feat: add with_content for pairing structured results with a custom rendering

content and structuredContent need not match: with_content replaces the
content blocks on any result, so a structured_only result can carry a
short text summary while structuredContent holds the full value.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: olaservo

crates/rmcp/src/model.rs

@@ -3869,6 +3869,9 @@ impl CallToolResult {
     /// but such clients will receive an empty `content` array — only opt out
     /// when you know your callers consume `structuredContent`.
     ///
+    /// To send a custom rendering instead of none (for example a short text
+    /// summary of a large value), chain [`CallToolResult::with_content`].
+    ///
     /// # Example
     ///
     /// ```rust,ignore
@@ -3906,6 +3909,27 @@ impl CallToolResult {
         }
     }
 
+    /// Replace the `content` blocks on this result.
+    ///
+    /// The `content` and `structuredContent` fields need not carry the same
+    /// data: pairing a hand-written rendering with a structured value is
+    /// valid, e.g. a short text summary in `content` while
+    /// `structured_content` holds the full value.
+    ///
+    /// # Example
+    ///
+    /// ```rust,ignore
+    /// use rmcp::model::{CallToolResult, ContentBlock};
+    /// use serde_json::json;
+    ///
+    /// let result = CallToolResult::structured_only(json!({"rows": [1, 2, 3]}))
+    ///     .with_content(vec![ContentBlock::text("3 rows matched")]);
+    /// ```
+    pub fn with_content(mut self, content: Vec<ContentBlock>) -> Self {
+        self.content = content;
+        self
+    }
+
     /// Set the metadata on this result
     pub fn with_meta(mut self, meta: Option<MetaObject>) -> Self {
         self.meta = meta;

crates/rmcp/tests/test_structured_output.rs

@@ -238,6 +238,24 @@ async fn test_structured_only_in_call_result() {
     assert_eq!(serialized["structuredContent"]["sum"], 7);
 }
 
+#[tokio::test]
+async fn test_structured_only_with_divergent_content() {
+    // with_content pairs structured_content with a custom rendering instead of
+    // the serialized mirror
+    let structured_data = json!({"rows": [1, 2, 3]});
+
+    let result = CallToolResult::structured_only(structured_data.clone())
+        .with_content(vec![ContentBlock::text("3 rows matched")]);
+
+    assert_eq!(result.content.len(), 1);
+    assert_eq!(
+        result.content.first().unwrap().as_text().unwrap().text,
+        "3 rows matched"
+    );
+    assert_eq!(result.structured_content, Some(structured_data));
+    assert_eq!(result.is_error, Some(false));
+}
+
 #[tokio::test]
 async fn test_structured_error_only_in_call_result() {
     let error_data = json!({