diff --git a/src/app/main.py b/src/app/main.py
index 0d09438..b944c19 100644
--- a/src/app/main.py
+++ b/src/app/main.py
@@ -23,7 +23,7 @@
     RerankRequest,
     RerankResponse,
 )
-from .models import get_model
+from .models import get_model as get_model, get_model_or_400
 from .config import (
     EMBEDDING_MODELS,
     RERANK_MODELS,
@@ -170,16 +170,7 @@ def _get_model_or_400(model_name: str, model_type: str) -> Any:
     Helper for backwards compatibility with legacy tests calling _get_model_or_400.
     """
     supported_models = EMBEDDING_MODELS if model_type == "embedding" else RERANK_MODELS
-    if model_name not in supported_models:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Model '{model_name}' not found for {model_type}s.",
-        )
-
-    try:
-        return get_model(model_name)
-    except ValueError as e:
-        raise HTTPException(status_code=400, detail=str(e))
+    return get_model_or_400(model_name, supported_models, model_type)
 
 
 def _determine_ruri_prefix(request: EmbeddingRequest) -> str:
diff --git a/src/app/models.py b/src/app/models.py
index 492495c..e30b007 100644
--- a/src/app/models.py
+++ b/src/app/models.py
@@ -120,3 +120,33 @@ def get_model(model_name: str, device: str | None = None):
         _model_cache[model_name] = model
         logging.info(f"Model '{model_name}' loaded successfully.")
         return model
+
+
+def get_model_or_400(
+    model_name: str, valid_models: Any, model_type: str = "model"
+) -> Any:
+    """
+    Validates model_name against valid_models and fetches model instance via get_model.
+    Raises HTTPException(400) if model is invalid or if get_model raises a ValueError.
+    """
+    from fastapi import HTTPException
+
+    if model_name not in valid_models:
+        suffix = "s" if not model_type.endswith("s") else ""
+        raise HTTPException(
+            status_code=400,
+            detail=f"Model '{model_name}' not found for {model_type}{suffix}.",
+        )
+
+    try:
+        # Dynamically import main to support patches on app.main.get_model in tests
+        try:
+            import app.main as main_mod
+
+            fetch_func = getattr(main_mod, "get_model", get_model)
+        except Exception:
+            fetch_func = get_model
+
+        return fetch_func(model_name)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
diff --git a/src/app/services/embedding.py b/src/app/services/embedding.py
index a93eb19..c74f253 100644
--- a/src/app/services/embedding.py
+++ b/src/app/services/embedding.py
@@ -7,6 +7,7 @@
 
 from .base import BaseEmbeddingService
 from ..image_utils import load_image_from_source
+from ..models import get_model_or_400
 from ..schemas import (
     EmbeddingRequest,
     EmbeddingResponse,
@@ -143,17 +144,7 @@ def _tokenize_and_truncate_embeddings(
 
 
 def _get_model_or_400(model_name: str) -> Any:
-    from app.main import get_model
-
-    if model_name not in EMBEDDING_MODELS:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Model '{model_name}' not found for embeddings.",
-        )
-    try:
-        return get_model(model_name)
-    except ValueError as e:
-        raise HTTPException(status_code=400, detail=str(e))
+    return get_model_or_400(model_name, EMBEDDING_MODELS, "embedding")
 
 
 class EmbeddingService(BaseEmbeddingService):
diff --git a/src/app/services/rerank.py b/src/app/services/rerank.py
index 105212f..6aab873 100644
--- a/src/app/services/rerank.py
+++ b/src/app/services/rerank.py
@@ -3,6 +3,7 @@
 from fastapi import HTTPException
 
 from .base import BaseRerankService
+from ..models import get_model_or_400
 from ..schemas import RerankRequest, RerankResponse, RerankData, Usage
 from ..config import RERANK_MODELS
 
@@ -47,17 +48,7 @@ def _sort_and_format_rerank_results(
 
 
 def _get_model_or_400(model_name: str) -> Any:
-    from app.main import get_model
-
-    if model_name not in RERANK_MODELS:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Model '{model_name}' not found for reranks.",
-        )
-    try:
-        return get_model(model_name)
-    except ValueError as e:
-        raise HTTPException(status_code=400, detail=str(e))
+    return get_model_or_400(model_name, RERANK_MODELS, "rerank")
 
 
 class RerankService(BaseRerankService):