diff --git a/app/services/backtesting/ui_service.py b/app/services/backtesting/ui_service.py
index 9415c9c..b6db36d 100644
--- a/app/services/backtesting/ui_service.py
+++ b/app/services/backtesting/ui_service.py
@@ -323,25 +323,12 @@ class BacktestPageService:
             margin_call_ltv=normalized_inputs.margin_call_ltv,
         )
 
-        # Get the appropriate provider based on data source
-        source_info = self.DATA_SOURCE_INFO.get(data_source, self.DATA_SOURCE_INFO["synthetic"])
-
-        # Use the injected provider's identity if available (for custom providers in tests)
-        if hasattr(self.backtest_service, "provider"):
-            injected_provider_id = getattr(self.backtest_service.provider, "provider_id", None)
-            injected_pricing_mode = getattr(self.backtest_service.provider, "pricing_mode", None)
-            # Only use injected identity if it differs from known providers
-            if injected_provider_id and injected_provider_id not in [
-                info.provider_id for info in self.DATA_SOURCE_INFO.values()
-            ]:
-                provider_id = injected_provider_id
-                pricing_mode = injected_pricing_mode or source_info.pricing_mode
-            else:
-                provider_id = source_info.provider_id
-                pricing_mode = source_info.pricing_mode
-        else:
-            provider_id = source_info.provider_id
-            pricing_mode = source_info.pricing_mode
+        # CRITICAL: The backtest engine uses a fixture provider (synthetic_v1), 
+        # regardless of the data_source used for price fetching.
+        # We must use the fixture provider's ID for the scenario, not the data source's ID.
+        # The data_source parameter only affects price data fetching, not the backtest engine.
+        provider_id = self.backtest_service.provider.provider_id
+        pricing_mode = self.backtest_service.provider.pricing_mode
 
         # For now, always use the synthetic engine (which uses fixture data for demo)
         # In a full implementation, we would create different engines for different providers