Strictly verify UI attachment during fallback pipe discovery

nvim-core/src/lib.rs

@@ -59,6 +59,28 @@ pub async fn send_error(id: Value, code: i32, message: &str) {
     .await;
 }
 
+async fn is_interactive(socket_path: &str) -> bool {
+    // If it's our explicitly named headless fallback instance, skip the shell-out and reject it immediately
+    if socket_path.contains("agy-headless-nvim") {
+        return false;
+    }
+    
+    if let Ok(output) = tokio::process::Command::new("nvim")
+        .arg("--server")
+        .arg(socket_path)
+        .arg("--headless")
+        .arg("--remote-expr")
+        .arg("empty(nvim_list_uis())")
+        .output()
+        .await
+    {
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        // empty(nvim_list_uis()) returns 1 if it IS headless/empty, and 0 if it has a UI
+        return stdout.trim() == "0";
+    }
+    false
+}
+
 #[cfg(windows)]
 async fn get_socket_path() -> Result<String, String> {
     let profile =
@@ -83,7 +105,10 @@ async fn get_socket_path() -> Result<String, String> {
             let name = entry.file_name();
             let name_str = name.to_string_lossy();
             if name_str.starts_with("nvim.") {
-                return Ok(format!(r"\\.\pipe\{name_str}"));
+                let candidate = format!(r"\\.\pipe\{name_str}");
+                if is_interactive(&candidate).await {
+                    return Ok(candidate);
+                }
             }
         }
     }
@@ -108,7 +133,10 @@ async fn get_socket_path() -> Result<String, String> {
                 if name.starts_with("nvim.") {
                     let path = entry.path().join("0");
                     if tokio::fs::try_exists(&path).await.unwrap_or(false) {
-                        return Ok(path.to_string_lossy().to_string());
+                        let candidate = path.to_string_lossy().to_string();
+                        if is_interactive(&candidate).await {
+                            return Ok(candidate);
+                        }
                     }
                 }
             }