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fix(server): keep discover lifecycle bootstrap-neutral #1248

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10e2798
fix(server): keep discover lifecycle bootstrap-neutral
Sep 7, 2026
68b1f4a
fix(server): isolate discover peer and drop panic
DaleSeo Oct 2, 2026
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131 changes: 90 additions & 41 deletions crates/rmcp/src/service/server.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -568,9 +568,14 @@ where
let mut transport = transport.into_transport();
let id_provider = <Arc<AtomicU32RequestIdProvider>>::default();

// Get initialize request; the MCP spec permits ping before initialize.
let (peer, peer_rx) = Peer::new(id_provider.clone(), None);

// Select the lifecycle only after an initialize request or the first valid
// non-discover request with complete inline metadata. A discover request is
// a bootstrap probe: respond to it, but remain open to either lifecycle.
// The MCP spec also 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)
Expand All @@ -589,56 +594,100 @@ where
)
})?;
}
ClientJsonRpcMessage::Request(req) => break (req.request, req.id),
other => {
return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
other,
)));
}
}
};

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| {
ClientJsonRpcMessage::Request(req) => {
let id = req.id;
let mut request = match req.request {
ClientRequest::InitializeRequest(request) => break (request, id),
request => request,
};
let missing_metadata = request
.get_meta()
.missing_required_keys(&ProtocolVersion::V_2026_07_28);
let requested_version = match request.get_meta().protocol_version() {
Some(version) if missing_metadata.is_empty() => version,
_ => {
transport
.send(ServerJsonRpcMessage::error(
missing_request_metadata_error(&missing_metadata),
Some(id),
))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending pre-init metadata error response",
)
})?;
continue;
}
};

if matches!(request, ClientRequest::DiscoverRequest(_)) {
// No lifecycle exists yet, so the handler gets a peer that
// cannot reach the client.
let (bootstrap_peer, _) = Peer::new(id_provider.clone(), None);
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: bootstrap_peer,
};
let response = match service.handle_request(request, context).await {
Ok(result) => ServerJsonRpcMessage::response(result, id),
Err(error) => ServerJsonRpcMessage::error(error, Some(id)),
};
transport.send(response).await.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending pre-init metadata error response",
"sending bootstrap request response",
)
})?;
continue;
}

let supported_versions = service.supported_protocol_versions();
if !supported_versions.contains(&requested_version) {
transport
.send(ServerJsonRpcMessage::error(
ErrorData::unsupported_protocol_version(
requested_version,
&supported_versions,
),
Some(id),
))
.await
.map_err(|error| {
ServerInitializeError::transport::<T>(
error,
"sending unsupported inline version response",
)
})?;
continue;
}
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,
));
}
other => {
return Err(ServerInitializeError::ExpectedInitializeRequest(Some(
ClientJsonRpcMessage::request(request, id),
other,
)));
}
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();
let mut negotiated_peer_info = initialize_request.params.clone();
let (peer, peer_rx) = Peer::new(id_provider, Some(negotiated_peer_info.clone()));
peer.set_peer_info(negotiated_peer_info.clone());
let request = ClientRequest::InitializeRequest(initialize_request);
let context = RequestContext {
ct: ct.child_token(),
Expand Down
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