feat(api): add POST /fusion/predict route for multi-modal fusion

src/api/main.py

@@ -12,6 +12,7 @@ from src.api.routes import (
     explain_router,
     experiments_router,
     agent_router,
+    fusion_router,   # NEW
 )
 from src.api.schemas import HealthResponse
 
@@ -26,6 +27,7 @@ app.include_router(predict_router)
 app.include_router(explain_router)
 app.include_router(experiments_router)
 app.include_router(agent_router)
+app.include_router(fusion_router)
 
 
 @app.get("/health", response_model=HealthResponse)

src/api/routes.py

@@ -28,6 +28,8 @@ from src.api.schemas import (
     BBBRequest,
     CalibrationContext,
     EEGExplainRequest,
+    FusionRequest,
+    FusionResponse,
     EEGExplainResponse,
     EEGRequest,
     FeatureAttribution,
@@ -632,3 +634,15 @@ def run_agent(req: AgentRunRequest) -> AgentRunResponse:
         model=result.model,
         finish_reason=result.finish_reason,
     )
+
+
+# --- Fusion router ---------------------------------------------------------
+
+fusion_router = APIRouter(prefix="/fusion")
+
+
+@fusion_router.post("/predict", response_model=FusionResponse)
+def fusion_predict(req: FusionRequest) -> FusionResponse:
+    """Combine MRI, EEG, and clinical scores into per-disease confidence."""
+    from src.fusion.engine import fuse as fuse_engine
+    return fuse_engine(req)

src/api/schemas.py

@@ -288,3 +288,16 @@ class AgentRunResponse(BaseModel):
     trace: list[AgentToolTraceItem] = Field(default_factory=list)
     model: str | None = None
     finish_reason: str = "complete"
+
+
+# --- Fusion engine surface --------------------------------------------------
+
+# Re-export the fusion types so the API surface lives in one file but the
+# implementation stays in src/fusion. This keeps `from src.api.schemas import *`
+# style imports stable for the frontend layer.
+from src.fusion.types import (  # noqa: E402,F401
+    ClinicalScores as FusionClinicalScores,
+    FusionInput as FusionRequest,
+    FusionOutput as FusionResponse,
+    ModalityPrediction as FusionModalityPrediction,
+)

tests/api/test_fusion_route.py

@@ -0,0 +1,50 @@
+"""Integration test for POST /fusion/predict."""
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+from src.api.main import app
+
+
+client = TestClient(app)
+
+
+class TestFusionRoute:
+    def test_happy_path_mri_only(self) -> None:
+        body = {
+            "mri": {
+                "label_text": "alzheimers",
+                "label": 1,
+                "confidence": 0.88,
+                "probabilities": [
+                    {"label_text": "control", "probability": 0.12},
+                    {"label_text": "alzheimers", "probability": 0.88},
+                ],
+            },
+        }
+        r = client.post("/fusion/predict", json=body)
+        assert r.status_code == 200, r.text
+        data = r.json()
+        assert "diseases" in data
+        assert any(d["disease"] == "alzheimers" for d in data["diseases"])
+        assert data["top_disease"] in {"alzheimers", "parkinsons", "other"}
+
+    def test_empty_input_returns_baseline(self) -> None:
+        r = client.post("/fusion/predict", json={})
+        assert r.status_code == 200
+        data = r.json()
+        for d in data["diseases"]:
+            assert abs(d["probability"] - 0.5) < 1e-6
+        assert "mri" in data["missing_inputs"]
+
+    def test_invalid_probability_returns_422(self) -> None:
+        body = {
+            "mri": {
+                "label_text": "x",
+                "label": 0,
+                "confidence": 1.5,
+                "probabilities": [{"label_text": "x", "probability": 1.5}],
+            },
+        }
+        r = client.post("/fusion/predict", json=body)
+        assert r.status_code == 422