feat!: relax tool result structuredContent type

Author: DaleSeo

crates/rmcp/src/model/content.rs

@@ -2,7 +2,7 @@
 //! The various content types can be display to humans but also understood by models
 //! They include optional annotations used to help inform agent usage
 use serde::{Deserialize, Serialize};
-use serde_json::json;
+use serde_json::{Value, json};
 
 use super::{AnnotateAble, Annotated, resource::ResourceContents};
 
@@ -107,7 +107,7 @@ pub struct ToolResultContent {
     pub content: Vec<Content>,
     /// Optional structured result
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub structured_content: Option<super::JsonObject>,
+    pub structured_content: Option<Value>,
     /// Whether tool execution failed
     #[serde(skip_serializing_if = "Option::is_none")]
     pub is_error: Option<bool>,

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

@@ -2118,12 +2118,7 @@
           ]
         },
         "structuredContent": {
-          "description": "Optional structured result",
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
+          "description": "Optional structured result"
         },
         "toolUseId": {
           "description": "ID of the corresponding tool use",

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

@@ -2118,12 +2118,7 @@
           ]
         },
         "structuredContent": {
-          "description": "Optional structured result",
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
+          "description": "Optional structured result"
         },
         "toolUseId": {
           "description": "ID of the corresponding tool use",

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -3467,12 +3467,7 @@
           ]
         },
         "structuredContent": {
-          "description": "Optional structured result",
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
+          "description": "Optional structured result"
         },
         "toolUseId": {
           "description": "ID of the corresponding tool use",

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -3467,12 +3467,7 @@
           ]
         },
         "structuredContent": {
-          "description": "Optional structured result",
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
+          "description": "Optional structured result"
         },
         "toolUseId": {
           "description": "ID of the corresponding tool use",

crates/rmcp/tests/test_sampling.rs

@@ -359,6 +359,38 @@ async fn test_tool_result_content_serialization() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn test_tool_result_content_with_array_structured_content() -> Result<()> {
+    let structured =
+        serde_json::json!([{ "city": "SF", "temp": 72 }, { "city": "NY", "temp": 65 }]);
+    let mut tool_result = ToolResultContent::new("call_123", vec![Content::text("forecast")]);
+    tool_result.structured_content = Some(structured);
+
+    let json = serde_json::to_string(&tool_result)?;
+    let deserialized: ToolResultContent = serde_json::from_str(&json)?;
+    assert_eq!(tool_result, deserialized);
+    assert!(deserialized.structured_content.unwrap().is_array());
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn test_tool_result_content_with_primitive_structured_content() -> Result<()> {
+    let structured = serde_json::json!(42);
+    let mut tool_result = ToolResultContent::new("call_123", vec![Content::text("count")]);
+    tool_result.structured_content = Some(structured);
+
+    let json = serde_json::to_string(&tool_result)?;
+    let deserialized: ToolResultContent = serde_json::from_str(&json)?;
+    assert_eq!(tool_result, deserialized);
+    assert!(matches!(
+        deserialized.structured_content,
+        Some(serde_json::Value::Number(_))
+    ));
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_sampling_message_with_tool_use() -> Result<()> {
     let message = SamplingMessage::assistant_tool_use(