fix: return tool errors for invalid arguments (modelcontextprotocol#894)

Author: 0xWeakSheep

crates/rmcp/src/handler/server/router/tool.rs

@@ -137,10 +137,24 @@ use crate::{
         tool::{CallToolHandler, DynCallToolHandler, ToolCallContext},
         tool_name_validation::validate_and_warn_tool_name,
     },
-    model::{CallToolResult, Tool, ToolAnnotations},
+    model::{CallToolResult, Content, ErrorCode, Tool, ToolAnnotations},
     service::{MaybeBoxFuture, MaybeSend},
 };
 
+const TOOL_ARGUMENT_DESERIALIZATION_ERROR_PREFIX: &str = "failed to deserialize parameters:";
+
+fn into_tool_argument_error(error: crate::ErrorData) -> Result<CallToolResult, crate::ErrorData> {
+    if error.code == ErrorCode::INVALID_PARAMS
+        && error
+            .message
+            .starts_with(TOOL_ARGUMENT_DESERIALIZATION_ERROR_PREFIX)
+    {
+        return Ok(CallToolResult::error(vec![Content::text(error.message)]));
+    }
+
+    Err(error)
+}
+
 #[non_exhaustive]
 pub struct ToolRoute<S> {
     #[allow(clippy::type_complexity)]
@@ -555,7 +569,10 @@ where
             .get(name)
             .ok_or_else(|| crate::ErrorData::invalid_params("tool not found", None))?;
 
-        let result = (item.call)(context).await?;
+        let result = match (item.call)(context).await {
+            Ok(result) => result,
+            Err(error) => return into_tool_argument_error(error),
+        };
 
         Ok(result)
     }
@@ -611,13 +628,65 @@ mod tests {
     use super::*;
     use crate::{
         RoleServer,
+        handler::server::wrapper::Parameters,
         model::{CallToolRequestParams, ErrorCode, NumberOrString},
         service::{AtomicU32RequestIdProvider, Peer, RequestContext},
     };
 
     struct DummyService;
     impl crate::handler::server::ServerHandler for DummyService {}
 
+    #[derive(serde::Deserialize, schemars::JsonSchema)]
+    struct RequiredParams {
+        project: String,
+    }
+
+    fn requires_params(Parameters(params): Parameters<RequiredParams>) -> String {
+        params.project
+    }
+
+    #[tokio::test]
+    async fn test_argument_deserialization_error_returns_tool_error_result() {
+        let service = DummyService;
+        let router = ToolRouter::new().with_route(ToolRoute::new(
+            crate::model::Tool::new(
+                "requires_params",
+                "requires params",
+                Arc::new(Default::default()),
+            ),
+            requires_params,
+        ));
+
+        let id_provider: Arc<dyn crate::service::RequestIdProvider> =
+            Arc::new(AtomicU32RequestIdProvider::default());
+        let (peer, _rx) = Peer::<RoleServer>::new(id_provider, None);
+        let ctx = crate::handler::server::tool::ToolCallContext::new(
+            &service,
+            CallToolRequestParams {
+                meta: None,
+                name: Cow::Borrowed("requires_params"),
+                arguments: Some(Default::default()),
+                task: None,
+            },
+            RequestContext::new(NumberOrString::Number(1), peer),
+        );
+
+        let result = router
+            .call(ctx)
+            .await
+            .expect("argument validation should be a tool result");
+        assert_eq!(result.is_error, Some(true));
+
+        let text = result
+            .content
+            .first()
+            .and_then(|content| content.raw.as_text())
+            .map(|text| text.text.as_str())
+            .expect("tool error result should include text");
+        assert!(text.contains("failed to deserialize parameters"));
+        assert!(text.contains("missing field `project`"));
+    }
+
     #[tokio::test]
     async fn test_call_disabled_tool_returns_error() {
         let service = DummyService;