diff --git a/pos/services/ai_chat.py b/pos/services/ai_chat.py
index d7cbd52..65aa86e 100644
--- a/pos/services/ai_chat.py
+++ b/pos/services/ai_chat.py
@@ -6,7 +6,16 @@ import json
 from config import OPENROUTER_API_KEY
 
 OPENROUTER_URL = "https://openrouter.ai/api/v1/chat/completions"
-MODEL = "qwen/qwen3-6b-preview:free"  # Qwen 3.6 Plus Preview (free on OpenRouter)
+
+# ⚠️ SOLO MODELOS GRATUITOS — No cambiar a modelos de pago.
+# El modelo DEBE terminar en ":free" para garantizar costo $0.
+# Alternativas gratuitas: "meta-llama/llama-4-scout:free", "google/gemma-3-27b-it:free"
+MODEL = "qwen/qwen3-6b-preview:free"
+
+def _validate_model(model_id):
+    """Ensure only free models are used. Raises if model is not free."""
+    if not model_id.endswith(':free'):
+        raise ValueError(f"BLOQUEADO: Solo se permiten modelos gratuitos (:free). Modelo '{model_id}' no es gratuito.")
 
 SYSTEM_PROMPT = """Eres un asistente de refaccionaria automotriz mexicana. Tu trabajo es ayudar a encontrar autopartes.
 
@@ -45,6 +54,7 @@ Reglas OBLIGATORIAS:
 
 def chat(user_message, conversation_history=None):
     """Send a message to the AI and get a response with search suggestions."""
+    _validate_model(MODEL)  # Block paid models
     messages = [{"role": "system", "content": SYSTEM_PROMPT}]
     if conversation_history:
         messages.extend(conversation_history)