diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index dc78ec75a6e..216a146143d 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -776,6 +776,7 @@ from litellm.types.router import (
     RoutingPlugin,
     SearchToolTypedDict,
     updateDeployment,
+    validate_max_parallel_requests_queue_size,
 )
 from litellm.types.router import ModelInfo as RouterModelInfo
 from litellm.types.scheduler import DefaultPriorities
@@ -16936,6 +16937,17 @@ async def update_config(
                         )
                     },
                 )
+            raw_queue_size: Final = raw_router_settings.get("default_max_parallel_requests_queue_size")
+            try:
+                validate_max_parallel_requests_queue_size(raw_queue_size)
+            except ValueError as invalid_queue_size:
+                raise HTTPException(
+                    status_code=400,
+                    detail=(
+                        f"default_max_parallel_requests_queue_size={raw_queue_size!r} is not valid, "
+                        "it must be a non-negative integer or null"
+                    ),
+                ) from invalid_queue_size
 
         if prisma_client is None:
             raise Exception("No DB Connected")
diff --git a/litellm/router.py b/litellm/router.py
index 132f48730df..97325e6c450 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -263,6 +263,7 @@ from litellm.types.router import (
     RoutingStrategy,
     SearchToolTypedDict,
     TaggedPreRoutingStrategy,
+    validate_max_parallel_requests_queue_size,
 )
 from litellm.types.services import ServiceTypes
 from litellm.types.utils import (
@@ -936,7 +937,9 @@ class Router:
             None  # use this to track the users default deployment, when they want to use model = *
         )
         self.default_max_parallel_requests = default_max_parallel_requests
-        self._default_max_parallel_requests_queue_size = default_max_parallel_requests_queue_size
+        self._default_max_parallel_requests_queue_size = validate_max_parallel_requests_queue_size(
+            default_max_parallel_requests_queue_size
+        )
         self.provider_default_deployment_ids: list[str] = []
         self.pattern_router = PatternMatchRouter()
         self.team_pattern_routers: dict[str, PatternMatchRouter] = {}  # {"TEAM_ID": PatternMatchRouter}
@@ -11852,7 +11855,7 @@ class Router:
 
     @default_max_parallel_requests_queue_size.setter
     def default_max_parallel_requests_queue_size(self, queue_size: int | None) -> None:
-        self._default_max_parallel_requests_queue_size = None if queue_size is None else int(queue_size)
+        self._default_max_parallel_requests_queue_size = validate_max_parallel_requests_queue_size(queue_size)
         InitalizeCachedClient.apply_default_max_parallel_requests_queue_size(
             litellm_router_instance=self, queue_size=self._default_max_parallel_requests_queue_size
         )
diff --git a/litellm/router_utils/client_initalization_utils.py b/litellm/router_utils/client_initalization_utils.py
index 72854cba028..be5f71a4e70 100644
--- a/litellm/router_utils/client_initalization_utils.py
+++ b/litellm/router_utils/client_initalization_utils.py
@@ -5,7 +5,7 @@ from typing import TYPE_CHECKING, Any, Final
 
 from litellm._logging import verbose_router_logger
 from litellm.exceptions import RateLimitError, RateLimitErrorCategory, RateLimitType
-from litellm.types.router import RouterErrors
+from litellm.types.router import RouterErrors, validate_max_parallel_requests_queue_size
 from litellm.utils import calculate_max_parallel_requests
 
 if TYPE_CHECKING:
@@ -26,7 +26,7 @@ class DeploymentSemaphore:
         self.max_parallel_requests: Final = max_parallel_requests
         self.model_id: Final = model_id
         self.model_group: Final = model_group
-        self.queue_size = queue_size
+        self.queue_size = validate_max_parallel_requests_queue_size(queue_size)
         self.waiting = 0
 
     def locked(self) -> bool:
diff --git a/litellm/types/router.py b/litellm/types/router.py
index 8f788b5f933..848dd28aaac 100644
--- a/litellm/types/router.py
+++ b/litellm/types/router.py
@@ -6,10 +6,10 @@ import datetime
 import enum
 from collections.abc import Mapping, Sequence
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, Any, ClassVar, Final, Generic, Literal, TypeVar, get_type_hints
+from typing import TYPE_CHECKING, Annotated, Any, ClassVar, Final, Generic, Literal, TypeVar, get_type_hints
 
 import httpx
-from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
+from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, field_validator, model_validator
 from typing_extensions import Protocol, ReadOnly, Required, TypedDict, runtime_checkable
 
 from litellm._logging import verbose_logger
@@ -314,6 +314,14 @@ class CredentialLiteLLMParams(BaseModel):
 _RESERVED_INIT_KEYS: Final = frozenset({"self", "params", "__class__"})
 
