fix: align protocol version negotiation (#855)

* fix: align protocol version negotiation

* ci: relax semver-checks to allow minor changes

.github/workflows/ci.yml

@@ -85,6 +85,7 @@ jobs:
           cargo semver-checks \
             --package rmcp \
             --baseline-rev ${{ github.event.pull_request.base.sha }} \
+            --release-type minor \
             --only-explicit-features \
             --features default
 
@@ -97,6 +98,7 @@ jobs:
           cargo semver-checks \
             --package rmcp \
             --baseline-rev ${{ github.event.pull_request.base.sha }} \
+            --release-type minor \
             --only-explicit-features \
             --features "$FEATURES"
 

crates/rmcp/src/service/server.rs

@@ -69,6 +69,10 @@ pub enum ServerInitializeError {
     #[error("initialize failed: {0}")]
     InitializeFailed(ErrorData),
 
+    #[deprecated(
+        since = "1.8.0",
+        note = "Negotiation now falls back to the server-configured version. This variant is never constructed and will be removed in a future major release."
+    )]
     #[error("unsupported protocol version: {0}")]
     UnsupportedProtocolVersion(ProtocolVersion),
 
@@ -155,6 +159,23 @@ where
     }
 }
 
+/// Echoes the client-requested version if known; otherwise returns `server_fallback`.
+fn negotiate_protocol_version(
+    client_requested: &ProtocolVersion,
+    server_fallback: ProtocolVersion,
+) -> ProtocolVersion {
+    if ProtocolVersion::KNOWN_VERSIONS.contains(client_requested) {
+        client_requested.clone()
+    } else {
+        tracing::warn!(
+            client_requested = %client_requested,
+            server_fallback = %server_fallback,
+            "client requested unsupported protocol version; falling back to server default"
+        );
+        server_fallback
+    }
+}
+
 async fn serve_server_with_ct_inner<S, T>(
     service: S,
     transport: T,
@@ -227,16 +248,10 @@ where
             return Err(ServerInitializeError::InitializeFailed(e));
         }
     };
-    let peer_protocol_version = peer_info.params.protocol_version.clone();
+    init_response.protocol_version = negotiate_protocol_version(
-    let protocol_version = match peer_protocol_version
+        &peer_info.params.protocol_version,
-        .partial_cmp(&init_response.protocol_version)
+        init_response.protocol_version,
-        .ok_or(ServerInitializeError::UnsupportedProtocolVersion(
+    );
-            peer_protocol_version,
-        ))? {
-        std::cmp::Ordering::Less => peer_info.params.protocol_version.clone(),
-        _ => init_response.protocol_version,
-    };
-    init_response.protocol_version = protocol_version;
     transport
         .send(ServerJsonRpcMessage::response(
             ServerResult::InitializeResult(init_response),

crates/rmcp/tests/test_server_initialization.rs

@@ -4,8 +4,11 @@ mod common;
 
 use common::handlers::TestServer;
 use rmcp::{
-    ServiceExt,
+    ServerHandler, ServiceExt,
-    model::{ClientJsonRpcMessage, ServerJsonRpcMessage, ServerResult},
+    model::{
+        ClientJsonRpcMessage, ProtocolVersion, ServerCapabilities, ServerInfo,
+        ServerJsonRpcMessage, ServerResult,
+    },
     transport::{IntoTransport, Transport},
 };
 
@@ -220,6 +223,82 @@ async fn server_init_buffers_request_before_initialized() {
     result.unwrap().cancel().await.unwrap();
 }
 
+fn init_request_with_version(v: &str) -> ClientJsonRpcMessage {
+    msg(&format!(
+        r#"{{
+            "jsonrpc": "2.0",
+            "id": 1,
+            "method": "initialize",
+            "params": {{
+                "protocolVersion": "{v}",
+                "capabilities": {{}},
+                "clientInfo": {{ "name": "test-client", "version": "0.0.1" }}
+            }}
+        }}"#
+    ))
+}
+
+async fn negotiate_version<H>(handler: H, client_version: &str) -> ProtocolVersion
+where
+    H: ServerHandler + 'static,
+{
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    let _server = tokio::spawn(async move { handler.serve(server_transport).await });
+    let mut client = IntoTransport::<rmcp::RoleClient, _, _>::into_transport(client_transport);
+
+    client
+        .send(init_request_with_version(client_version))
+        .await
+        .unwrap();
+    let response = client.receive().await.unwrap();
+    let ServerJsonRpcMessage::Response(r) = response else {
+        panic!("expected initialize response, got {response:?}");
+    };
+    let ServerResult::InitializeResult(init) = r.result else {
+        panic!("expected InitializeResult");
+    };
+    init.protocol_version
+}
+
+#[tokio::test]
+async fn server_echoes_client_protocol_version_when_known_old() {
+    let negotiated = negotiate_version(TestServer::new(), "2024-11-05").await;
+    assert_eq!(negotiated, ProtocolVersion::V_2024_11_05);
+}
+
+#[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);
+}
+
+#[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);
+}
+
+struct PinnedServer;
+
+impl ServerHandler for PinnedServer {
+    fn get_info(&self) -> ServerInfo {
+        ServerInfo::new(ServerCapabilities::builder().build())
+            .with_protocol_version(ProtocolVersion::V_2025_06_18)
+    }
+}
+
+#[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);
+}
+
+#[tokio::test]
+async fn server_pinned_version_used_as_fallback_for_unknown_client_request() {
+    let negotiated = negotiate_version(PinnedServer, "2099-99-99").await;
+    assert_eq!(negotiated, ProtocolVersion::V_2025_06_18);
+}
+
 // Server buffers multiple requests before initialized and processes them in order.
 #[tokio::test]
 async fn server_init_buffers_multiple_requests_before_initialized() {