fix(fusion): correct logit-scale comment; top_disease=None on empty input

src/fusion/engine.py

@@ -21,7 +21,7 @@ from src.fusion.types import (
 
 logger = get_logger(__name__)
 
-_LOGIT_SCALE = 4.0  # tuned so a single saturated modality maps to ~0.88
+_LOGIT_SCALE = 4.0  # a single saturated modality maps to ~0.77-0.80 (depending on its weight)
 
 
 # Clinical-test name -> (signal_fn, attribute_on_ClinicalScores)
@@ -46,7 +46,10 @@ def fuse(inp: FusionInput) -> FusionOutput:
     for disease in weight_registry.available_diseases():
         diseases.append(_score_one_disease(disease, inp))
 
-    top = max(diseases, key=lambda d: d.probability).disease
+    if any(d.contributions for d in diseases):
+        top: str | None = max(diseases, key=lambda d: d.probability).disease
+    else:
+        top = None
     return FusionOutput(diseases=diseases, top_disease=top, missing_inputs=missing)
 
 

src/fusion/types.py

@@ -51,5 +51,5 @@ class DiseaseScore(BaseModel):
 
 class FusionOutput(BaseModel):
     diseases: list[DiseaseScore]
-    top_disease: str
+    top_disease: str | None
     missing_inputs: list[str] = Field(default_factory=list)

tests/api/test_fusion_route.py

@@ -36,6 +36,7 @@ class TestFusionRoute:
         for d in data["diseases"]:
             assert abs(d["probability"] - 0.5) < 1e-6
         assert "mri" in data["missing_inputs"]
+        assert data["top_disease"] is None
 
     def test_invalid_probability_returns_422(self) -> None:
         body = {

tests/fusion/test_engine.py

@@ -40,6 +40,7 @@ class TestFuse:
             assert ds.contributions == []
         assert "mri" in out.missing_inputs
         assert "eeg" in out.missing_inputs
+        assert out.top_disease is None
 
     def test_mri_only_alzheimers_high(self) -> None:
         inp = FusionInput(mri=_mri(prob_alz=0.9))