 
+MaxParallelRequestsQueueSize = Annotated[int, Field(strict=True, ge=0)]
+_MAX_PARALLEL_REQUESTS_QUEUE_SIZE_ADAPTER: Final = TypeAdapter(MaxParallelRequestsQueueSize | None)
+
+
+def validate_max_parallel_requests_queue_size(value: object) -> int | None:
+    return _MAX_PARALLEL_REQUESTS_QUEUE_SIZE_ADAPTER.validate_python(value)
+
+
 class GenericLiteLLMParams(CredentialLiteLLMParams, CustomPricingLiteLLMParams):
     """
     LiteLLM Params without 'model' arg (used across completion / assistants api)
@@ -324,6 +332,7 @@ class GenericLiteLLMParams(CredentialLiteLLMParams, CustomPricingLiteLLMParams):
     rpm: int | None = None
     itpm: int | None = None
     otpm: int | None = None
+    max_parallel_requests_queue_size: MaxParallelRequestsQueueSize | None = None
     timeout: float | str | httpx.Timeout | None = None  # if str, pass in as os.environ/
     stream_timeout: float | str | None = None  # timeout when making stream=True calls, if str, pass in as os.environ/
     max_retries: int | None = None
@@ -497,7 +506,7 @@ class LiteLLMParamsTypedDict(TypedDict, total=False):
     order: int | None
     weight: int | None
     max_parallel_requests: int | None
-    max_parallel_requests_queue_size: ReadOnly[int | None]
+    max_parallel_requests_queue_size: ReadOnly[MaxParallelRequestsQueueSize | None]
     api_key: str | None
     api_base: str | None
     api_version: str | None
diff --git a/tests/code_coverage_tests/router_code_coverage.py b/tests/code_coverage_tests/router_code_coverage.py
index 057e82a24c8..582977d613b 100644
--- a/tests/code_coverage_tests/router_code_coverage.py
+++ b/tests/code_coverage_tests/router_code_coverage.py
@@ -88,7 +88,7 @@ ignored_function_names = [
     "_resolve_claude_code_session_router",  # Tested through Claude Code session routing in test_router.py
     "_get_claude_code_session_router_binding",  # Tested through the two-worker session routing test in test_router.py
     "_apply_updated_routing_strategy_args",  # Tested via update_settings in test_lowest_latency.py (file lacks "router" in name)
-    "default_max_parallel_requests_queue_size",  # Property, so its reads and assignments in test_router.py are never an ast.Call
+    "default_max_parallel_requests_queue_size",
 ]
 
 
diff --git a/tests/test_litellm/proxy/test_proxy_server.py b/tests/test_litellm/proxy/test_proxy_server.py
index 5754301ac4a..5a2e76039e8 100644
--- a/tests/test_litellm/proxy/test_proxy_server.py
+++ b/tests/test_litellm/proxy/test_proxy_server.py
@@ -9393,6 +9393,25 @@ def test_update_config_router_settings_null_clears_max_parallel_requests_queue_s
         restore()
 
