diff --git a/routes/translate_routes.py b/routes/translate_routes.py
index fcba732..8a9a7e3 100644
--- a/routes/translate_routes.py
+++ b/routes/translate_routes.py
@@ -573,13 +573,28 @@ async def translate_document_v1(
         if prompt_id in ("null", "undefined", ""): prompt_id = None
         if file_url in ("null", "undefined", ""): file_url = None
 
-        # Story 3.12 / Bugfix: If user is on the free tier, they might have stale Pro features 
+        # Story 3.12 / Bugfix: If user is on the free tier, they might have stale Pro features
         # in their frontend localStorage (e.g., they downgraded, or switched accounts).
-        # Instead of hard-blocking the translation, we simply strip the Pro features.
+        # These features require a paid plan.
         if tier == "free":
-            glossary_id = None
-            custom_prompt = None
-            prompt_id = None
+            if glossary_id:
+                raise TranslateEndpointError(
+                    code=TranslateEndpointError.PRO_FEATURE_REQUIRED,
+                    message="Glossaries are reserved for Pro users.",
+                    details={"feature": "glossary_id", "tier": tier},
+                )
+            if custom_prompt:
+                raise TranslateEndpointError(
+                    code=TranslateEndpointError.PRO_FEATURE_REQUIRED,
+                    message="Custom prompts are reserved for Pro users.",
+                    details={"feature": "custom_prompt", "tier": tier},
+                )
+            if prompt_id:
+                raise TranslateEndpointError(
+                    code=TranslateEndpointError.PRO_FEATURE_REQUIRED,
+                    message="Saved prompts are reserved for Pro users.",
+                    details={"feature": "prompt_id", "tier": tier},
+                )
 
         if file_url:
             if tier == "free":