feat!: relax tool result structuredContent type (modelcontextprotocol#919)

Author: DaleSeo

crates/rmcp/src/model/content.rs

@@ -10,7 +10,7 @@
 // ToolUseContent/ToolResultContent are SEP-2577-deprecated; internal references are expected.
 #![expect(deprecated)]
 use serde::{Deserialize, Serialize};
-use serde_json::json;
+use serde_json::{Value, json};
 
 use super::{Annotations, Meta, resource::ResourceContents};
 
@@ -207,7 +207,7 @@ pub struct ToolResultContent {
     pub tool_use_id: String,
     pub content: Vec<ContentBlock>,
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub structured_content: Option<super::JsonObject>,
+    pub structured_content: Option<Value>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub is_error: Option<bool>,
 }

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

@@ -2352,13 +2352,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

@@ -2352,13 +2352,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -3454,13 +3454,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -3454,13 +3454,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_sampling.rs

@@ -370,6 +370,38 @@ fn test_tool_result_content_requires_content() {
     assert!(err.to_string().contains("missing field `content`"));
 }
 
+#[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![ContentBlock::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![ContentBlock::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(