Add support for t-distributed innovations in SARIXModel

- Extend _get_extra_sarix_params() to pass innovation_dist and innovation_df_prior_scale
- Update SARIXFourierModel to properly merge base and Fourier parameters
- Add test_sarix_tdist_innovations() integration test
- Validates model runs successfully with t-distributed errors
- Verifies output structure and prediction quality

Depends on sarix library support for t-distributed innovations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: nickreich

src/idmodels/sarix.py

@@ -14,7 +14,15 @@ def __init__(self, model_config):
 
     def _get_extra_sarix_params(self, df):
         """Return extra parameters to pass to SARIX constructor. Returns empty dict by default."""
-        return {}
+        extra_params = {}
+
+        # Add innovation distribution parameters if specified
+        if hasattr(self.model_config, 'innovation_dist'):
+            extra_params['innovation_dist'] = self.model_config.innovation_dist
+        if hasattr(self.model_config, 'innovation_df_prior_scale'):
+            extra_params['innovation_df_prior_scale'] = self.model_config.innovation_df_prior_scale
+
+        return extra_params
 
     def run(self, run_config):
         fdl = DiseaseDataLoader()
@@ -118,15 +126,21 @@ class SARIXFourierModel(SARIXModel):
     """
     def _get_extra_sarix_params(self, df):
         """Return Fourier-specific parameters for SARIX constructor."""
+        # Get base parameters (includes innovation_dist if specified)
+        extra_params = super()._get_extra_sarix_params(df)
+
         # Extract day-of-year from dates for Fourier features
         # Take the first location's dates (same for all locations after reshaping)
         day_of_year = df.groupby("location")["wk_end_date"].apply(lambda x: x.dt.dayofyear.values).iloc[0]
 
-        return {
+        # Add Fourier-specific parameters
+        extra_params.update({
             "day_of_year": day_of_year,
             "fourier_K": self.model_config.fourier_K,
             "fourier_pooling": self.model_config.fourier_pooling
-        }
+        })
+
+        return extra_params
 
 
 def _np_percentile(predictions, q_levels, axis):

tests/integration/test_sarix.py

@@ -333,6 +333,78 @@ def test_sarix_fourier_missing_pooling_parameter():
             f"Error should mention fourier_pooling, got: {str(e)}"
 
 
+def test_sarix_tdist_innovations(tmp_path):
+    """Test SARIX model with t-distributed innovations."""
+    model_config = SimpleNamespace(
+        model_class="sarix",
+        model_name="sarix_p6_4rt_thetashared_sigmanone_tdist",
+
+        # data sources and adjustments for reporting issues
+        sources=["nhsn"],
+
+        # fit locations separately or jointly
+        fit_locations_separately=False,
+
+        # SARI model parameters
+        p=6,
+        P=0,
+        d=0,
+        D=0,
+        season_period=1,
+
+        # power transform applied to surveillance signals
+        power_transform="4rt",
+
+        # sharing of information about parameters
+        theta_pooling="shared",
+        sigma_pooling="none",
+
+        # innovation distribution parameters
+        innovation_dist="t",
+        innovation_df_prior_scale=10.0,
+
+        # covariates
+        x=[]
+    )
+
+    run_config = SimpleNamespace(
+        disease="flu",
+        ref_date=datetime.date.fromisoformat("2024-01-06"),
+        output_root=tmp_path / "model-output",
+        artifact_store_root=tmp_path / "artifact-store",
+        save_feat_importance=False,
+        locations=["US", "01", "02", "04", "05"],  # Reduced for faster testing
+        max_horizon=2,  # Reduced for faster testing
+        q_levels=[0.025, 0.50, 0.975],
+        q_labels=["0.025", "0.5", "0.975"],
+        num_warmup=100,  # Reduced for faster testing
+        num_samples=100,
+        num_chains=1
+    )
+
+    model = SARIXModel(model_config)
+    model.run(run_config)
+
+    # Verify output structure
+    actual_df = pd.read_csv(
+        run_config.output_root / "UMass-sarix_p6_4rt_thetashared_sigmanone_tdist" /
+        "2024-01-06-UMass-sarix_p6_4rt_thetashared_sigmanone_tdist.csv"
+    )
+
+    # Assertions
+    assert len(actual_df) > 0, "Output dataframe should not be empty"
+    assert set(actual_df["location"].unique()) == set(run_config.locations), \
+        "Output should contain predictions for all input locations"
+    assert all(actual_df["output_type"] == "quantile"), \
+        "All outputs should be quantiles"
+    assert set(actual_df["output_type_id"].astype(str).unique()) == set(run_config.q_labels), \
+        "Output should contain all specified quantile levels"
+    assert actual_df["value"].notna().all(), \
+        "All predictions should be non-null"
+    assert (actual_df["value"] >= 0).all(), \
+        "All predictions should be non-negative"
+
+
 def _np_percentile_val():
     return numpy.array(
         [[[2.22541624e-01, 1.82324940e-01, 1.27709944e-01],