diff --git a/api/main.py b/api/main.py
index bd45121..6d05be8 100644
--- a/api/main.py
+++ b/api/main.py
@@ -23,7 +23,7 @@ from api.simulation_worker import SimulationWorker
 from api.database import get_db_connection
 from api.price_data_manager import PriceDataManager
 from api.date_utils import validate_date_range, expand_date_range, get_max_simulation_days
-from tools.deployment_config import get_deployment_mode_dict
+from tools.deployment_config import get_deployment_mode_dict, log_dev_mode_startup_warning
 import threading
 import time
 
@@ -506,4 +506,8 @@ app = create_app()
 
 if __name__ == "__main__":
     import uvicorn
+
+    # Display DEV mode warning if applicable
+    log_dev_mode_startup_warning()
+
     uvicorn.run(app, host="0.0.0.0", port=8080)
diff --git a/main.py b/main.py
index a40234b..29a56b1 100644
--- a/main.py
+++ b/main.py
@@ -12,7 +12,8 @@ from prompts.agent_prompt import all_nasdaq_100_symbols
 from tools.deployment_config import (
     is_dev_mode,
     get_deployment_mode,
-    log_api_key_warning
+    log_api_key_warning,
+    log_dev_mode_startup_warning
 )
 from api.database import initialize_dev_database
 
@@ -108,16 +109,13 @@ async def main(config_path=None):
 
     # Initialize dev environment if needed
     if is_dev_mode():
-        print("=" * 60)
-        print("🛠️  DEVELOPMENT MODE ACTIVE")
-        print("=" * 60)
+        log_dev_mode_startup_warning()
         log_api_key_warning()
 
         # Initialize dev database (reset unless PRESERVE_DEV_DATA=true)
         from tools.deployment_config import get_db_path
         dev_db_path = get_db_path("data/jobs.db")
         initialize_dev_database(dev_db_path)
-        print("=" * 60)
 
     # Get Agent type
     agent_type = config.get("agent_type", "BaseAgent")
diff --git a/tools/deployment_config.py b/tools/deployment_config.py
index 9e0297e..79dd66f 100644
--- a/tools/deployment_config.py
+++ b/tools/deployment_config.py
@@ -119,6 +119,43 @@ def log_api_key_warning() -> None:
         print("   This is expected if you're testing dev mode with existing .env file")
 
 
+def log_dev_mode_startup_warning() -> None:
+    """
+    Display prominent warning when server starts in DEV mode
+
+    Warns users that:
+    - AI calls will be simulated/mocked
+    - Data may not be retained between runs
+    - This is a development environment
+    """
+    if not is_dev_mode():
+        return
+
+    preserve_data = should_preserve_dev_data()
+
+    print()
+    print("=" * 70)
+    print("⚠️  " + "DEVELOPMENT MODE WARNING".center(64) + "  ⚠️")
+    print("=" * 70)
+    print()
+    print("  🚧 This server is running in DEVELOPMENT mode (DEPLOYMENT_MODE=DEV)")
+    print()
+    print("  📌 IMPORTANT:")
+    print("     • AI API calls will be SIMULATED (mock responses)")
+    print("     • No real AI model costs will be incurred")
+    if preserve_data:
+        print("     • Dev data WILL BE PRESERVED between runs (PRESERVE_DEV_DATA=true)")
+    else:
+        print("     • Dev data WILL BE RESET on each startup (PRESERVE_DEV_DATA=false)")
+    print("     • Using isolated dev database and data paths")
+    print()
+    print("  💡 To use PRODUCTION mode:")
+    print("     Set environment variable: DEPLOYMENT_MODE=PROD")
+    print()
+    print("=" * 70)
+    print()
+
+
 def get_deployment_mode_dict() -> dict:
     """
     Get deployment mode information as dictionary (for API responses)