feat: update LATEST

crates/rmcp/src/model.rs

@@ -172,11 +172,35 @@ impl ProtocolVersion {
     pub const V_2025_06_18: Self = Self(Cow::Borrowed("2025-06-18"));
     pub const V_2025_03_26: Self = Self(Cow::Borrowed("2025-03-26"));
     pub const V_2024_11_05: Self = Self(Cow::Borrowed("2024-11-05"));
-    pub const LATEST: Self = Self::V_2025_11_25;
+
+    /// The newest protocol version known to this SDK.
+    pub const LATEST: Self = Self::V_2026_07_28;
 
     /// First protocol version that requires SEP-2243 standard HTTP headers.
     pub const STANDARD_HEADERS: Self = Self::V_2026_07_28;
 
+    /// First protocol version that replaced the `initialize` handshake with
+    /// per-request `_meta` (SEP-2567).
+    pub const NO_INITIALIZE: Self = Self::V_2026_07_28;
+
+    /// The newest protocol version that still has an `initialize` handshake.
+    ///
+    /// From [`Self::NO_INITIALIZE`] onward the lifecycle moved into per-request
+    /// `_meta`, so there is no handshake left to answer. A server replying to
+    /// `initialize` can therefore never name [`Self::LATEST`] once `LATEST`
+    /// reaches that revision — it has to name this one instead.
+    ///
+    /// Use this, not `LATEST`, whenever the subject is the handshake itself:
+    /// the version a server can echo from `initialize`, or the version a test
+    /// needs in order to exercise session-based behavior.
+    ///
+    /// ```
+    /// # use rmcp::model::ProtocolVersion;
+    /// assert!(ProtocolVersion::LATEST_WITH_INITIALIZE.has_initialize());
+    /// assert!(!ProtocolVersion::LATEST.has_initialize());
+    /// ```
+    pub const LATEST_WITH_INITIALIZE: Self = Self::V_2025_11_25;
+
     /// All protocol versions known to this SDK, oldest first.
     pub const KNOWN_VERSIONS: &[Self] = &[
         Self::V_2024_11_05,
@@ -191,6 +215,23 @@ impl ProtocolVersion {
         &self.0
     }
 
+    /// Whether this revision negotiates its lifecycle over the `initialize`
+    /// handshake.
+    ///
+    /// `false` from [`Self::NO_INITIALIZE`] onward, where SEP-2567 moved the
+    /// lifecycle into per-request `_meta`. Unknown versions are compared
+    /// lexically, which orders correctly because every revision is dated
+    /// `YYYY-MM-DD`.
+    ///
+    /// ```
+    /// # use rmcp::model::ProtocolVersion;
+    /// assert!(ProtocolVersion::V_2025_11_25.has_initialize());
+    /// assert!(!ProtocolVersion::V_2026_07_28.has_initialize());
+    /// ```
+    pub fn has_initialize(&self) -> bool {
+        self.as_str() < Self::NO_INITIALIZE.as_str()
+    }
+
     /// The known versions up to and including `max`, oldest first.
     ///
     /// Servers that implement every revision up to some ceiling can return
@@ -4741,6 +4782,34 @@ mod tests {
         );
     }
 
+    // Guards the pair of constants this SDK's negotiation rests on. `LATEST`
+    // and `LATEST_WITH_INITIALIZE` were the same value until `LATEST` reached
+    // `NO_INITIALIZE`; conflating them is what made bumping `LATEST` expensive.
+    #[test]
+    fn latest_with_initialize_is_the_newest_known_version_that_has_one() {
+        let derived = ProtocolVersion::KNOWN_VERSIONS
+            .iter()
+            .filter(|version| version.has_initialize())
+            .max_by(|left, right| left.as_str().cmp(right.as_str()))
+            .expect("some known version should have an initialize handshake");
+        assert_eq!(
+            derived,
+            &ProtocolVersion::LATEST_WITH_INITIALIZE,
+            "LATEST_WITH_INITIALIZE must track KNOWN_VERSIONS"
+        );
+    }
+
+    #[test]
+    fn has_initialize_splits_known_versions_at_no_initialize() {
+        for version in ProtocolVersion::KNOWN_VERSIONS {
+            assert_eq!(
+                version.has_initialize(),
+                version.as_str() < ProtocolVersion::NO_INITIALIZE.as_str(),
+                "{version} classified inconsistently"
+            );
+        }
+    }
+
     #[test]
     fn known_up_to_includes_the_ceiling_itself() {
         assert_eq!(

crates/rmcp/src/service.rs

@@ -201,17 +201,11 @@ pub enum PeerRequestAssociation {
     Unknown { has_pending_outbound_request: bool },
 }
 
