diff --git a/src/void_tools/constitutional_linter.py b/src/void_tools/constitutional_linter.py
index fcc8b3d..6987012 100755
--- a/src/void_tools/constitutional_linter.py
+++ b/src/void_tools/constitutional_linter.py
@@ -27,9 +27,26 @@ def lint_outbox(filepath):
     except Exception as e:
         logging.error(f"Failed to read outbox: {e}")
         return False
-        
-    if not data or 'actions' not in data:
-        return True
+
+    if not data:
+        logging.error("Outbox file is empty or invalid YAML.")
+        return False
+
+    # Schema validation: must have 'actions' key
+    if 'actions' not in data:
+        # Check for legacy schema (posts: at root with reply_to)
+        if 'posts' in data:
+            logging.error("Schema mismatch: Outbox uses legacy format ('posts:' at root).")
+            logging.error("  Expected: 'config:' and 'actions:' structure")
+            logging.error("  Migrate: wrap posts in action blocks with 'action: reply' and 'target_id'")
+        else:
+            logging.error("Schema validation failed: 'actions' key not found in outbox.")
+        return False
+
+    actions = data.get('actions', [])
+    if not isinstance(actions, list):
+        logging.error("Schema validation failed: 'actions' must be a list.")
+        return False
         
     has_errors = False