use serde_json::{json, Value as JsonValue};
pub const PROTOCOL_VERSION: &str = "2025-11-25";
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct UnsupportedMcpMethod {
pub method: &'static str,
pub feature: &'static str,
pub role: &'static str,
pub reason: &'static str,
}
pub const UNSUPPORTED_LATEST_SPEC_METHODS: &[UnsupportedMcpMethod] = &[
UnsupportedMcpMethod {
method: "completion/complete",
feature: "completions",
role: "server",
reason: "Harn does not expose prompt or resource-template argument completion.",
},
UnsupportedMcpMethod {
method: "resources/subscribe",
feature: "resource subscriptions",
role: "server",
reason: "Harn resources are read on demand and are not backed by subscription state.",
},
UnsupportedMcpMethod {
method: "resources/unsubscribe",
feature: "resource subscriptions",
role: "server",
reason: "Harn resources are read on demand and are not backed by subscription state.",
},
UnsupportedMcpMethod {
method: "roots/list",
feature: "roots",
role: "client",
reason: "Harn does not currently expose host root discovery to MCP servers.",
},
UnsupportedMcpMethod {
method: "sampling/createMessage",
feature: "sampling",
role: "client",
reason: "Harn does not currently let MCP servers invoke a client-side model sampler.",
},
UnsupportedMcpMethod {
method: "elicitation/create",
feature: "elicitation",
role: "client",
reason: "Harn does not currently let MCP servers request client-side user elicitation.",
},
UnsupportedMcpMethod {
method: "tasks/get",
feature: "tasks",
role: "server",
reason: "Harn MCP tools execute inline; MCP task polling is not implemented.",
},
UnsupportedMcpMethod {
method: "tasks/result",
feature: "tasks",
role: "server",
reason: "Harn MCP tools execute inline; MCP task result retrieval is not implemented.",
},
UnsupportedMcpMethod {
method: "tasks/list",
feature: "tasks",
role: "server",
reason: "Harn MCP tools execute inline; MCP task listing is not implemented.",
},
UnsupportedMcpMethod {
method: "tasks/cancel",
feature: "tasks",
role: "server",
reason: "Harn MCP cancellation uses notifications/cancelled instead of MCP tasks.",
},
];
pub fn unsupported_latest_spec_method(method: &str) -> Option<&'static UnsupportedMcpMethod> {
UNSUPPORTED_LATEST_SPEC_METHODS
.iter()
.find(|entry| entry.method == method)
}
pub fn unsupported_latest_spec_method_response(
id: impl Into<JsonValue>,
method: &str,
) -> Option<JsonValue> {
unsupported_latest_spec_method(method).map(|entry| {
crate::jsonrpc::error_response_with_data(
id,
-32601,
&format!("Unsupported MCP method: {method}"),
unsupported_method_data(entry),
)
})
}
pub fn unsupported_task_augmentation_response(id: impl Into<JsonValue>, method: &str) -> JsonValue {
crate::jsonrpc::error_response_with_data(
id,
-32602,
"MCP task-augmented execution is not supported",
json!({
"type": "mcp.unsupportedFeature",
"protocolVersion": PROTOCOL_VERSION,
"method": method,
"feature": "tasks",
"status": "unsupported",
"reason": "Harn MCP tools execute inline and do not advertise taskSupport.",
}),
)
}
pub fn requests_task_augmentation(params: &JsonValue) -> bool {
params.get("task").is_some()
}
fn unsupported_method_data(entry: &UnsupportedMcpMethod) -> JsonValue {
json!({
"type": "mcp.unsupportedFeature",
"protocolVersion": PROTOCOL_VERSION,
"method": entry.method,
"feature": entry.feature,
"role": entry.role,
"status": "unsupported",
"reason": entry.reason,
})
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn latest_spec_gap_methods_are_explicit() {
for method in [
"completion/complete",
"resources/subscribe",
"resources/unsubscribe",
"roots/list",
"sampling/createMessage",
"elicitation/create",
"tasks/get",
"tasks/result",
"tasks/list",
"tasks/cancel",
] {
let response = unsupported_latest_spec_method_response(json!(1), method)
.expect("expected explicit unsupported method");
assert_eq!(response["error"]["code"], json!(-32601));
assert_eq!(response["error"]["data"]["method"], json!(method));
assert_eq!(response["error"]["data"]["status"], json!("unsupported"));
}
}
#[test]
fn task_augmentation_error_is_json_rpc_shaped() {
let response = unsupported_task_augmentation_response(json!("call-1"), "tools/call");
assert_eq!(response["jsonrpc"], json!("2.0"));
assert_eq!(response["id"], json!("call-1"));
assert_eq!(response["error"]["code"], json!(-32602));
assert_eq!(response["error"]["data"]["feature"], json!("tasks"));
}
}