From bf9066a602a3ebf9c130b977511a3fb3b5b2ea2c Mon Sep 17 00:00:00 2001
From: Philippe Branchu <philippe@branchu.com>
Date: Sun, 22 Mar 2026 16:49:28 +0000
Subject: [PATCH] Fix MCP bridge dropping tool results from servers that send
 notifications

Read response lines until finding a JSON-RPC response matching the
request ID. Previously, the bridge read one line and assumed it was
the response, causing "No result from MCP tools/call" when MCP
servers send notifications or log lines before the actual result.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 crates/openfang-runtime/src/mcp.rs | 61 +++++++++++++++++++++---------
 1 file changed, 44 insertions(+), 17 deletions(-)

diff --git a/crates/openfang-runtime/src/mcp.rs b/crates/openfang-runtime/src/mcp.rs
index 0fbfbb98..c77f1c7f 100644
--- a/crates/openfang-runtime/src/mcp.rs
+++ b/crates/openfang-runtime/src/mcp.rs
@@ -328,24 +328,51 @@ impl McpConnection {
                     .await
                     .map_err(|e| format!("Failed to flush stdin: {e}"))?;
 
-                // Read response line
-                let mut line = String::new();
-                let timeout = tokio::time::Duration::from_secs(self.config.timeout_secs);
-                match tokio::time::timeout(timeout, stdout.read_line(&mut line)).await {
-                    Ok(Ok(0)) => return Err("MCP server closed connection".to_string()),
-                    Ok(Ok(_)) => {}
-                    Ok(Err(e)) => return Err(format!("Failed to read MCP response: {e}")),
-                    Err(_) => return Err("MCP request timed out".to_string()),
+                // Read response lines until we find one matching our request ID.
+                // MCP servers may send notifications or log lines before the
+                // actual response.
+                let timeout_dur = tokio::time::Duration::from_secs(self.config.timeout_secs);
+                let deadline = tokio::time::Instant::now() + timeout_dur;
+
+                loop {
+                    let mut line = String::new();
+                    let remaining = deadline.saturating_duration_since(tokio::time::Instant::now());
+                    if remaining.is_zero() {
+                        return Err("MCP request timed out".to_string());
+                    }
+                    match tokio::time::timeout(remaining, stdout.read_line(&mut line)).await {
+                        Ok(Ok(0)) => return Err("MCP server closed connection".to_string()),
+                        Ok(Ok(_)) => {}
+                        Ok(Err(e)) => return Err(format!("Failed to read MCP response: {e}")),
+                        Err(_) => return Err("MCP request timed out".to_string()),
+                    }
+
+                    let trimmed = line.trim();
+                    if trimmed.is_empty() {
+                        continue;
+                    }
+
+                    // Try to parse as JSON-RPC response
+                    let parsed: Result<JsonRpcResponse, _> = serde_json::from_str(trimmed);
+                    match parsed {
+                        Ok(response) if response.id == Some(id) => {
+                            if let Some(err) = response.error {
+                                return Err(format!("{err}"));
+                            }
+                            return Ok(response.result);
+                        }
+                        Ok(_other) => {
+                            // Response for a different ID or notification — skip
+                            debug!("MCP: skipping non-matching response line");
+                            continue;
+                        }
+                        Err(_) => {
+                            // Not valid JSON-RPC — skip (could be a log line)
+                            debug!("MCP: skipping non-JSON line from server");
+                            continue;
+                        }
+                    }
                 }
-
-                let response: JsonRpcResponse = serde_json::from_str(line.trim())
-                    .map_err(|e| format!("Invalid MCP JSON-RPC response: {e}"))?;
-
-                if let Some(err) = response.error {
-                    return Err(format!("{err}"));
-                }
-
-                Ok(response.result)
             }
             McpTransportHandle::Sse { client, url } => {
                 let response = client