Match the shape of CallToolResult schema

crates/rmcp/src/model.rs

@@ -181,7 +181,7 @@ impl<'de> Deserialize<'de> for ProtocolVersion {
 pub enum NumberOrString {
     /// A numeric identifier
     Number(u32),
-    /// A string identifier  
+    /// A string identifier
     String(Arc<str>),
 }
 
@@ -1186,8 +1186,7 @@ pub type RootsListChangedNotification = NotificationNoParam<RootsListChangedNoti
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct CallToolResult {
     /// The content returned by the tool (text, images, etc.)
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub content: Option<Vec<Content>>,
+    pub content: Vec<Content>,
     /// An optional JSON object that represents the structured result of the tool call
     #[serde(skip_serializing_if = "Option::is_none")]
     pub structured_content: Option<Value>,
@@ -1200,15 +1199,15 @@ impl CallToolResult {
     /// Create a successful tool result with unstructured content
     pub fn success(content: Vec<Content>) -> Self {
         CallToolResult {
-            content: Some(content),
+            content,
             structured_content: None,
             is_error: Some(false),
         }
     }
     /// Create an error tool result with unstructured content
     pub fn error(content: Vec<Content>) -> Self {
         CallToolResult {
-            content: Some(content),
+            content,
             structured_content: None,
             is_error: Some(true),
         }
@@ -1229,7 +1228,7 @@ impl CallToolResult {
     /// ```
     pub fn structured(value: Value) -> Self {
         CallToolResult {
-            content: Some(vec![Content::text(value.to_string())]),
+            content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(false),
         }
@@ -1254,7 +1253,7 @@ impl CallToolResult {
     /// ```
     pub fn structured_error(value: Value) -> Self {
         CallToolResult {
-            content: Some(vec![Content::text(value.to_string())]),
+            content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(true),
         }
@@ -1270,10 +1269,10 @@ impl CallToolResult {
     where
         T: DeserializeOwned,
     {
-        let raw_text = match (self.structured_content, &self.content) {
+        let raw_text = match (self.structured_content, &self.content.first()) {
             (Some(value), _) => return serde_json::from_value(value),
             (None, Some(contents)) => {
-                if let Some(text) = contents.first().and_then(|c| c.as_text()) {
+                if let Some(text) = contents.as_text() {
                     let text = &text.text;
                     Some(text)
                 } else {
@@ -1308,13 +1307,13 @@ impl<'de> Deserialize<'de> for CallToolResult {
 
         let helper = CallToolResultHelper::deserialize(deserializer)?;
         let result = CallToolResult {
-            content: helper.content,
+            content: helper.content.unwrap_or_default(),
             structured_content: helper.structured_content,
             is_error: helper.is_error,
         };
 
         // Validate mutual exclusivity
-        if result.content.is_none() && result.structured_content.is_none() {
+        if result.content.is_empty() && result.structured_content.is_none() {
             return Err(serde::de::Error::custom(
                 "CallToolResult must have either content or structured_content",
             ));

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -304,10 +304,7 @@
       "properties": {
         "content": {
           "description": "The content returned by the tool (text, images, etc.)",
-          "type": [
-            "array",
-            "null"
-          ],
+          "type": "array",
           "items": {
             "$ref": "#/definitions/Annotated"
           }
@@ -322,7 +319,10 @@
         "structuredContent": {
           "description": "An optional JSON object that represents the structured result of the tool call"
         }
-      }
+      },
+      "required": [
+        "content"
+      ]
     },
     "CancelledNotificationMethod": {
       "type": "string",

crates/rmcp/tests/test_structured_output.rs

@@ -122,10 +122,10 @@ async fn test_structured_content_in_call_result() {
 
     let result = CallToolResult::structured(structured_data.clone());
 
-    assert!(result.content.is_some());
+    assert!(!result.content.is_empty());
     assert!(result.structured_content.is_some());
 
-    let contents = result.content.unwrap();
+    let contents = result.content;
 
     assert_eq!(contents.len(), 1);
 
@@ -150,10 +150,10 @@ async fn test_structured_error_in_call_result() {
 
     let result = CallToolResult::structured_error(error_data.clone());
 
-    assert!(result.content.is_some());
+    assert!(!result.content.is_empty());
     assert!(result.structured_content.is_some());
 
-    let contents = result.content.unwrap();
+    let contents = result.content;
 
     assert_eq!(contents.len(), 1);
 
@@ -217,10 +217,10 @@ async fn test_structured_return_conversion() {
 
     // Tools which return structured content should also return a serialized version as
     // Content::text for backwards compatibility.
-    assert!(call_result.content.is_some());
+    assert!(!call_result.content.is_empty());
     assert!(call_result.structured_content.is_some());
 
-    let contents = call_result.content.unwrap();
+    let contents = call_result.content;
 
     assert_eq!(contents.len(), 1);
 
@@ -278,5 +278,5 @@ async fn test_output_schema_requires_structured_content() {
 
     // Verify it has structured_content and content
     assert!(call_result.structured_content.is_some());
-    assert!(call_result.content.is_some());
+    assert!(!call_result.content.is_empty());
 }

crates/rmcp/tests/test_tool_macros.rs

@@ -301,8 +301,7 @@ async fn test_optional_i64_field_with_null_input() -> anyhow::Result<()> {
 
     let result_text = result
         .content
-        .as_ref()
-        .and_then(|contents| contents.first())
+        .first()
         .and_then(|content| content.raw.as_text())
         .map(|text| text.text.as_str())
         .expect("Expected text content");
@@ -330,8 +329,7 @@ async fn test_optional_i64_field_with_null_input() -> anyhow::Result<()> {
 
     let some_result_text = some_result
         .content
-        .as_ref()
-        .and_then(|contents| contents.first())
+        .first()
         .and_then(|content| content.raw.as_text())
         .map(|text| text.text.as_str())
         .expect("Expected text content");

examples/simple-chat-client/src/chat.rs

@@ -85,8 +85,8 @@ impl ChatSession {
                         if result.is_error.is_some_and(|b| b) {
                             self.messages
                                 .push(Message::user("tool call failed, mcp call error"));
-                        } else if let Some(contents) = &result.content {
-                            contents.iter().for_each(|content| {
+                        } else {
+                            result.content.iter().for_each(|content| {
                                 if let Some(content_text) = content.as_text() {
                                     let json_result = serde_json::from_str::<serde_json::Value>(
                                         &content_text.text,