diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index 762e23c..d916d93 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -1463,6 +1463,10 @@ const synthGradium = async(logger, {
     params += `,write_cache_file=${disableTtsCache ? 0 : 1}`;
     if (model_id) params += `,model_id=${model_id}`;
     if (pronunciation_id) params += `,pronunciation_id=${pronunciation_id}`;
+    /* the say: param parser is brace-aware, so a nested json object survives intact */
+    if (json_config) {
+      params += `,json_config=${typeof json_config === 'string' ? json_config : JSON.stringify(json_config)}`;
+    }
     params += '}';
 
     return {