diff --git a/tools/subagent_generate b/tools/subagent_generate
index aa27699..ce697e1 100755
--- a/tools/subagent_generate
+++ b/tools/subagent_generate
@@ -54,6 +54,8 @@ def main():
     shared_task = raw.get("shared_task", "")
     protocol = raw.get("protocol", "")
     constraints = raw.get("constraints", [])
+    if not isinstance(constraints, list):
+        die("constraints must be an array of strings")
     legacy = raw.get("prompt", "")
 
     if not role and not legacy: