diff --git a/litellm/proxy/management_endpoints/common_utils.py b/litellm/proxy/management_endpoints/common_utils.py
index 98155ad6839..61d610babb1 100644
--- a/litellm/proxy/management_endpoints/common_utils.py
+++ b/litellm/proxy/management_endpoints/common_utils.py
@@ -23,6 +23,27 @@ def validate_finite_spend(spend: float | None) -> None:
         )
 
 
+def validate_key_duration(duration: str | None) -> None:
+    """Reject key-expiry durations that can't be parsed, so a bad value 400s
+    at the write boundary instead of 500ing inside duration math.
+
+    `None` (never expires) and `'-1'` (the update path's never-expire
+    sentinel) pass through.
+    """
+    if duration is None or duration == "-1":
+        return
+
+    from litellm.litellm_core_utils.duration_parser import duration_in_seconds
+
+    try:
+        duration_in_seconds(duration)
+    except (ValueError, OverflowError):
+        raise HTTPException(
+            status_code=400,
+            detail={"error": f"Invalid duration '{duration}'. Use a format like '1h', '24h', '7d', or '30d'."},
+        )
+
+
 def validate_budget_duration(budget_duration: str | None, status_code: int = 400) -> None:
     """Reject budget durations that can't be parsed, are non-positive, or
     overflow date math, so a bad value can't be persisted and later crash the
diff --git a/litellm/proxy/management_endpoints/key_management_endpoints.py b/litellm/proxy/management_endpoints/key_management_endpoints.py
index 749a940de0e..4fa6a1d0571 100644
--- a/litellm/proxy/management_endpoints/key_management_endpoints.py
+++ b/litellm/proxy/management_endpoints/key_management_endpoints.py
@@ -92,6 +92,7 @@ from litellm.proxy.management_endpoints.common_utils import (
     _user_has_admin_view,
     validate_budget_duration,
     validate_finite_spend,
+    validate_key_duration,
 )
 from litellm.proxy.management_endpoints.model_management_endpoints import (
     _add_model_to_db,
@@ -1002,6 +1003,7 @@ async def _common_key_generation_helper(
     )
 
     validate_budget_duration(data.budget_duration)
+    validate_key_duration(data.duration)
     raise_on_invalid_key_logging_config(data.metadata)
 
     if data.throttle_on_budget_exceeded is True and user_api_key_dict.user_role != LitellmUserRoles.PROXY_ADMIN.value:
@@ -2695,6 +2697,7 @@ async def _validate_update_key_data(
     # Reject NaN/±inf spend before it can reach the DB / spend counter.
     validate_finite_spend(data.spend)
     validate_budget_duration(data.budget_duration)
+    validate_key_duration(data.duration)
 
     _is_proxy_admin: Final = user_api_key_dict.user_role == LitellmUserRoles.PROXY_ADMIN.value
 
diff --git a/tests/test_litellm/proxy/management_endpoints/test_common_utils.py b/tests/test_litellm/proxy/management_endpoints/test_common_utils.py
index da8fc760787..634bb728f0b 100644
--- a/tests/test_litellm/proxy/management_endpoints/test_common_utils.py
+++ b/tests/test_litellm/proxy/management_endpoints/test_common_utils.py
@@ -29,6 +29,7 @@ from litellm.proxy.management_endpoints.common_utils import (
     _user_has_admin_privileges,
     _user_has_admin_view,
     admin_can_invite_user,
+    validate_key_duration,
 )
 from litellm.proxy.management_endpoints.common_utils import _has_non_empty_value
 
@@ -1120,3 +1121,23 @@ class TestUpdateMetadataFieldsPremiumCheck:
         }
         _update_metadata_fields(updated_kv)
         mock_check.assert_called()
+
+
+class TestValidateKeyDuration:
+    """Invalid key-expiry durations must 400, not 500 (#39710)."""
+
+    def test_none_and_never_expire_sentinel_pass(self):
+        validate_key_duration(None)
+        validate_key_duration("-1")
+
+    def test_valid_durations_pass(self):
+        validate_key_duration("7d")
+        validate_key_duration("1h")
+
+    def test_garbage_durations_raise_400(self):
+        from fastapi import HTTPException
+
+        for bad in ["banana", "1x", ""]:
+            with pytest.raises(HTTPException) as exc_info:
+                validate_key_duration(bad)
+            assert exc_info.value.status_code == 400