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fix(server): answer a known method with invalid params with -32602 #1375

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fix(server): answer a known method with invalid params with -32602 #1375

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569e864
fix(server): answer a known method with invalid params with -32602
talhayme Oct 17, 2026
0c01e98
fix(server): keep methods this lifecycle does not serve as -32601
talhayme Oct 17, 2026
201a516
fix(server): tell invalid params from unknown methods at deserialization
talhayme Oct 19, 2026
4b8df13
fix(model): keep the derived name of ClientRequest::deserialize's typ…
talhayme Oct 19, 2026
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199 changes: 121 additions & 78 deletions crates/rmcp/src/handler/server.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,12 +28,17 @@ pub mod wrapper;
/// the request's `_meta` per-request capabilities or, for session-mode
/// peers, at `initialize` time), the spec requires `-32021` Missing
/// Required Client Capability with the required capability in `data`.
fn validate_tasks_capability<M: ConstString, H: ServerHandler>(
fn validate_tasks_capability<H: ServerHandler>(
handler: &H,
method: &str,
context: &RequestContext<RoleServer>,
) -> Result<(), McpError> {
if !handler.get_info().capabilities.supports_tasks() {
return Err(McpError::method_not_found::<M>());
return Err(McpError::new(
ErrorCode::METHOD_NOT_FOUND,
method.to_owned(),
None,
));
}
let client_declared = context
.client_capabilities()
Expand All @@ -47,6 +52,56 @@ fn validate_tasks_capability<M: ConstString, H: ServerHandler>(
}
}

/// Decides, in one place and before dispatch, whether `method` is served on
/// this lifecycle (SEP-2575) and, for the task methods, whether both sides
/// declared the tasks capability.
///
/// The 2026-07-28 lifecycle has no handshake, so `initialize`, `ping`,
/// `logging/setLevel`, `resources/subscribe` and `resources/unsubscribe` are
/// not found there; the legacy lifecycle has no `subscriptions/listen`. These
/// answer `-32601` whatever the params carried, so a removed method with
/// malformed params is still "not found" rather than "invalid params".
///
/// `initialize` is judged by `inline_session` rather than `legacy_request`:
/// its own params supply the fallback protocol version while the handshake
/// is still open, so only a session already on the inline lifecycle, or a
/// request that declares 2026-07-28 in its `_meta`, has no `initialize`.
fn validate_method_served<H: ServerHandler>(
handler: &H,
method: &str,
legacy_request: bool,
inline_session: bool,
context: &RequestContext<RoleServer>,
) -> Result<(), McpError> {
let not_served: &[&str] = if legacy_request {
&[SubscriptionsListenRequestMethod::VALUE]
} else {
&[
PingRequestMethod::VALUE,
SetLevelRequestMethod::VALUE,
SubscribeRequestMethod::VALUE,
UnsubscribeRequestMethod::VALUE,
]
};
if not_served.contains(&method) || (inline_session && method == InitializeResultMethod::VALUE) {
return Err(McpError::new(
ErrorCode::METHOD_NOT_FOUND,
method.to_owned(),
None,
));
}
if [
GetTaskMethod::VALUE,
UpdateTaskMethod::VALUE,
CancelTaskMethod::VALUE,
]
.contains(&method)
{
validate_tasks_capability(handler, method, context)?;
}
Ok(())
}

