diff --git a/app/routers/recipes.py b/app/routers/recipes.py
index 290efdd..38d1eff 100644
--- a/app/routers/recipes.py
+++ b/app/routers/recipes.py
@@ -73,7 +73,9 @@ async def upload_recipe(
     variable_inputs = []
     fixed_inputs = []
 
-    if is_sexp and SEXP_AVAILABLE:
+    if is_sexp:
+        if not SEXP_AVAILABLE:
+            raise HTTPException(500, "S-expression recipes require artdag.sexp module (not installed on server)")
         # Parse S-expression
         try:
             compiled = compile_string(content)
diff --git a/app/services/recipe_service.py b/app/services/recipe_service.py
index fb3c1a4..119e4b2 100644
--- a/app/services/recipe_service.py
+++ b/app/services/recipe_service.py
@@ -68,7 +68,9 @@ class RecipeService:
         recipe_data = None
         is_sexp = _is_sexp_format(content)
 
-        if is_sexp and SEXP_AVAILABLE:
+        if is_sexp:
+            if not SEXP_AVAILABLE:
+                return {"error": "S-expression recipes require artdag.sexp module (not installed)", "recipe_id": recipe_id}
             # Parse as S-expression
             try:
                 compiled = compile_string(content)
@@ -153,7 +155,9 @@ class RecipeService:
         # Detect format
         is_sexp = _is_sexp_format(content)
 
-        if is_sexp and SEXP_AVAILABLE:
+        if is_sexp:
+            if not SEXP_AVAILABLE:
+                return None, "S-expression recipes require artdag.sexp module (not installed on server)"
             # Validate S-expression
             try:
                 compiled = compile_string(content)