-/// Whether `version` predates `2026-07-28`, the revision that replaced the
-/// `initialize` handshake with per-request metadata.
-pub(crate) fn is_legacy_version(version: &ProtocolVersion) -> bool {
-    version.as_str() < ProtocolVersion::V_2026_07_28.as_str()
-}
-
 pub(crate) fn uses_legacy_lifecycle(
     protocol_version: Option<&ProtocolVersion>,
     uses_discover_lifecycle: bool,
 ) -> bool {
-    !uses_discover_lifecycle && protocol_version.is_none_or(is_legacy_version)
+    !uses_discover_lifecycle && protocol_version.is_none_or(ProtocolVersion::has_initialize)
 }
 
 pub(crate) fn peer_request_association<Req: crate::model::GetExtensions, V>(

crates/rmcp/src/service/client.rs

@@ -275,7 +275,7 @@ impl ServiceRole for RoleClient {
         let Some(server_info) = peer.peer_info() else {
             return;
         };
-        if server_info.protocol_version.as_str() < ProtocolVersion::V_2026_07_28.as_str() {
+        if server_info.protocol_version.has_initialize() {
             return;
         }
         peer.set_client_request_metadata(ClientRequestMetadata {
@@ -898,6 +898,7 @@ where
     T: Transport<RoleClient> + 'static,
 {
     let id = id_provider.next_request_id();
+    let client_info_for_meta = client_info.clone();
     let init_request = InitializeRequest {
         method: Default::default(),
         params: client_info,
@@ -920,6 +921,17 @@ where
     let ServerResult::InitializeResult(initialize_result) = response else {
         return Err(ClientInitializeError::ExpectedInitResult(Some(response)));
     };
+    // A server may answer `initialize` with a version that has no handshake
+    // (e.g. a client pinned past `NO_INITIALIZE` talking to a server that
+    // honors it). From there the lifecycle lives in per-request `_meta`, so the
+    // metadata has to be seeded now or every later request omits it.
+    if !initialize_result.protocol_version.has_initialize() {
+        peer.set_client_request_metadata(ClientRequestMetadata {
+            protocol_version: initialize_result.protocol_version.clone(),
+            client_info: client_info_for_meta.client_info,
+            client_capabilities: client_info_for_meta.capabilities,
+        });
+    }
     peer.set_peer_info(initialize_result.into());
 
     // send notification

crates/rmcp/src/service/server.rs

@@ -467,9 +467,15 @@ where
 ///
 /// `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. `2026-07-28` replaced the handshake with
-/// per-request metadata, so a client naming that revision or later is answered
-/// with the server's newest legacy version instead.
+/// version it cannot serve. [`ProtocolVersion::NO_INITIALIZE`] replaced the
+/// handshake with per-request metadata, so a client naming that revision or
+/// later is answered with the server's newest handshake version instead.
+///
+/// `preferred_fallback` is a *preference*, not a guarantee: it is honored only
+/// when it has an `initialize` handshake of its own. A server that pins itself
+/// past [`ProtocolVersion::NO_INITIALIZE`] still has to answer the handshake
+/// with something, so the newest handshake version in `server_supported` is
+/// substituted.
 ///
 /// # Errors
 ///
@@ -479,18 +485,18 @@ where
 /// [`ErrorCode::UNSUPPORTED_PROTOCOL_VERSION`]: crate::model::ErrorCode::UNSUPPORTED_PROTOCOL_VERSION
 pub(crate) fn negotiate_protocol_version(
     client_requested: &ProtocolVersion,
-    server_fallback: ProtocolVersion,
+    preferred_fallback: ProtocolVersion,
     server_supported: &[ProtocolVersion],
 ) -> Result<ProtocolVersion, ErrorData> {
-    if is_legacy_version(client_requested) && server_supported.contains(client_requested) {
+    if client_requested.has_initialize() && server_supported.contains(client_requested) {
         return Ok(client_requested.clone());
     }
-    let legacy_fallback = if is_legacy_version(&server_fallback) {
-        Some(server_fallback)
+    let fallback = if preferred_fallback.has_initialize() {
+        Some(preferred_fallback)
     } else {
-        newest_legacy_version(server_supported)
+        newest_version_with_initialize(server_supported)
     };
-    let Some(legacy_fallback) = legacy_fallback else {
+    let Some(negotiated) = fallback else {
         tracing::warn!(
             client_requested = %client_requested,
             "server supports no protocol version with an initialize handshake; rejecting"
@@ -504,17 +510,17 @@ pub(crate) fn negotiate_protocol_version(
     // 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"
+        negotiated = %negotiated,
+        "client requested a protocol version unavailable over initialize; falling back to a supported version"
     );
-    Ok(legacy_fallback)
+    Ok(negotiated)
 }
 
