fix: add compatibility handling for non-standard notifications in async_rw (#247)

- Introduced `is_standard_notification` function to check for standard MCP notifications.
- Added `try_parse_with_compatibility` function to handle parsing messages with compatibility for non-standard notifications.
- Updated the decoder implementation to utilize the new compatibility handling.
- Added unit tests for standard notification checks and compatibility function to ensure correct behavior.

crates/rmcp/src/transport/async_rw.rs

@@ -168,6 +168,68 @@ fn without_carriage_return(s: &[u8]) -> &[u8] {
     }
 }
 
+/// Check if a notification method is a standard MCP notification
+/// should update this when MCP spec is updated about new notifications
+fn is_standard_notification(method: &str) -> bool {
+    matches!(
+        method,
+        "notifications/cancelled"
+            | "notifications/initialized"
+            | "notifications/message"
+            | "notifications/progress"
+            | "notifications/prompts/list_changed"
+            | "notifications/resources/list_changed"
+            | "notifications/resources/updated"
+            | "notifications/roots/list_changed"
+            | "notifications/tools/list_changed"
+    )
+}
+
+/// Try to parse a message with compatibility handling for non-standard notifications
+fn try_parse_with_compatibility<T: serde::de::DeserializeOwned>(
+    line: &[u8],
+    context: &str,
+) -> Result<Option<T>, JsonRpcMessageCodecError> {
+    if let Ok(line_str) = std::str::from_utf8(line) {
+        match serde_json::from_slice(line) {
+            Ok(item) => Ok(Some(item)),
+            Err(e) => {
+                // Check if this is a non-standard notification that should be ignored
+                if line_str.contains("\"method\":\"notifications/") {
+                    // Extract the method name to check if it's standard
+                    if let Ok(json_value) = serde_json::from_str::<serde_json::Value>(line_str) {
+                        if let Some(method) = json_value.get("method").and_then(|m| m.as_str()) {
+                            if method.starts_with("notifications/")
+                                && !is_standard_notification(method)
+                            {
+                                tracing::debug!(
+                                    "Ignoring non-standard notification {} {}: {}",
+                                    method,
+                                    context,
+                                    line_str
+                                );
+                                return Ok(None); // Skip this message
+                            }
+                        }
+                    }
+                }
+
+                tracing::debug!(
+                    "Failed to parse message {}: {} | Error: {}",
+                    context,
+                    line_str,
+                    e
+                );
+                Err(JsonRpcMessageCodecError::Serde(e))
+            }
+        }
+    } else {
+        serde_json::from_slice(line)
+            .map(Some)
+            .map_err(JsonRpcMessageCodecError::Serde)
+    }
+}
+
 #[derive(Debug, Error)]
 pub enum JsonRpcMessageCodecError {
     #[error("max line length exceeded")]
@@ -234,8 +296,12 @@ impl<T: DeserializeOwned> Decoder for JsonRpcMessageCodec<T> {
                     let line = buf.split_to(newline_index + 1);
                     let line = &line[..line.len() - 1];
                     let line = without_carriage_return(line);
-                    let item =
-                        serde_json::from_slice(line).map_err(JsonRpcMessageCodecError::Serde)?;
+
+                    // Use compatibility handling function
+                    let item = match try_parse_with_compatibility(line, "decode")? {
+                        Some(item) => item,
+                        None => return Ok(None), // Skip non-standard message
+                    };
                     return Ok(Some(item));
                 }
                 (false, None) if buf.len() > self.max_length => {
@@ -266,8 +332,12 @@ impl<T: DeserializeOwned> Decoder for JsonRpcMessageCodec<T> {
                 } else {
                     let line = buf.split_to(buf.len());
                     let line = without_carriage_return(&line);
-                    let item =
-                        serde_json::from_slice(line).map_err(JsonRpcMessageCodecError::Serde)?;
+
+                    // Use compatibility handling function
+                    let item = match try_parse_with_compatibility(line, "decode_eof")? {
+                        Some(item) => item,
+                        None => return Ok(None), // Skip non-standard message
+                    };
                     Some(item)
                 }
             }
@@ -319,7 +389,7 @@ mod test {
     {"jsonrpc":"2.0","method":"subtract","params":[23,42],"id":8}
     {"jsonrpc":"2.0","method":"subtract","params":[42,23],"id":9}
     {"jsonrpc":"2.0","method":"subtract","params":[23,42],"id":10}
-    
+
     "#;
 
         let mut cursor = BufReader::new(data.as_bytes());
@@ -379,4 +449,61 @@ mod test {
         // Make sure there are no extra lines
         assert!(lines.next().is_none());
     }
+
+    #[test]
+    fn test_standard_notification_check() {
+        // Test that all standard notifications are recognized
+        assert!(is_standard_notification("notifications/cancelled"));
+        assert!(is_standard_notification("notifications/initialized"));
+        assert!(is_standard_notification("notifications/progress"));
+        assert!(is_standard_notification(
+            "notifications/resources/list_changed"
+        ));
+        assert!(is_standard_notification("notifications/resources/updated"));
+        assert!(is_standard_notification(
+            "notifications/prompts/list_changed"
+        ));
+        assert!(is_standard_notification("notifications/tools/list_changed"));
+        assert!(is_standard_notification("notifications/message"));
+        assert!(is_standard_notification("notifications/roots/list_changed"));
+
+        // Test that non-standard notifications are not recognized
+        assert!(!is_standard_notification("notifications/stderr"));
+        assert!(!is_standard_notification("notifications/custom"));
+        assert!(!is_standard_notification("notifications/debug"));
+        assert!(!is_standard_notification("some/other/method"));
+    }
+
+    #[test]
+    fn test_compatibility_function() {
+        // Test the compatibility function directly
+        let stderr_message =
+            r#"{"method":"notifications/stderr","params":{"content":"stderr message"}}"#;
+        let custom_message = r#"{"method":"notifications/custom","params":{"data":"custom"}}"#;
+        let standard_message =
+            r#"{"method":"notifications/message","params":{"level":"info","data":"standard"}}"#;
+        let progress_message = r#"{"method":"notifications/progress","params":{"progressToken":"token","progress":50}}"#;
+
+        // Test with valid JSON - all should parse successfully
+        let result1 =
+            try_parse_with_compatibility::<serde_json::Value>(stderr_message.as_bytes(), "test");
+        let result2 =
+            try_parse_with_compatibility::<serde_json::Value>(custom_message.as_bytes(), "test");
+        let result3 =
+            try_parse_with_compatibility::<serde_json::Value>(standard_message.as_bytes(), "test");
+        let result4 =
+            try_parse_with_compatibility::<serde_json::Value>(progress_message.as_bytes(), "test");
+
+        // All should parse successfully since they're valid JSON
+        assert!(result1.is_ok());
+        assert!(result2.is_ok());
+        assert!(result3.is_ok());
+        assert!(result4.is_ok());
+
+        // Standard notifications should return Some(value)
+        assert!(result3.unwrap().is_some());
+        assert!(result4.unwrap().is_some());
+
+        println!("Standard notifications are preserved, non-standard are handled gracefully");
+    }
 }