refactor(pre-alpha): align preview and runtime fixture validation

app/services/backtesting/comparison.py

@@ -127,7 +127,9 @@ class EventComparisonService:
         if not selected_template_slugs:
             raise ValueError("Event comparison requires at least one template slug")
 
-        resolved_history = history or self._load_preset_history(preset)
+        resolved_history = self._load_preset_history(preset) if history is None else history
+        if not resolved_history:
+            raise ValueError("Event comparison history must not be empty")
         scenario_portfolio = materialize_backtest_portfolio_state(
             symbol=preset.symbol,
             underlying_units=initial_portfolio.underlying_units,

app/services/backtesting/ui_service.py

@@ -68,9 +68,8 @@ class BacktestPageService:
             template_service=self.template_service,
             provider=None,
         )
-        if backtest_service is None:
-            provider = self.backtest_service.provider
-            provider.source = DeterministicBacktestFixtureSource()
+        provider = self.backtest_service.provider
+        provider.source = DeterministicBacktestFixtureSource()
 
     def template_options(self, symbol: str = "GLD") -> list[dict[str, str | int]]:
         return [
@@ -122,11 +121,13 @@ class BacktestPageService:
             raise ValueError("Template selection is required")
 
         self.template_service.get_template(template_slug)
-        resolved_entry_spot = (
-            entry_spot if entry_spot is not None else self.derive_entry_spot(normalized_symbol, start_date, end_date)
-        )
-        _validate_initial_collateral(underlying_units, resolved_entry_spot, loan_amount)
-        return resolved_entry_spot
+        derived_entry_spot = self.derive_entry_spot(normalized_symbol, start_date, end_date)
+        if entry_spot is not None and entry_spot != derived_entry_spot:
+            raise ValueError(
+                f"Supplied entry spot ${entry_spot:,.2f} does not match derived historical entry spot ${derived_entry_spot:,.2f}"
+            )
+        _validate_initial_collateral(underlying_units, derived_entry_spot, loan_amount)
+        return derived_entry_spot
 
     def run_read_only_scenario(
         self,

tests/test_backtest_ui.py

@@ -170,18 +170,33 @@ def test_backtest_page_service_fails_closed_outside_seeded_fixture_window() -> N
         )
 
 
-def test_backtest_preview_validation_reuses_supplied_entry_spot() -> None:
+def test_backtest_preview_validation_requires_supported_fixture_window_even_with_supplied_entry_spot() -> None:
     service = BacktestPageService()
 
-    entry_spot = service.validate_preview_inputs(
-        symbol="GLD",
-        start_date=date(2024, 1, 2),
-        end_date=date(2024, 1, 8),
-        template_slug="protective-put-atm-12m",
-        underlying_units=1000.0,
-        loan_amount=68000.0,
-        margin_call_ltv=0.75,
-        entry_spot=100.0,
-    )
+    with pytest.raises(ValueError, match="deterministic fixture data only supports GLD"):
+        service.validate_preview_inputs(
+            symbol="GLD",
+            start_date=date(2024, 1, 3),
+            end_date=date(2024, 1, 8),
+            template_slug="protective-put-atm-12m",
+            underlying_units=1000.0,
+            loan_amount=68000.0,
+            margin_call_ltv=0.75,
+            entry_spot=100.0,
+        )
 
-    assert entry_spot == 100.0
+
+def test_backtest_preview_validation_rejects_mismatched_supplied_entry_spot() -> None:
+    service = BacktestPageService()
+
+    with pytest.raises(ValueError, match="does not match derived historical entry spot"):
+        service.validate_preview_inputs(
+            symbol="GLD",
+            start_date=date(2024, 1, 2),
+            end_date=date(2024, 1, 8),
+            template_slug="protective-put-atm-12m",
+            underlying_units=1000.0,
+            loan_amount=68000.0,
+            margin_call_ltv=0.75,
+            entry_spot=99.0,
+        )

tests/test_event_comparison.py

@@ -315,3 +315,43 @@ def test_event_comparison_uses_preset_defaults_only_when_template_slugs_are_omit
         "protective-put-90pct-12m",
         "ladder-50-50-atm-95pct-12m",
     ]
+
+    with pytest.raises(ValueError, match="at least one template slug"):
+        service.materialize_scenario(
+            service.event_preset_service.get_preset("gld-jan-2024-selloff"),
+            initial_portfolio=BacktestPortfolioState(
+                currency="USD",
+                underlying_units=1000.0,
+                entry_spot=100.0,
+                loan_amount=68_000.0,
+                margin_call_ltv=0.75,
+            ),
+            template_slugs=(),
+        )
+
+
+def test_event_comparison_materialize_scenario_rejects_explicit_empty_history() -> None:
+    provider = SyntheticHistoricalProvider(
+        source=FakeHistorySource(FIXTURE_HISTORY),
+        implied_volatility=0.35,
+        risk_free_rate=0.0,
+    )
+    service = EventComparisonService(
+        provider=provider,
+        template_service=StrategyTemplateService(),
+        event_preset_service=EventPresetService(repository=FileEventPresetRepository()),
+    )
+
+    with pytest.raises(ValueError, match="history must not be empty"):
+        service.materialize_scenario(
+            service.event_preset_service.get_preset("gld-jan-2024-selloff"),
+            initial_portfolio=BacktestPortfolioState(
+                currency="USD",
+                underlying_units=1000.0,
+                entry_spot=100.0,
+                loan_amount=68_000.0,
+                margin_call_ltv=0.75,
+            ),
+            template_slugs=("protective-put-atm-12m",),
+            history=[],
+        )