diff --git a/litellm/litellm_core_utils/get_litellm_params.py b/litellm/litellm_core_utils/get_litellm_params.py
index edd2e88f95c..49fc9abc525 100644
--- a/litellm/litellm_core_utils/get_litellm_params.py
+++ b/litellm/litellm_core_utils/get_litellm_params.py
@@ -41,6 +41,7 @@ OPTIONAL_KWARGS_KEYS: Final = (
             "azure_password",
             "azure_scope",
             "timeout",
+            "client_side_timeout",
             "gcs_bucket_name",
             "bucket_name",
             "vertex_credentials",
diff --git a/litellm/router.py b/litellm/router.py
index ef89d611075..2c20e810839 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -167,6 +167,7 @@ from litellm.router_utils.cooldown_handlers import (
     _get_cooldown_deployments,
     _set_cooldown_deployments,
     is_advisor_orchestration_failure,
+    is_caller_timeout_408,
 )
 from litellm.router_utils.fallback_event_handlers import (
     AttemptedFallbackTargets,
@@ -8297,6 +8298,13 @@ class Router:
             litellm_params: Final = kwargs.get("litellm_params", {})
             _model_info: Final = litellm_params.get("model_info", {})
 
+            if is_caller_timeout_408(kwargs, exception_status):
+                verbose_router_logger.debug(
+                    "Router: Exiting 'deployment_callback_on_failure' without cooldown. "
+                    "A timeout the caller set caused this 408, not the deployment's health."
+                )
+                return False
+
             exception_headers: Final = litellm.litellm_core_utils.exception_mapping_utils._get_response_headers(
                 original_exception=exception
             )
diff --git a/litellm/router_utils/cooldown_handlers.py b/litellm/router_utils/cooldown_handlers.py
index 027f0a9ca05..6e6d4c253e9 100644
--- a/litellm/router_utils/cooldown_handlers.py
+++ b/litellm/router_utils/cooldown_handlers.py
@@ -9,6 +9,7 @@ Router cooldown handlers
 import asyncio
 import math
 from collections.abc import Mapping
+from datetime import datetime
 from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Final
 
@@ -637,3 +638,23 @@ def cast_exception_status_to_int(exception_status: str | int) -> int:
             )
             exception_status = 500
     return exception_status
+
+
+def is_caller_timeout_408(
+    model_call_details: Mapping[str, object], exception_status: str | int, ended: datetime | None = None
+) -> bool:
+    """A 408 that arrives before the caller-set timeout could have fired came from the provider.
+
+    ``ended`` overrides ``model_call_details["end_time"]`` for callers that run before the
+    failure logger has stamped the current API call's end time."""
+    if cast_exception_status_to_int(exception_status) != 408:
+        return False
+    litellm_params: Final = model_call_details.get("litellm_params")
+    if not isinstance(litellm_params, Mapping) or not litellm_params.get("client_side_timeout"):
+        return False
+    timeout: Final = litellm_params.get("timeout")
+    started: Final = model_call_details.get("api_call_start_time") or model_call_details.get("start_time")
+    finished: Final = ended if ended is not None else model_call_details.get("end_time")
+    if not isinstance(timeout, (int, float)) or not isinstance(started, datetime) or not isinstance(finished, datetime):
+        return False
+    return (finished - started).total_seconds() >= timeout
diff --git a/litellm/router_utils/fallback_event_handlers.py b/litellm/router_utils/fallback_event_handlers.py
index 7fda5d96fb0..94164d0ea0c 100644
--- a/litellm/router_utils/fallback_event_handlers.py
+++ b/litellm/router_utils/fallback_event_handlers.py
@@ -2,7 +2,9 @@ import hashlib
 import json
 from collections.abc import Mapping, Sequence
 from dataclasses import dataclass
+from datetime import datetime
 from enum import Enum
+from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Final
 
 import litellm
@@ -20,6 +22,7 @@ from litellm.router_utils.cooldown_handlers import (
     _set_cooldown_deployments,  # pyright: ignore[reportPrivateUsage] - shared helper, used across router_utils
     cast_exception_status_to_int,
     is_advisor_orchestration_failure,
+    is_caller_timeout_408,
 )
 from litellm.router_utils.router_callbacks.track_deployment_metrics import (
     increment_deployment_failures_for_current_minute,
@@ -36,12 +39,14 @@ else:
 # Status codes a generic API call's caller-supplied resource id can trigger on its own
 # (e.g. a nonexistent file/batch/thread id), independent of the selected deployment's health.
 _REQUEST_SCOPED_STATUS_CODES: Final = frozenset((404,))
+_NO_MODEL_CALL_DETAILS: Final[Mapping[str, object]] = MappingProxyType({})
 
