diff --git a/api/simulation_worker.py b/api/simulation_worker.py
index a03a83e..1643ba6 100644
--- a/api/simulation_worker.py
+++ b/api/simulation_worker.py
@@ -236,6 +236,52 @@ class SimulationWorker:
             warnings.append(msg)
             logger.warning(f"Job {self.job_id}: {msg}")
 
+    def _filter_completed_dates(
+        self,
+        available_dates: List[str],
+        models: List[str]
+    ) -> List[str]:
+        """
+        Filter out dates that are already completed for all models.
+
+        Implements idempotent job behavior - skip model-days that already
+        have completed data.
+
+        Args:
+            available_dates: List of dates with complete price data
+            models: List of model signatures
+
+        Returns:
+            List of dates that need processing
+        """
+        if not available_dates:
+            return []
+
+        # Get completed dates from job_manager
+        start_date = available_dates[0]
+        end_date = available_dates[-1]
+
+        completed_dates = self.job_manager.get_completed_model_dates(
+            models,
+            start_date,
+            end_date
+        )
+
+        # Build list of dates that need processing
+        dates_to_process = []
+        for date in available_dates:
+            # Check if any model needs this date
+            needs_processing = False
+            for model in models:
+                if date not in completed_dates.get(model, []):
+                    needs_processing = True
+                    break
+
+            if needs_processing:
+                dates_to_process.append(date)
+
+        return dates_to_process
+
     def get_job_info(self) -> Dict[str, Any]:
         """
         Get job information.
diff --git a/tests/unit/test_simulation_worker.py b/tests/unit/test_simulation_worker.py
index 1deaabd..9f9ea91 100644
--- a/tests/unit/test_simulation_worker.py
+++ b/tests/unit/test_simulation_worker.py
@@ -334,5 +334,53 @@ class TestSimulationWorkerHelperMethods:
         assert len(warnings) == 1
         assert "Rate limit" in warnings[0]
 
+    def test_filter_completed_dates_all_new(self, clean_db):
+        """Test filtering when no dates are completed."""
+        from api.simulation_worker import SimulationWorker
+        from api.database import initialize_database
+
+        db_path = clean_db
+        initialize_database(db_path)
+
+        worker = SimulationWorker(job_id="test-789", db_path=db_path)
+
+        # Mock job_manager to return empty completed dates
+        mock_job_manager = Mock()
+        mock_job_manager.get_completed_model_dates.return_value = {}
+        worker.job_manager = mock_job_manager
+
+        available_dates = ["2025-10-01", "2025-10-02"]
+        models = ["gpt-5"]
+
+        result = worker._filter_completed_dates(available_dates, models)
+
+        # All dates should be returned
+        assert result == available_dates
+
+    def test_filter_completed_dates_some_completed(self, clean_db):
+        """Test filtering when some dates are completed."""
+        from api.simulation_worker import SimulationWorker
+        from api.database import initialize_database
+
+        db_path = clean_db
+        initialize_database(db_path)
+
+        worker = SimulationWorker(job_id="test-abc", db_path=db_path)
+
+        # Mock job_manager to return one completed date
+        mock_job_manager = Mock()
+        mock_job_manager.get_completed_model_dates.return_value = {
+            "gpt-5": ["2025-10-01"]
+        }
+        worker.job_manager = mock_job_manager
+
+        available_dates = ["2025-10-01", "2025-10-02", "2025-10-03"]
+        models = ["gpt-5"]
+
+        result = worker._filter_completed_dates(available_dates, models)
+
+        # Should exclude completed date
+        assert result == ["2025-10-02", "2025-10-03"]
+
 
 # Coverage target: 90%+ for api/simulation_worker.py