 
+@pytest.mark.parametrize("invalid_queue_size", [-1, 2.5, "3"])
+def test_update_config_rejects_invalid_max_parallel_requests_queue_size_before_persisting(
+    _update_config_setup, invalid_queue_size
+):
+    client, prisma, restore = _update_config_setup(
+        initial_rows={"router_settings": {"default_max_parallel_requests_queue_size": 3}},
+    )
+    try:
+        resp = client.post(
+            "/config/update",
+            json={"router_settings": {"default_max_parallel_requests_queue_size": invalid_queue_size}},
+        )
+        assert resp.status_code == 400
+        assert "default_max_parallel_requests_queue_size" in resp.json()["error"]["message"]
+        assert prisma.db.litellm_config.rows["router_settings"] == {"default_max_parallel_requests_queue_size": 3}
+    finally:
+        restore()
+
+
 def test_update_config_success_callback_normalizes_existing_mixed_case(
     _update_config_setup,
 ):
diff --git a/tests/test_litellm/router_utils/test_client_initalization_utils.py b/tests/test_litellm/router_utils/test_client_initalization_utils.py
index 332f2f1503a..a6626d2f975 100644
--- a/tests/test_litellm/router_utils/test_client_initalization_utils.py
+++ b/tests/test_litellm/router_utils/test_client_initalization_utils.py
@@ -2,6 +2,7 @@ import asyncio
 from typing import Final
 
 import pytest
+from pydantic import ValidationError
 
 import litellm
 from litellm import Router
@@ -111,6 +112,37 @@ def _router_semaphore(router: Router, model_name: str) -> DeploymentSemaphore:
     return client
 
 
+@pytest.mark.parametrize("invalid_queue_size", [-1, 2.5, True, "3"])
+def test_invalid_queue_sizes_are_rejected_instead_of_coerced(invalid_queue_size: object):
+    """A negative bound would reject every busy request and a fraction would be truncated, so
+    neither may reach a semaphore, the router default, or a live update of that default."""
+    with pytest.raises(ValidationError):
+        _semaphore(queue_size=invalid_queue_size)
+    model_list: Final = [{"model_name": "gpt-5.6", "litellm_params": {"model": "openai/gpt-5.6", "rpm": 1}}]
+    with pytest.raises(ValidationError):
+        Router(model_list=model_list, default_max_parallel_requests_queue_size=invalid_queue_size)
+    with pytest.raises(ValidationError):
+        Router(
+            model_list=[
+                {
+                    "model_name": "gpt-5.6",
+                    "litellm_params": {
+                        "model": "openai/gpt-5.6",
+                        "rpm": 1,
+                        "max_parallel_requests_queue_size": invalid_queue_size,
+                    },
+                }
+            ]
+        )
+
+    router: Final = Router(model_list=model_list, default_max_parallel_requests_queue_size=4)
+    semaphore: Final = _router_semaphore(router, "gpt-5.6")
+    with pytest.raises(ValidationError):
+        router.update_settings(default_max_parallel_requests_queue_size=invalid_queue_size)
+    assert router.default_max_parallel_requests_queue_size == 4
+    assert semaphore.queue_size == 4
+
+
 @pytest.mark.asyncio
 async def test_deployment_queue_size_overrides_router_default_and_zero_is_honored():
     router: Final = Router(
@@ -170,7 +202,7 @@ async def test_update_settings_applies_default_queue_size_to_live_semaphores_wit
     pinned: Final = _router_semaphore(router, "pinned")
     assert router.get_settings()["default_max_parallel_requests_queue_size"] is None
 
-    router.update_settings(default_max_parallel_requests_queue_size="0")
+    router.update_settings(default_max_parallel_requests_queue_size=0)
     assert router.get_settings()["default_max_parallel_requests_queue_size"] == 0
     assert (inherits.queue_size, pinned.queue_size) == (0, 5)
 
diff --git a/ui/litellm-dashboard/src/lib/http/schema.d.ts b/ui/litellm-dashboard/src/lib/http/schema.d.ts
index 872875cc535..84ca93eccfe 100644
--- a/ui/litellm-dashboard/src/lib/http/schema.d.ts
+++ b/ui/litellm-dashboard/src/lib/http/schema.d.ts
@@ -30441,6 +30441,8 @@ export interface components {
             max_budget?: number | null;
             /** Max File Size Mb */
             max_file_size_mb?: number | null;
+            /** Max Parallel Requests Queue Size */
+            max_parallel_requests_queue_size?: number | null;
             /** Max Retries */
             max_retries?: number | null;
             /**
@@ -40893,6 +40895,8 @@ export interface components {
             max_budget?: number | null;
             /** Max File Size Mb */
             max_file_size_mb?: number | null;
+            /** Max Parallel Requests Queue Size */
+            max_parallel_requests_queue_size?: number | null;
             /** Max Retries */
             max_retries?: number | null;
             /**