 
 def _trigger_cooldown_for_failed_deployment(
     litellm_router: LitellmRouter,
     kwargs: Mapping[str, object],
     exception: Exception,
+    model_call_details: Mapping[str, object] = _NO_MODEL_CALL_DETAILS,
 ) -> None:
     """
     Trigger cooldown for a failed fallback deployment.
@@ -80,7 +85,11 @@ def _trigger_cooldown_for_failed_deployment(
         # timeout, which litellm.Timeout reports as status 408 regardless of the deployment's
         # actual health. Left unguarded, a caller could force a 408 on every deployment in
         # the fallback chain from a single request with a near-zero timeout.
-        if kwargs.get("client_side_timeout") and cast_exception_status_to_int(exception_status) == 408:
+        if is_caller_timeout_408(
+            model_call_details,
+            exception_status,
+            ended=datetime.now(),  # noqa: DTZ005  # naive to match the logging pipeline's api_call_start_time
+        ):
             verbose_router_logger.debug(
                 "Not triggering cooldown for fallback deployment: a caller-supplied "
                 "x-litellm-timeout caused this 408, not deployment health."
@@ -579,6 +588,7 @@ async def run_async_fallback(
                     litellm_router=litellm_router,
                     kwargs=kwargs,
                     exception=e,
+                    model_call_details=logging_obj.model_call_details,
                 )
     raise error_from_fallbacks
 
diff --git a/tests/test_litellm/router_utils/test_fallback_event_handlers.py b/tests/test_litellm/router_utils/test_fallback_event_handlers.py
index a4965c49f07..9318f306c89 100644
--- a/tests/test_litellm/router_utils/test_fallback_event_handlers.py
+++ b/tests/test_litellm/router_utils/test_fallback_event_handlers.py
@@ -1,4 +1,5 @@
 import json
+from datetime import datetime, timedelta
 from typing import NoReturn
 from unittest.mock import MagicMock, patch
 
@@ -955,7 +956,11 @@ class TestTriggerCooldownForFailedDeployment:
         """The proxy's x-litellm-timeout header lets a caller set an arbitrarily short
         timeout, which litellm.Timeout reports as status 408 regardless of the
         deployment's actual health. Without this guard, a caller could force a 408 on
-        every deployment in the fallback chain from a single request."""
+        every deployment in the fallback chain from a single request.
+
+        The failure logger never stamps end_time for a fallback hop (has_logged_async_failure
+        is already set), so model_call_details still carries the previous hop's end_time, which
+        predates this hop's api_call_start_time. The guard must not trust it."""
         mock_router = MagicMock()
         mock_router.cooldown_time = 60.0
         mock_router.get_model_info.return_value = None
@@ -973,11 +978,61 @@ class TestTriggerCooldownForFailedDeployment:
                 litellm_router=mock_router,
                 kwargs={"client_side_timeout": True},
                 exception=exc,
+                model_call_details={
+                    "litellm_params": {"client_side_timeout": True, "timeout": 0.5},
+                    "api_call_start_time": datetime.now() - timedelta(seconds=1),
+                    "end_time": datetime.now() - timedelta(seconds=5),
+                },
             )
 
             mock_set_cooldown.assert_not_called()
             mock_increment.assert_not_called()
 
+    @pytest.mark.asyncio
+    async def test_still_cools_down_provider_408_before_caller_deadline(self):
+        """client_side_timeout only records that the caller configured a timeout. A 408
+        that comes back before that deadline was raised by the provider itself, so it is
+        a real health signal and must still cool the deployment down."""
+        from litellm.router_utils.router_callbacks.track_deployment_metrics import (
+            get_deployment_failures_for_current_minute,
+        )
+
+        router = litellm.Router(
+            model_list=[
+                {
+                    "model_name": "fallback-model",
+                    "litellm_params": {"model": "openai/gpt-5.6", "api_key": "sk-fake"},
+                    "model_info": {"id": "fallback-deployment"},
+                }
+            ],
+            allowed_fails=0,
+            cooldown_time=60,
+            num_retries=0,
+        )
+        exc = litellm.Timeout(message="timeout", model="gpt-5.6", llm_provider="openai")
+        exc.failed_deployment_id = "fallback-deployment"
+        started = datetime.now()
+
+        _trigger_cooldown_for_failed_deployment(
+            litellm_router=router,
+            kwargs={"client_side_timeout": True},
+            exception=exc,
+            model_call_details={
+                "litellm_params": {"client_side_timeout": True, "timeout": 30},
+                "api_call_start_time": started,
+                "end_time": started + timedelta(seconds=1),
+            },
+        )
+
+        assert (
+            get_deployment_failures_for_current_minute(
+                litellm_router_instance=router, deployment_id="fallback-deployment"
+            )
+            == 1
+        )
+        active = router.cooldown_cache.get_active_cooldowns(model_ids=["fallback-deployment"], parent_otel_span=None)
+        assert [entry[0] for entry in active] == ["fallback-deployment"]
+
     def test_still_cools_down_408_without_client_side_timeout_flag(self):
         """The client-side-timeout guard is scoped to caller-supplied timeouts only: a
         408 that did not come from x-litellm-timeout (no client_side_timeout in kwargs)
diff --git a/tests/test_litellm/test_router.py b/tests/test_litellm/test_router.py
index c46a080976c..fe01df04351 100644
--- a/tests/test_litellm/test_router.py
+++ b/tests/test_litellm/test_router.py
@@ -7,7 +7,7 @@ import os
 import sys
 import threading
 from collections.abc import Awaitable, Callable, Mapping
-from datetime import datetime
+from datetime import datetime, timedelta
 from types import SimpleNamespace
 from typing import Final, Literal
 from unittest.mock import AsyncMock, MagicMock, patch
@@ -8520,6 +8520,106 @@ class TestAdvisorSubCallCooldown:
         assert "dep-1" not in self._cooled_down_ids(router)
 