impl<H: ServerHandler> Service<RoleServer> for H {
async fn handle_request(
&self,
Expand Down Expand Up @@ -100,6 +155,17 @@ impl<H: ServerHandler> Service<RoleServer> for H {
}
let legacy_request =
uses_legacy_lifecycle(protocol_version.as_ref(), requires_request_metadata);
let inline_session = context.peer.request_metadata_required()
|| requested_version
.as_ref()
.is_some_and(|version| version.as_str() >= ProtocolVersion::V_2026_07_28.as_str());
validate_method_served(
self,
request.method(),
legacy_request,
inline_session,
&context,
)?;
let result = match request {
ClientRequest::InitializeRequest(request) => self
.initialize(request.params, context)
Expand All @@ -110,11 +176,7 @@ impl<H: ServerHandler> Service<RoleServer> for H {
.await
.map(ServerResult::DiscoverResult),
ClientRequest::PingRequest(_request) => {
if !legacy_request {
Err(McpError::method_not_found::<PingRequestMethod>())
} else {
self.ping(context).await.map(ServerResult::empty)
}
self.ping(context).await.map(ServerResult::empty)
}
ClientRequest::CompleteRequest(request) => self
.complete(request.params, context)
Expand Down Expand Up @@ -145,61 +207,45 @@ impl<H: ServerHandler> Service<RoleServer> for H {
.await
.map(ServerResult::from),
ClientRequest::SubscriptionsListenRequest(request) => {
if legacy_request {
Err(McpError::method_not_found::<SubscriptionsListenRequestMethod>())
} else {
let requested = request.params.notifications;
let Some(candidate) = self.accepted_subscription_filter(&requested) else {
return Err(
McpError::method_not_found::<SubscriptionsListenRequestMethod>(),
);
};
let server_info = self.get_info();
let advertised = requested.supported_by(&server_info.capabilities);
let handler_accepted = requested.intersection(&candidate);
let accepted = handler_accepted.intersection(&advertised);
if accepted != handler_accepted {
tracing::debug!(
requested_resource_count = requested
.resource_subscriptions
.as_ref()
.map_or(0, Vec::len),
accepted_resource_count =
accepted.resource_subscriptions.as_ref().map_or(0, Vec::len),
"subscription filter reduced to advertised server capabilities"
);
}
let server_implementation = server_info.server_info;
let subscription_id = context.id.clone();
let subscription =
SubscriptionContext::establish(context, requested, accepted).await?;
// The 2026-07-28 schema defines a final result for graceful
// server teardown; explicit stdio cancellation remains a notification.
self.listen(subscription).await.map(|()| {
let mut result = SubscriptionsListenResult::complete(subscription_id);
result.meta.set_server_info(server_implementation);
ServerResult::SubscriptionsListenResult(result)
})
let requested = request.params.notifications;
let Some(candidate) = self.accepted_subscription_filter(&requested) else {
return Err(McpError::method_not_found::<SubscriptionsListenRequestMethod>());
};
let server_info = self.get_info();
let advertised = requested.supported_by(&server_info.capabilities);
let handler_accepted = requested.intersection(&candidate);
let accepted = handler_accepted.intersection(&advertised);
if accepted != handler_accepted {
tracing::debug!(
requested_resource_count = requested
.resource_subscriptions
.as_ref()
.map_or(0, Vec::len),
accepted_resource_count =
accepted.resource_subscriptions.as_ref().map_or(0, Vec::len),
"subscription filter reduced to advertised server capabilities"
);
}
}
ClientRequest::SubscribeRequest(request) => {
if !legacy_request {
Err(McpError::method_not_found::<SubscribeRequestMethod>())
} else {
self.subscribe(request.params, context)
.await
.map(ServerResult::empty)
}
}
ClientRequest::UnsubscribeRequest(request) => {
if !legacy_request {
Err(McpError::method_not_found::<UnsubscribeRequestMethod>())
} else {
self.unsubscribe(request.params, context)
.await
.map(ServerResult::empty)
}
}
let server_implementation = server_info.server_info;
let subscription_id = context.id.clone();
let subscription =
SubscriptionContext::establish(context, requested, accepted).await?;
// The 2026-07-28 schema defines a final result for graceful
// server teardown; explicit stdio cancellation remains a notification.
self.listen(subscription).await.map(|()| {
let mut result = SubscriptionsListenResult::complete(subscription_id);
result.meta.set_server_info(server_implementation);
ServerResult::SubscriptionsListenResult(result)
})
}
ClientRequest::SubscribeRequest(request) => self
.subscribe(request.params, context)
.await
.map(ServerResult::empty),
ClientRequest::UnsubscribeRequest(request) => self
.unsubscribe(request.params, context)
.await
.map(ServerResult::empty),
ClientRequest::CallToolRequest(request) => {
let client_declared_tasks = context
.client_capabilities()
Expand All @@ -224,24 +270,21 @@ impl<H: ServerHandler> Service<RoleServer> for H {
.on_custom_request(request, context)
.await
.map(ServerResult::CustomResult),
ClientRequest::GetTaskRequest(request) => {
validate_tasks_capability::<GetTaskMethod, _>(self, &context)?;
self.get_task(request.params, context)
.await
.map(ServerResult::GetTaskResult)
}
ClientRequest::UpdateTaskRequest(request) => {
validate_tasks_capability::<UpdateTaskMethod, _>(self, &context)?;
self.update_task(request.params, context)
.await
.map(ServerResult::task_ack)
}
ClientRequest::CancelTaskRequest(request) => {
validate_tasks_capability::<CancelTaskMethod, _>(self, &context)?;
self.cancel_task(request.params, context)
.await
.map(ServerResult::task_ack)
ClientRequest::InvalidParams(request) => {
Err(McpError::invalid_params(request.to_string(), None))
}
ClientRequest::GetTaskRequest(request) => self
.get_task(request.params, context)
.await
.map(ServerResult::GetTaskResult),
ClientRequest::UpdateTaskRequest(request) => self
.update_task(request.params, context)
.await
.map(ServerResult::task_ack),
ClientRequest::CancelTaskRequest(request) => self
.cancel_task(request.params, context)
.await
.map(ServerResult::task_ack),
};
let result = result.and_then(|mut result| {
if matches!(result, ServerResult::InputRequiredResult(_)) && !is_2026_07_28_or_later {
Expand Down
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