diff --git a/src/core/errors.py b/src/core/errors.py
index dea1f50..69d7e0c 100644
--- a/src/core/errors.py
+++ b/src/core/errors.py
@@ -7,6 +7,7 @@
 from http import HTTPStatus
 
 from fastapi import Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
 
 # =============================================================================
@@ -89,6 +90,27 @@ def problem_detail_exception_handler(
     )
 
 
+def validation_exception_handler(
+    request: Request,  # noqa: ARG001
+    exc: RequestValidationError,
+) -> JSONResponse:
+    """FastAPI exception handler for RequestValidationError.
+
+    Returns a RFC 9457 compliant response for input validation failures.
+    """
+    return JSONResponse(
+        status_code=HTTPStatus.UNPROCESSABLE_ENTITY,
+        content={
+            "type": "https://openml.org/problems/validation-error",
+            "title": "Validation Error",
+            "status": HTTPStatus.UNPROCESSABLE_ENTITY,
+            "detail": "Input validation failed.",
+            "errors": exc.errors(),
+        },
+        media_type="application/problem+json",
+    )
+
+
 # =============================================================================
 # Dataset Errors
 # =============================================================================
diff --git a/src/main.py b/src/main.py
index 3be2c5c..46cd79c 100644
--- a/src/main.py
+++ b/src/main.py
@@ -7,10 +7,15 @@
 
 import uvicorn
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 from loguru import logger
 
 from config import load_configuration
-from core.errors import ProblemDetailError, problem_detail_exception_handler
+from core.errors import (
+    ProblemDetailError,
+    problem_detail_exception_handler,
+    validation_exception_handler,
+)
 from core.logging import (
     add_request_context_to_log,
     log_request_duration,
@@ -87,6 +92,7 @@ def create_api(configuration_file: Path | None = None) -> FastAPI:
     app.middleware("http")(add_request_context_to_log)
 
     app.add_exception_handler(ProblemDetailError, problem_detail_exception_handler)  # type: ignore[arg-type]
+    app.add_exception_handler(RequestValidationError, validation_exception_handler)  # type: ignore[arg-type]
 
     logger.info("Adding routers to app")
     app.include_router(datasets_router)
diff --git a/tests/routers/openml/dataset_tag_test.py b/tests/routers/openml/dataset_tag_test.py
index cddd0d8..d11fc96 100644
--- a/tests/routers/openml/dataset_tag_test.py
+++ b/tests/routers/openml/dataset_tag_test.py
@@ -42,7 +42,7 @@ async def test_dataset_tag_invalid_tag_is_rejected(
     )
 
     assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
-    assert response.json()["detail"][0]["loc"] == ["body", "tag"]
+    assert response.json()["errors"][0]["loc"] == ["body", "tag"]
 
 
 # ── Direct call tests: tag_dataset ──
diff --git a/tests/routers/openml/task_list_test.py b/tests/routers/openml/task_list_test.py
index 4667967..67d8539 100644
--- a/tests/routers/openml/task_list_test.py
+++ b/tests/routers/openml/task_list_test.py
@@ -135,9 +135,9 @@ async def test_list_tasks_invalid_pagination_type(
     )
     assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
     # Verify that the error points to the correct field
-    detail = response.json()["detail"][0]
-    assert detail["loc"][-2:] == ["pagination", expected_field]
-    assert detail["type"] in {"type_error.integer", "int_parsing", "int_type"}
+    error = response.json()["errors"][0]
+    assert error["loc"][-2:] == ["pagination", expected_field]
+    assert error["type"] in {"type_error.integer", "int_parsing", "int_type"}
 
 
 @pytest.mark.parametrize(
@@ -150,8 +150,8 @@ async def test_list_tasks_invalid_range(value: str, py_api: httpx.AsyncClient) -
     response = await py_api.post("/tasks/list", json={"number_instances": value})
     assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
     # Verify the error is for the correct field
-    detail = response.json()["detail"][0]
-    assert detail["loc"][-1] == "number_instances"
+    error = response.json()["errors"][0]
+    assert error["loc"][-1] == "number_instances"
 
 
 @pytest.mark.parametrize(
@@ -171,9 +171,9 @@ async def test_list_tasks_invalid_inputs(
     response = await py_api.post("/tasks/list", json=payload)
     assert response.status_code == HTTPStatus.UNPROCESSABLE_ENTITY
     # Ensure we are failing for the field we provided
-    detail = response.json()["detail"][0]
+    error = response.json()["errors"][0]
     expected_field = next(iter(payload))
-    assert detail["loc"][-1] == expected_field
+    assert error["loc"][-1] == expected_field
 
 
 async def test_list_tasks_no_results_api_mapping(py_api: httpx.AsyncClient) -> None: