fix(upstream): capture stderr output before MCP initialization

Previously, stderr monitoring started only after successful MCP initialize() handshake. This caused startup errors (like missing API keys or credentials) to be invisible in logs when initialization timed out.

Now stderr monitoring begins immediately after process start, before the initialize() call. This ensures all process output is captured and logged, making debugging connection failures much easier.

Author: DarkGenius

.gitignore

@@ -79,3 +79,4 @@ data/
 .cursor/
 assets/mcpproxy.icns
 build-info.json
+.claude/

internal/upstream/core/connection.go

@@ -358,6 +358,17 @@ func (c *Client) connectStdio(ctx context.Context) error {
 		return fmt.Errorf("failed to start stdio client: %w", err)
 	}
 
+	// CRITICAL FIX: Enable stderr monitoring IMMEDIATELY after starting the process
+	// This ensures we capture startup errors (like missing API keys) even if
+	// initialization fails with timeout. Previously, stderr monitoring started
+	// after successful initialization, so early errors were never logged.
+	c.stderr = stdioTransport.Stderr()
+	if c.stderr != nil {
+		c.StartStderrMonitoring()
+		c.logger.Debug("Started early stderr monitoring to capture startup errors",
+			zap.String("server", c.config.Name))
+	}
+
 	// IMPORTANT: Perform MCP initialize() handshake for stdio transports as well,
 	// so c.serverInfo is populated and tool discovery/search can proceed.
 	// Use the caller's context with timeout to avoid hanging.
@@ -459,11 +470,8 @@ func (c *Client) connectStdio(ctx context.Context) error {
 		}
 	}
 
-	// Enable stderr monitoring for Docker containers
-	c.stderr = stdioTransport.Stderr()
-	if c.stderr != nil {
-		c.StartStderrMonitoring()
-	}
+	// Note: stderr monitoring was already started earlier (right after Start())
+	// to capture startup errors before initialization completes
 
 	// Start process monitoring if we have the process reference OR it's a Docker command
 	if c.processCmd != nil {