 /// The newest of `versions` that still has an `initialize` handshake.
-fn newest_legacy_version(versions: &[ProtocolVersion]) -> Option<ProtocolVersion> {
+fn newest_version_with_initialize(versions: &[ProtocolVersion]) -> Option<ProtocolVersion> {
     versions
         .iter()
-        .filter(|version| is_legacy_version(version))
+        .filter(|version| version.has_initialize())
         .max_by(|left, right| left.as_str().cmp(right.as_str()))
         .cloned()
 }

crates/rmcp/tests/test_notification.rs

@@ -5,7 +5,7 @@ use std::sync::Arc;
 use rmcp::{
     ClientHandler, ServerHandler, ServiceExt,
     model::{
-        ClientNotification, CustomNotification, ResourceUpdatedNotificationParam,
+        ClientNotification, CustomNotification, ProtocolVersion, ResourceUpdatedNotificationParam,
         ServerCapabilities, ServerConfig, ServerNotification, SubscribeRequestParams,
     },
 };
@@ -55,6 +55,13 @@ pub struct Client {
 }
 
 impl ClientHandler for Client {
+    // Pinned to the handshake lifecycle: this test drives subscriptions over a
+    // session, which `LATEST` no longer establishes.
+    fn get_info(&self) -> rmcp::model::ClientInfo {
+        rmcp::model::ClientInfo::default()
+            .with_protocol_version(ProtocolVersion::LATEST_WITH_INITIALIZE)
+    }
+
     async fn on_resource_updated(
         &self,
         params: rmcp::model::ResourceUpdatedNotificationParam,

crates/rmcp/tests/test_protocol_version_negotiation.rs

@@ -32,8 +32,9 @@ impl ServerHandler for EchoServer {
 }
 
 /// Every known version whose lifecycle still runs the `initialize` handshake.
-/// `2026-07-28` replaced the handshake with per-request metadata, so this is
-/// also the list a server that has not implemented that revision supports.
+/// [`ProtocolVersion::NO_INITIALIZE`] replaced the handshake with per-request
+/// metadata, so this is also the list a server that has not implemented that
+/// revision supports.
 const HANDSHAKE_VERSIONS: &[ProtocolVersion] = &[
     ProtocolVersion::V_2024_11_05,
     ProtocolVersion::V_2025_03_26,
@@ -168,8 +169,8 @@ async fn handshake_never_agrees_to_a_version_that_dropped_it() {
     let negotiated = negotiated_version(ProtocolVersion::V_2026_07_28).await;
     assert_eq!(
         negotiated,
-        ProtocolVersion::LATEST,
-        "a version that dropped the handshake should fall back to the server's own"
+        ProtocolVersion::LATEST_WITH_INITIALIZE,
+        "a version that dropped the handshake should fall back to the server's newest handshake version"
     );
 }
 
@@ -197,13 +198,14 @@ async fn modern_only_server_rejects_the_handshake() {
 }
 
 #[tokio::test]
-async fn unknown_version_falls_back_to_latest() {
-    let unknown: ProtocolVersion = serde_json::from_str(r#""1999-01-01""#).unwrap();
+async fn unknown_version_falls_back_to_newest_handshake_version() {
+    let unknown: ProtocolVersion =
+        serde_json::from_str(r#""1999-01-01""#).expect("version literal should deserialize");
     let negotiated = negotiated_version(unknown).await;
     assert_eq!(
         negotiated,
-        ProtocolVersion::LATEST,
-        "unknown version should fall back to LATEST"
+        ProtocolVersion::LATEST_WITH_INITIALIZE,
+        "unknown version should fall back to the newest handshake version"
     );
 }
 

crates/rmcp/tests/test_server_initialization.rs

@@ -266,16 +266,18 @@ async fn server_echoes_client_protocol_version_when_known_old() {
     assert_eq!(negotiated, ProtocolVersion::V_2024_11_05);
 }
 
