diff --git a/scripts/opencode-builder.sh b/scripts/opencode-builder.sh
index f7d549de7..ff1c42af7 100755
--- a/scripts/opencode-builder.sh
+++ b/scripts/opencode-builder.sh
@@ -251,8 +251,11 @@ send_message_capture() {
   local body="$2"
   local out_file="$3"
 
+  # Use prompt_async — fire-and-forget; server returns 204 immediately.
+  # The synchronous /message endpoint would block until the agent finishes
+  # its full response (potentially many minutes for a busy worker).
   ( trap '' INT
-    exec curl -sf -X POST "${BASE}/session/${sid}/message" \
+    exec curl -sf -X POST "${BASE}/session/${sid}/prompt_async" \
       -H "Content-Type: application/json" \
       -d "$body" \
       -w "\n"