diff --git a/src/routers/openml/tasktype.py b/src/routers/openml/tasktype.py
index 5355e45..44228fa 100644
--- a/src/routers/openml/tasktype.py
+++ b/src/routers/openml/tasktype.py
@@ -1,4 +1,6 @@
 import json
+import logging
+from collections.abc import Mapping
 from typing import Annotated, Any, Literal, cast
 
 from fastapi import APIRouter, Depends
@@ -11,6 +13,7 @@
 from routers.dependencies import expdb_connection
 
 router = APIRouter(prefix="/tasktype", tags=["tasks"])
+logger = logging.getLogger(__name__)
 
 
 def _normalize_task_type(task_type: Row[Any]) -> dict[str, str | None | list[Any]]:
@@ -26,6 +29,36 @@ def _normalize_task_type(task_type: Row[Any]) -> dict[str, str | None | list[Any
     return ttype
 
 
+def _extract_data_type_from_api_constraints(
+    api_constraints: Mapping[str, Any] | str | None,
+    input_name: str,
+) -> str | None:
+    """Extract string data_type from api_constraints safely."""
+    constraint: Mapping[str, Any] | None = None
+
+    if isinstance(api_constraints, str):
+        try:
+            loaded = json.loads(api_constraints)
+        except json.JSONDecodeError:
+            logger.warning(
+                "Failed to decode legacy api_constraints JSON for task_type_input '%s'; value=%r",
+                input_name,
+                api_constraints,
+                exc_info=True,
+            )
+            return None
+        if isinstance(loaded, Mapping):
+            constraint = loaded
+    elif isinstance(api_constraints, Mapping):
+        constraint = api_constraints
+
+    if not constraint:
+        return None
+
+    data_type = constraint.get("data_type")
+    return data_type if isinstance(data_type, str) else None
+
+
 @router.get(path="/list")
 async def list_task_types(
     expdb: Annotated[AsyncConnection, Depends(expdb_connection)],
@@ -59,6 +92,7 @@ async def get_task_type(
             creator.strip(' "') for creator in cast("str", contributors).split(",")
         ]
     task_type["creation_date"] = task_type.pop("creationDate")
+
     task_type_inputs = await get_input_for_task_type(task_type_id, expdb)
     input_types = []
     for task_type_input in task_type_inputs:
@@ -66,10 +100,15 @@ async def get_task_type(
         if task_type_input.requirement == "required":
             input_["requirement"] = task_type_input.requirement
         input_["name"] = task_type_input.name
-        # api_constraints is for one input only in the test database (TODO: patch db)
-        if isinstance(task_type_input.api_constraints, str):
-            constraint = json.loads(task_type_input.api_constraints)
-            input_["data_type"] = constraint["data_type"]
+
+        data_type = _extract_data_type_from_api_constraints(
+            task_type_input.api_constraints,
+            task_type_input.name,
+        )
+        if data_type is not None:
+            input_["data_type"] = data_type
+
         input_types.append(input_)
+
     task_type["input"] = input_types
     return {"task_type": task_type}