+// `initialize` can only answer with a version that still has a handshake, so a
+// client naming `LATEST` gets `LATEST_WITH_INITIALIZE` instead.
 #[tokio::test]
-async fn server_echoes_client_protocol_version_when_latest() {
-    let negotiated = negotiate_version(TestServer::new(), "2025-11-25").await;
-    assert_eq!(negotiated, ProtocolVersion::LATEST);
+async fn server_falls_back_to_newest_handshake_version_when_client_requests_latest() {
+    let negotiated = negotiate_version(TestServer::new(), ProtocolVersion::LATEST.as_str()).await;
+    assert_eq!(negotiated, ProtocolVersion::LATEST_WITH_INITIALIZE);
 }
 
 #[tokio::test]
 async fn server_falls_back_when_client_protocol_version_unknown() {
     let negotiated = negotiate_version(TestServer::new(), "2099-99-99").await;
-    assert_eq!(negotiated, ProtocolVersion::LATEST);
+    assert_eq!(negotiated, ProtocolVersion::LATEST_WITH_INITIALIZE);
 }
 
 struct PinnedServer;
@@ -289,8 +291,18 @@ impl ServerHandler for PinnedServer {
 
 #[tokio::test]
 async fn server_pinned_version_does_not_override_known_client_request() {
-    let negotiated = negotiate_version(PinnedServer, "2025-11-25").await;
-    assert_eq!(negotiated, ProtocolVersion::LATEST);
+    let negotiated = negotiate_version(
+        PinnedServer,
+        ProtocolVersion::LATEST_WITH_INITIALIZE.as_str(),
+    )
+    .await;
+    assert_eq!(negotiated, ProtocolVersion::LATEST_WITH_INITIALIZE);
+}
+
+#[tokio::test]
+async fn server_pinned_version_used_as_fallback_when_client_requests_latest() {
+    let negotiated = negotiate_version(PinnedServer, ProtocolVersion::LATEST.as_str()).await;
+    assert_eq!(negotiated, ProtocolVersion::V_2025_06_18);
 }
 
 #[tokio::test]

crates/rmcp/tests/test_stateless_protocol_version.rs

@@ -39,8 +39,9 @@ impl ServerHandler for OverridingInitialize {
 }
 
 /// Every known version whose lifecycle still runs the `initialize` handshake.
-/// `2026-07-28` replaced the handshake with per-request metadata, so this is
-/// also the list a server that has not implemented that revision supports.
+/// [`ProtocolVersion::NO_INITIALIZE`] replaced the handshake with per-request
+/// metadata, so this is also the list a server that has not implemented that
+/// revision supports.
 const HANDSHAKE_VERSIONS: &[ProtocolVersion] = &[
     ProtocolVersion::V_2024_11_05,
     ProtocolVersion::V_2025_03_26,
@@ -183,14 +184,15 @@ async fn stateless_sse_init_echoes_handshake_versions_when_handler_overrides_ini
 }
 
 #[tokio::test]
-async fn stateless_json_init_preserves_handler_fallback_for_unknown_version() {
+async fn stateless_json_init_substitutes_a_handshake_version_for_unknown_version() {
     let (client, url, ct) = spawn_server(stateless_json_config()).await;
 
     let resp = post_init(&client, &url, "1999-01-01").await;
     assert_eq!(
         resp["result"]["protocolVersion"],
-        ProtocolVersion::LATEST.as_str(),
-        "unknown version should preserve the handler's fallback"
+        ProtocolVersion::LATEST_WITH_INITIALIZE.as_str(),
+        "the handler's `LATEST` fallback has no handshake, so the newest version \
+         that does is substituted"
     );
 
     ct.cancel();
@@ -221,7 +223,7 @@ async fn stateless_json_init_does_not_agree_to_version_outside_supported_list()
     let resp = post_init(&client, &url, ProtocolVersion::V_2026_07_28.as_str()).await;
     assert_eq!(
         resp["result"]["protocolVersion"],
-        ProtocolVersion::V_2025_11_25.as_str(),
+        ProtocolVersion::LATEST_WITH_INITIALIZE.as_str(),
         "a version outside supported_protocol_versions should not be echoed back"
     );