fix: default CallToolResult content to empty vec on missing field

Author: DaleSeo

crates/rmcp/src/model.rs

@@ -2697,6 +2697,7 @@ pub type ElicitationCompletionNotification =
 #[non_exhaustive]
 pub struct CallToolResult {
     /// The content returned by the tool (text, images, etc.)
+    #[serde(default)]
     pub content: Vec<Content>,
     /// An optional JSON object that represents the structured result of the tool call
     #[serde(skip_serializing_if = "Option::is_none")]

crates/rmcp/tests/test_structured_output.rs

@@ -298,18 +298,20 @@ async fn test_empty_content_array_with_is_error() {
     assert_eq!(result.is_error, Some(false));
 }
 
-#[tokio::test]
-async fn test_missing_content_is_rejected() {
+#[test]
+fn test_missing_content_defaults_to_empty() {
     let raw = json!({ "isError": false });
-    let result: Result<CallToolResult, _> = serde_json::from_value(raw);
-    assert!(result.is_err());
+    let result: CallToolResult = serde_json::from_value(raw).unwrap();
+    assert!(result.content.is_empty());
+    assert_eq!(result.is_error, Some(false));
 }
 
-#[tokio::test]
-async fn test_missing_content_with_structured_content_is_rejected() {
+#[test]
+fn test_missing_content_with_structured_content_deserializes() {
     let raw = json!({ "structuredContent": {"key": "value"}, "isError": false });
-    let result: Result<CallToolResult, _> = serde_json::from_value(raw);
-    assert!(result.is_err());
+    let result: CallToolResult = serde_json::from_value(raw).unwrap();
+    assert!(result.content.is_empty());
+    assert_eq!(result.structured_content.unwrap()["key"], "value");
 }
 
 #[tokio::test]
@@ -333,3 +335,13 @@ async fn test_empty_content_roundtrip() {
     let deserialized: CallToolResult = serde_json::from_value(v).unwrap();
     assert_eq!(deserialized, result);
 }
+
+#[test]
+fn test_call_tool_result_deserialize_without_content() {
+    let json = json!({
+        "structuredContent": {"message": "Hello"}
+    });
+    let result: CallToolResult = serde_json::from_value(json).unwrap();
+    assert!(result.content.is_empty());
+    assert!(result.structured_content.is_some());
+}