions`]: crate::handler::server::ServerHandler::supported_protocol_versions
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
+    }
 }
 
 #[cfg(feature = "local")]
@@ -299,6 +312,12 @@ pub trait Service<R: ServiceRole>: 'static {
         context: NotificationContext<R>,
     ) -> impl Future<Output = Result<(), McpError>> + MaybeSendFuture + '_;
     fn get_info(&self) -> R::Info;
+    /// The protocol versions this service can speak.
+    ///
+    /// See the non-`local` variant of this trait for details.
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
+    }
 }
 
 pub trait ServiceExt<R: ServiceRole>: Service<R> + Sized {
@@ -350,6 +369,10 @@ impl<R: ServiceRole> Service<R> for Box<dyn DynService<R>> {
     fn get_info(&self) -> R::Info {
         DynService::get_info(self.as_ref())
     }
+
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        DynService::supported_protocol_versions(self.as_ref())
+    }
 }
 
 #[cfg(not(feature = "local"))]
@@ -365,6 +388,10 @@ pub trait DynService<R: ServiceRole>: Send + Sync {
         context: NotificationContext<R>,
     ) -> MaybeBoxFuture<'_, Result<(), McpError>>;
     fn get_info(&self) -> R::Info;
+    /// See [`Service::supported_protocol_versions`].
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
+    }
 }
 
 #[cfg(feature = "local")]
@@ -380,6 +407,10 @@ pub trait DynService<R: ServiceRole> {
         context: NotificationContext<R>,
     ) -> MaybeBoxFuture<'_, Result<(), McpError>>;
     fn get_info(&self) -> R::Info;
+    /// See [`Service::supported_protocol_versions`].
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Cow::Borrowed(ProtocolVersion::KNOWN_VERSIONS)
+    }
 }
 
 impl<R: ServiceRole, S: Service<R>> DynService<R> for S {
@@ -400,6 +431,9 @@ impl<R: ServiceRole, S: Service<R>> DynService<R> for S {
     fn get_info(&self) -> R::Info {
         self.get_info()
     }
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        Service::supported_protocol_versions(self)
+    }
 }
 
 use std::{

crates/rmcp/src/service/server.rs

@@ -460,12 +460,18 @@ where
     }
 }
 
-/// Echoes the client-requested version if known; otherwise returns `server_fallback`.
+/// Echoes the client-requested version if the server supports it; otherwise
+/// returns `server_fallback`.
+///
+/// `server_supported` comes from [`Service::supported_protocol_versions`], so a
+/// server that narrows that list is never made to answer `initialize` with a
+/// version it cannot serve.
 pub(crate) fn negotiate_protocol_version(
     client_requested: &ProtocolVersion,
     server_fallback: ProtocolVersion,
+    server_supported: &[ProtocolVersion],
 ) -> ProtocolVersion {
-    if ProtocolVersion::KNOWN_VERSIONS.contains(client_requested) {
+    if server_supported.contains(client_requested) {
         client_requested.clone()
     } else {
         tracing::warn!(
@@ -578,8 +584,11 @@ where
             return Err(ServerInitializeError::InitializeFailed(e));
         }
     };
-    init_response.protocol_version =
-        negotiate_protocol_version(&requested_protocol_version, init_response.protocol_version);
+    init_response.protocol_version = negotiate_protocol_version(
+        &requested_protocol_version,
+        init_response.protocol_version,
+        &service.supported_protocol_versions(),
+    );
     // Update peer_info so context.protocol_version() reflects the negotiated
     // version in all subsequent request handlers.
     negotiated_peer_info.protocol_version = init_response.protocol_version.clone();

crates/rmcp/src/transport/streamable_http_server/tower.rs

@@ -279,8 +279,11 @@ impl<S: Service<RoleServer>> Service<RoleServer> for NegotiatingStatelessHttpSer
         if let (Some(requested), ServerResult::InitializeResult(result)) =
             (requested_protocol_version, &mut response)
         {
-            result.protocol_version =
-                negotiate_protocol_version(&requested, result.protocol_version.clone());
+            result.protocol_version = negotiate_protocol_version(
+                &requested,
+                result.protocol_version.clone(),
+                &self.0.supported_protocol_versions(),
+            );
             if let Some(peer_info) = peer.peer_info() {
                 let mut peer_info = (*peer_info).clone();
                 peer_info.protocol_version = result.protocol_version.clone();
@@ -301,6 +304,10 @@ impl<S: Service<RoleServer>> Service<RoleServer> for NegotiatingStatelessHttpSer
     fn get_info(&self) -> ServerInfo {
         self.0.get_info()
     }
+
+    fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
+        self.0.supported_protocol_versions()
+    }
 }
 
 #[expect(