fix: prevent double-starting stdio transport in client (#564)

* fix: prevent double-starting stdio transport in client

Avoid starting stdio transport twice by checking transport type before
calling Start(). The stdio transport from NewStdioMCPClientWithOptions
is already started and doesn't need to be started again.

* docs: update NewStdioMCPClient comment for clarity

Author: linw1995

client/client.go

@@ -77,9 +77,16 @@ func (c *Client) Start(ctx context.Context) error {
 	if c.transport == nil {
 		return fmt.Errorf("transport is nil")
 	}
-	err := c.transport.Start(ctx)
-	if err != nil {
-		return err
+
+	if _, ok := c.transport.(*transport.Stdio); !ok {
+		// the stdio transport from NewStdioMCPClientWithOptions
+		// is already started, dont start again.
+		//
+		// Start the transport for other transport types
+		err := c.transport.Start(ctx)
+		if err != nil {
+			return err
+		}
 	}
 
 	c.transport.SetNotificationHandler(func(notification mcp.JSONRPCNotification) {

client/stdio.go

@@ -12,7 +12,7 @@ import (
 // It launches the specified command with given arguments and sets up stdin/stdout pipes for communication.
 // Returns an error if the subprocess cannot be started or the pipes cannot be created.
 //
-// NOTICE: NewStdioMCPClient will start the connection automatically. Don't call the Start method manually.
+// NOTICE: NewStdioMCPClient will start the connection automatically.
 // This is for backward compatibility.
 func NewStdioMCPClient(
 	command string,
@@ -28,7 +28,6 @@ func NewStdioMCPClient(
 // such as setting a custom command function.
 //
 // NOTICE: NewStdioMCPClientWithOptions automatically starts the underlying transport.
-// Don't call the Start method manually.
 // This is for backward compatibility.
 func NewStdioMCPClientWithOptions(
 	command string,