 
+class TestCallerTimeoutCooldown:
+    """A timeout the caller set (the proxy's `timeout` body field or x-litellm-timeout
+    header) comes back as a 408 whatever the deployment's health, so it must neither
+    count toward allowed_fails nor bench the deployment. A 408 without that marker, or
+    one that arrives before the caller's deadline could have fired, is the provider's
+    and keeps cooling the deployment down."""
+
+    def _router(self):
+        return litellm.Router(
+            model_list=[
+                {
+                    "model_name": "slow-model",
+                    "litellm_params": {"model": "openai/gpt-5.6", "api_key": "sk-fake"},
+                    "model_info": {"id": "dep-1"},
+                }
+            ],
+            allowed_fails=0,
+            cooldown_time=120,
+            num_retries=0,
+        )
+
+    def _kwargs(self, marker, started=None, ended=None):
+        exception = litellm.Timeout(message="Request timed out", model="gpt-5.6", llm_provider="openai")
+        return {
+            "exception": exception,
+            "api_call_start_time": started,
+            "end_time": ended,
+            "litellm_params": {"model_info": {"id": "dep-1"}, "metadata": {}, **marker},
+        }
+
+    def _fail_count(self, router):
+        from litellm.router_utils.router_callbacks.track_deployment_metrics import (
+            get_deployment_failures_for_current_minute,
+        )
+
+        return get_deployment_failures_for_current_minute(litellm_router_instance=router, deployment_id="dep-1")
+
+    def _cooled_down_ids(self, router):
+        active = router.cooldown_cache.get_active_cooldowns(model_ids=["dep-1"], parent_otel_span=None)
+        return [entry[0] for entry in active]
+
+    @pytest.mark.asyncio
+    async def test_caller_timeout_408_leaves_failure_counter_and_cooldown_untouched(self):
+        router = self._router()
+        started = datetime.now()
+        ended = started + timedelta(seconds=2.05)
+        kwargs = self._kwargs({"client_side_timeout": True, "timeout": 2}, started=started, ended=ended)
+        assert router.deployment_callback_on_failure(kwargs, None, started, ended) is False
+        assert self._fail_count(router) == 0
+        assert self._cooled_down_ids(router) == []
+
+    @pytest.mark.asyncio
+    async def test_provider_timeout_408_still_counts_and_cools_down(self):
+        router = self._router()
+        now = datetime.now()
+        assert router.deployment_callback_on_failure(self._kwargs({}), None, now, now) is True
+        assert self._fail_count(router) == 1
+        assert self._cooled_down_ids(router) == ["dep-1"]
+
+    @pytest.mark.asyncio
+    async def test_provider_408_before_caller_deadline_still_counts_and_cools_down(self):
+        """The marker only says the caller configured a timeout. A 408 that comes back
+        well before that deadline was raised by the provider, so it is a real health
+        signal and must not hide behind the caller's timeout."""
+        router = self._router()
+        started = datetime.now()
+        ended = started + timedelta(seconds=0.4)
+        kwargs = self._kwargs({"client_side_timeout": True, "timeout": 30}, started=started, ended=ended)
+        assert router.deployment_callback_on_failure(kwargs, None, started, ended) is True
+        assert self._fail_count(router) == 1
+        assert self._cooled_down_ids(router) == ["dep-1"]
+
+    @pytest.mark.asyncio
+    async def test_caller_timeout_marker_reaches_failure_callback_end_to_end(self):
+        router = self._router()
+        seen = []
+        recorded = threading.Event()
+
+        def record(kwargs, completion_response, start_time, end_time):
+            seen.append(kwargs)
+            recorded.set()
+
+        litellm.failure_callback.append(record)
+        try:
+            with pytest.raises(litellm.Timeout):
+                await router.acompletion(
+                    model="slow-model",
+                    messages=[{"role": "user", "content": "hello"}],
+                    mock_timeout=True,
+                    timeout=0.001,
+                    client_side_timeout=True,
+                )
+            assert await asyncio.to_thread(recorded.wait, 5)
+        finally:
+            litellm.failure_callback.remove(record)
+        assert seen[0]["litellm_params"]["client_side_timeout"] is True
+        assert self._fail_count(router) == 0
+        assert self._cooled_down_ids(router) == []
+
+
 def test_stream_chunks_have_generated_content_detects_text_and_non_text():
     from litellm.router import _stream_chunks_have_generated_content
     from litellm.types.utils import (