diff --git a/backend/app/services/ai_chat_service.py b/backend/app/services/ai_chat_service.py
index 8055331..9f97c7d 100644
--- a/backend/app/services/ai_chat_service.py
+++ b/backend/app/services/ai_chat_service.py
@@ -8,7 +8,7 @@ logger = logging.getLogger(__name__)
 
 # Configuration
 OPENROUTER_API_KEY = os.getenv("OPENROUTER_API_KEY", "")
-OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "meta-llama/llama-3-8b-instruct")
+OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "stepfun/step-3.5-flash:free")
 OPENROUTER_BASE_URL = os.getenv(
     "OPENROUTER_BASE_URL", "https://openrouter.ai/api/v1/chat/completions"
 )
diff --git a/backend/app/services/pdf_ai_service.py b/backend/app/services/pdf_ai_service.py
index ffd8975..90ca762 100644
--- a/backend/app/services/pdf_ai_service.py
+++ b/backend/app/services/pdf_ai_service.py
@@ -9,7 +9,7 @@ logger = logging.getLogger(__name__)
 
 # Configuration
 OPENROUTER_API_KEY = os.getenv("OPENROUTER_API_KEY", "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")
-OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "meta-llama/llama-3-8b-instruct")
+OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "stepfun/step-3.5-flash:free")
 OPENROUTER_BASE_URL = os.getenv(
     "OPENROUTER_BASE_URL", "https://openrouter.ai/api/v1/chat/completions"
 )
diff --git a/backend/celerybeat-schedule b/backend/celerybeat-schedule
index 4c9d4a7..3901fd6 100644
Binary files a/backend/celerybeat-schedule and b/backend/celerybeat-schedule differ
diff --git a/backend/config/__init__.py b/backend/config/__init__.py
index bbd1de7..6cfe42c 100644
--- a/backend/config/__init__.py
+++ b/backend/config/__init__.py
@@ -81,7 +81,7 @@ class BaseConfig:
 
     # OpenRouter AI
     OPENROUTER_API_KEY = os.getenv("OPENROUTER_API_KEY", "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")
-    OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "meta-llama/llama-3-8b-instruct")
+    OPENROUTER_MODEL = os.getenv("OPENROUTER_MODEL", "stepfun/step-3.5-flash:free")
     OPENROUTER_BASE_URL = os.getenv(
         "OPENROUTER_BASE_URL", "https://openrouter.ai/api/v1/chat/completions"
     )