fix(model): don't treat _meta alone as a CallToolResult

crates/rmcp/src/model.rs

@@ -4165,7 +4165,8 @@ pub struct CallToolResult {
 // 1. Defaults `content` to `[]` when the field is missing (lenient per Postel's law)
 // 2. Requires at least one known field to be present, so that `CallToolResult` doesn't
 //    greedily match arbitrary JSON objects when used inside `#[serde(untagged)]` enums
-//    (e.g. `ServerResult`), which would shadow `CustomResult`.
+//    (e.g. `ServerResult`), which would shadow `CustomResult`. `_meta` doesn't count,
+//    since any result type may carry it.
 // 3. Rejects non-`complete` result types so other `ServerResult` variants can match.
 // 4. Keeps a present `"structuredContent": null` as `Some(Value::Null)`, distinct
 //    from an absent field, since structured content can be any JSON value.
@@ -4202,11 +4203,10 @@ impl<'de> Deserialize<'de> for CallToolResult {
         if helper.content.is_none()
             && helper.structured_content.is_none()
             && helper.is_error.is_none()
-            && helper.meta.is_none()
         {
             return Err(serde::de::Error::custom(
                 "expected at least one known CallToolResult field \
-                 (content, structuredContent, isError, or _meta)",
+                 (content, structuredContent, or isError)",
             ));
         }
 

crates/rmcp/tests/test_deserialization.rs

@@ -105,6 +105,37 @@ mod untagged_server_result {
         );
     }
 
+    #[test]
+    fn meta_does_not_shadow_custom_result() {
+        // `_meta` can appear on any result, so it alone must not make
+        // CallToolResult swallow a custom result (e.g. SEP-2640 skills/list).
+        let payload = json!({
+            "resultType": "complete",
+            "skills": [{ "uri": "skill://a/SKILL.md" }],
+            "ttlMs": 0,
+            "cacheScope": "private",
+            "_meta": { "x": true }
+        });
+        let result = parse_result(wrap_response(payload.clone()));
+        let ServerResult::CustomResult(result) = result else {
+            panic!("expected CustomResult, got {result:?}");
+        };
+        assert_eq!(result.0, payload);
+    }
+
+    #[test]
+    fn call_tool_result_with_meta_deserializes_to_correct_variant() {
+        let result = parse_result(wrap_response(json!({
+            "content": [{ "type": "text", "text": "hi" }],
+            "_meta": {}
+        })));
+        let ServerResult::CallToolResult(result) = result else {
+            panic!("expected CallToolResult, got {result:?}");
+        };
+        assert_eq!(result.content.len(), 1);
+        assert!(result.meta.is_some());
+    }
+
     #[test]
     fn input_required_result_with_meta_deserializes_to_correct_variant() {
         let result = parse_result(wrap_response(json!({

crates/rmcp/tests/test_result_type_wire.rs

@@ -123,28 +123,21 @@ fn server_result_should_preserve_input_required_result_when_meta_present() {
 }
 
 #[test]
-fn server_result_should_accept_legacy_call_tool_result_when_only_meta_present() {
-    let legacy = json!({
+fn server_result_should_not_treat_meta_only_result_as_call_tool_result() {
+    // `_meta` may appear on any result, so on its own it says nothing about the
+    // result being a tool call.
+    let payload = json!({
         "_meta": {
             "example.com/replica": "r1",
         },
     });
 
     let result =
-        serde_json::from_value::<ServerResult>(legacy).expect("deserialize legacy CallToolResult");
+        serde_json::from_value::<ServerResult>(payload.clone()).expect("deserialize ServerResult");
 
     match result {
-        ServerResult::CallToolResult(result) => {
-            assert_eq!(
-                (
-                    result.result_type,
-                    result.content.is_empty(),
-                    result.meta.is_some()
-                ),
-                (None, true, true)
-            );
-        }
-        _ => panic!("expected CallToolResult"),
+        ServerResult::CustomResult(result) => assert_eq!(result.0, payload),
+        other => panic!("expected CustomResult, got {other:?}"),
     }
 }