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fix: preserve legacy fallback after rejected discovery #1269

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fix: preserve legacy fallback after rejected discovery #1269

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61e1d60
fix: preserve legacy fallback after rejected discovery
Sep 14, 2026
10d27a2
fix(test): import InitializeResult in no-local builds
Sep 14, 2026
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12 changes: 11 additions & 1 deletion crates/rmcp/src/handler/server.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use crate::{
model::*,
service::{
MaybeSendFuture, NotificationContext, RequestContext, RoleServer, Service, ServiceRole,
SubscriptionContext, negotiate_protocol_version, uses_legacy_lifecycle,
SubscriptionContext, is_legacy_version, negotiate_protocol_version, uses_legacy_lifecycle,
},
};

Expand Down Expand Up @@ -61,6 +61,16 @@ impl<H: ServerHandler> Service<RoleServer> for H {
.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(_));
// Legacy-only servers do not implement discovery. MethodNotFound tells
// dual-lifecycle clients to fall back to initialize.
if matches!(&request, ClientRequest::DiscoverRequest(_))
&& self
.supported_protocol_versions()
.iter()
.all(is_legacy_version)
{
return Err(McpError::method_not_found::<DiscoverRequestMethod>());
}
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) {
Expand Down
139 changes: 88 additions & 51 deletions crates/rmcp/src/service/server.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -564,70 +564,107 @@ where

// Get initialize request; the MCP spec permits ping before initialize.
// See: https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#initialization
let (request, id) = loop {
let (initialize_request, id) = loop {
let msg = expect_next_message(&mut transport, "initialize request").await?;
match msg {
ClientJsonRpcMessage::Request(req)
if matches!(req.request, ClientRequest::PingRequest(_)) =>
{
transport
.send(ServerJsonRpcMessage::response(
ServerResult::EmptyResult(EmptyResult {}),
req.id,
))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending pre-init ping response",
)
})?;
}
ClientJsonRpcMessage::Request(req) => break (req.request, req.id),
let request = match msg {
ClientJsonRpcMessage::Request(request) => request,
other => {
return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
other,
)));
}
};

if matches!(&request.request, ClientRequest::PingRequest(_)) {
transport
.send(ServerJsonRpcMessage::response(
ServerResult::EmptyResult(EmptyResult {}),
request.id,
))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(error, "sending pre-init ping response")
})?;
continue;
}
};

let initialize_request = match request {
ClientRequest::InitializeRequest(request) => request,
request => {
let missing_metadata = request
.get_meta()
.missing_required_keys(&ProtocolVersion::V_2026_07_28);
if !missing_metadata.is_empty() {
transport
.send(ServerJsonRpcMessage::error(
missing_request_metadata_error(&missing_metadata),
Some(id.clone()),
))
.await
.map_err(|error| {
let id = request.id;
match request.request {
ClientRequest::InitializeRequest(request) => break (request, id),
mut request => {
let missing_metadata = request
.get_meta()
.missing_required_keys(&ProtocolVersion::V_2026_07_28);
if !missing_metadata.is_empty() {
transport
.send(ServerJsonRpcMessage::error(
missing_request_metadata_error(&missing_metadata),
Some(id.clone()),
))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending pre-init metadata error response",
)
})?;
return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
ClientJsonRpcMessage::request(request, id),
)));
}

let (peer, peer_rx) = Peer::new(id_provider.clone(), None);
if matches!(&request, ClientRequest::DiscoverRequest(_)) {
let context = RequestContext {
ct: ct.child_token(),
id: id.clone(),
meta: std::mem::take(request.get_meta_mut()),
extensions: std::mem::take(request.extensions_mut()),
peer: peer.clone(),
};
let response = match service.handle_request(request, context).await {
Ok(result) => ServerJsonRpcMessage::response(result, id),
Err(error) => {
transport
.send(ServerJsonRpcMessage::error(error, Some(id)))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending rejected discover response",
)
})?;
continue;
}
};

peer.require_request_metadata();
transport.send(response).await.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending pre-init metadata error response",
"sending negotiated request response",
)
})?;
return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
ClientJsonRpcMessage::request(request, id),
)));
return Ok(serve_inner(
service,
transport,
peer,
peer_rx,
VecDeque::new(),
ct,
));
}

