diff --git a/hubzilla/addon/vs01/vs01_spool.php b/hubzilla/addon/vs01/vs01_spool.php
index f7b75c7..4ddaa40 100644
--- a/hubzilla/addon/vs01/vs01_spool.php
+++ b/hubzilla/addon/vs01/vs01_spool.php
@@ -206,6 +206,24 @@ function vs01_post_to_orchestrator($envelope, $config = null) {
     $curl_error = curl_error($ch);
     curl_close($ch);
 
+    // --- DEBUG LOGGING ---
+    // Writes every orchestrator POST attempt (request + response) to a
+    // dedicated debug log, independent of timing. Read with:
+    //   tail -50 /var/www/hubzilla/addon/vs01/debug.log
+    // Remove this block once the 400 root cause is found and fixed.
+    @file_put_contents(
+        __DIR__ . '/debug.log',
+        '[' . date('c') . "] vs01 POST to orchestrator\n" .
+        "  url: $receiver_url\n" .
+        "  http_code: $http_code\n" .
+        "  curl_error: " . ($curl_error ?: '(none)') . "\n" .
+        "  request_payload: $payload\n" .
+        "  response_body: " . ($response === false ? '(curl failed)' : $response) . "\n" .
+        "----\n",
+        FILE_APPEND
+    );
+    // --- END DEBUG LOGGING ---
+
     if ($curl_error) {
         logger('vs01_spool: curl error: ' . $curl_error);
         return 'Network error: ' . $curl_error;