fix: match shape of the calltoolresult schema (#377)

BREAKING CHANGE: makes the `content` field non-optional

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");