diff --git a/gliner_inference_server/main.py b/gliner_inference_server/main.py
index 0de10e4..2790c2b 100644
--- a/gliner_inference_server/main.py
+++ b/gliner_inference_server/main.py
@@ -1,4 +1,5 @@
 from fastapi import FastAPI, HTTPException, status
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 from starlette.requests import Request
 from typing import List, Optional, Union, Dict, Any, Tuple
@@ -71,6 +72,36 @@ async def http_exception_handler(request: Request, exc: HTTPException):
         content=response_body
     )
 
+
+@app.exception_handler(RequestValidationError)
+async def validation_exception_handler(request: Request, exc: RequestValidationError):
+    """
+    Handles Pydantic validation errors (normally 422) and returns them as 400 Bad Request
+    with a consistent error structure.
+
+    Return first validation error.
+    """
+    # Extract validation error details
+    errors = exc.errors()
+
+    # Get first error:
+    location = " -> ".join(str(l) for l in errors[0]["loc"])
+    error_message = f"{location}: {errors[0]['msg']}"
+
+    response_body = {
+        "error": {
+            "code": "INVALID_INPUT",
+            "message": error_message,
+            "details": {}
+        }
+    }
+
+    return JSONResponse(
+        # Changed from 422 to 400
+        status_code=400,
+        content=response_body
+    )
+
 # ==================== Endpoints ====================
 @app.get("/health")
 async def health_check():
diff --git a/tests/test_general_endpoint.py b/tests/test_general_endpoint.py
index a72acfa..cf31ddc 100644
--- a/tests/test_general_endpoint.py
+++ b/tests/test_general_endpoint.py
@@ -89,7 +89,7 @@ class TestGeneralEndpoint:
             "entities": []
         }
         response = api_client.post(f"{BASE_URL}/general", json=payload)
-        assert response.status_code == 422  # Pydantic validation error
+        assert response.status_code == 400
 
     def test_error_empty_texts_list(self, api_client):
         """Test error with an empty texts list"""
@@ -109,7 +109,7 @@ class TestGeneralEndpoint:
             "threshold": threshold
         }
         response = api_client.post(f"{BASE_URL}/general", json=payload)
-        assert response.status_code == 422
+        assert response.status_code == 400
 
     @pytest.mark.parametrize("threshold", [0.0, 0.5, 1.0])
     def test_valid_threshold_range(self, api_client, threshold):
diff --git a/tests/test_relation_extraction_endpoint.py b/tests/test_relation_extraction_endpoint.py
index 2e7eb88..52932bd 100644
--- a/tests/test_relation_extraction_endpoint.py
+++ b/tests/test_relation_extraction_endpoint.py
@@ -73,7 +73,7 @@ class TestRelationExtractionEndpoint:
             "relations": []
         }
         response = api_client.post(f"{BASE_URL}/relation-extraction", json=payload)
-        assert response.status_code == 422
+        assert response.status_code == 400
 
     def test_error_both_text_and_texts(self, api_client):
         """Test error when both text and texts are present"""