peer.require_request_metadata();
return Ok(serve_inner(
service,
transport,
peer,
peer_rx,
VecDeque::from([ClientJsonRpcMessage::request(request, id)]),
ct,
));
}
let (peer, peer_rx) = Peer::new(id_provider, None);
peer.require_request_metadata();
// Dispatch the request from inside the service loop rather than
// inline: its handler may send notifications through `peer`, which
// only complete once the loop drains `peer_rx`.
return Ok(serve_inner(
service,
transport,
peer,
peer_rx,
VecDeque::from([ClientJsonRpcMessage::request(request, id)]),
ct,
));
}
};
let requested_protocol_version = initialize_request.params.protocol_version.clone();
Expand Down
4 changes: 2 additions & 2 deletions crates/rmcp/tests/test_cancelled_response.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@ use rmcp::{
model::{
CancelledNotification, CancelledNotificationParam, ClientJsonRpcMessage, ClientRequest,
ClientResult, ElicitRequest, ElicitRequestParams, ElicitResult, ElicitationAction,
ElicitationSchema, PingRequest, RequestId, ServerJsonRpcMessage, ServerNotification,
ServerRequest, ServerResult,
ElicitationSchema, InitializeResult, PingRequest, RequestId, ServerJsonRpcMessage,
ServerNotification, ServerRequest, ServerResult,
},
service::{PeerRequestOptions, QuitReason, serve_directly},
transport::{IntoTransport, Transport},
Expand Down
16 changes: 8 additions & 8 deletions crates/rmcp/tests/test_server_discover_http.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,7 @@ impl ServerHandler for DiscoveryServer {
}

fn supported_protocol_versions(&self) -> Cow<'static, [ProtocolVersion]> {
Cow::Borrowed(&[ProtocolVersion::V_2025_11_25])
Cow::Borrowed(&[ProtocolVersion::V_2025_11_25, ProtocolVersion::V_2026_07_28])
}
}

Expand Down Expand Up @@ -120,7 +120,7 @@ async fn discover_returns_server_metadata_without_session() {
body["result"],
json!({
"resultType": "complete",
"supportedVersions": ["2025-11-25"],
"supportedVersions": ["2025-11-25", "2026-07-28"],
"capabilities": { "tools": {} },
"_meta": {
"io.modelcontextprotocol/serverInfo": {
Expand Down Expand Up @@ -153,16 +153,16 @@ async fn discover_does_not_require_initialization_in_legacy_session_mode() {
async fn discover_rejects_unsupported_version_with_http_400() {
let (client, url, cancellation_token) = spawn_server(true).await;

let response = post_discover(&client, &url, "2026-07-28", Some("2026-07-28")).await;
let response = post_discover(&client, &url, "2099-01-01", Some("2099-01-01")).await;

assert_eq!(response.status(), 400);
let body: serde_json::Value = response.json().await.expect("response should be JSON");
assert_eq!(body["error"]["code"], -32022);
assert_eq!(
body["error"]["data"],
json!({
"requested": "2026-07-28",
"supported": ["2025-11-25"]
"requested": "2099-01-01",
"supported": ["2025-11-25", "2026-07-28"]
})
);

Expand Down Expand Up @@ -191,7 +191,7 @@ async fn regular_request_rejects_server_unsupported_meta_version() {
"method": "tools/list",
"params": {
"_meta": {
"io.modelcontextprotocol/protocolVersion": "2026-07-28"
"io.modelcontextprotocol/protocolVersion": "2099-01-01"
}
}
});
Expand All @@ -200,7 +200,7 @@ async fn regular_request_rejects_server_unsupported_meta_version() {
.post(&url)
.header("Content-Type", "application/json")
.header("Accept", "application/json, text/event-stream")
.header("MCP-Protocol-Version", "2026-07-28")
.header("MCP-Protocol-Version", "2099-01-01")
.header("Mcp-Method", "tools/list")
.json(&body)
.send()
Expand Down Expand Up @@ -365,7 +365,7 @@ async fn discover_accepts_missing_optional_client_info() {
async fn discover_error_uses_http_400_when_sse_is_configured() {
let (client, url, cancellation_token) = spawn_server(false).await;

let response = post_discover(&client, &url, "2026-07-28", Some("2026-07-28")).await;
let response = post_discover(&client, &url, "2099-01-01", Some("2099-01-01")).await;

assert_eq!(response.status(), 400);
assert_eq!(
Expand Down
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