diff --git a/litellm/proxy/_types.py b/litellm/proxy/_types.py
index 57a7d860baa..fba968dd95d 100644
--- a/litellm/proxy/_types.py
+++ b/litellm/proxy/_types.py
@@ -2312,6 +2312,24 @@ class ConfigGeneralSettings(LiteLLMPydanticObjectBase):
         None,
         description="List of MCP server fields that must be filled in for a submission to pass standards checks (e.g. ['description', 'source_url', 'alias']).",
     )
+    disable_budget_reservation: Optional[bool] = Field(
+        None,
+        description=(
+            "If True, disables the optimistic per-request budget reservation "
+            "introduced in v1.84.0. "
+            "WARNING: This weakens hard budget enforcement. Without the reservation, "
+            "a burst of concurrent requests from a single key can each pass the "
+            "read-time spend check before any of them is charged, allowing a "
+            "configured budget to be exceeded under high concurrency. "
+            "Budgets are still evaluated on every request at read time, so "
+            "an already-exhausted budget is still rejected. "
+            "Enable only if your deployment is experiencing phantom "
+            "BudgetExceededError responses caused by leaked reservations "
+            "(see GitHub issue #27639). "
+            "A proxy-level WARNING is logged on every request while this flag "
+            "is active as a reminder that hard enforcement is relaxed."
+        ),
+    )
 
 
 class ConfigYAML(LiteLLMPydanticObjectBase):
diff --git a/litellm/proxy/auth/user_api_key_auth.py b/litellm/proxy/auth/user_api_key_auth.py
index a970e0ddee8..666c01562b5 100644
--- a/litellm/proxy/auth/user_api_key_auth.py
+++ b/litellm/proxy/auth/user_api_key_auth.py
@@ -2425,6 +2425,7 @@ async def _run_centralized_common_checks(  # noqa: PLR0915
         user_api_key_cache=user_api_key_cache,
         proxy_logging_obj=proxy_logging_obj,
         skip_budget_checks=skip_budget_checks,
+        general_settings=general_settings,
     )
 
 
@@ -2445,12 +2446,23 @@ async def _reserve_budget_after_common_checks(
     user_api_key_cache: UserApiKeyCache,
     proxy_logging_obj: ProxyLogging,
     skip_budget_checks: bool,
+    general_settings: dict,
     end_user_id: Optional[str] = None,
     end_user_object: Optional[LiteLLM_EndUserTable] = None,
 ) -> None:
     user_api_key_auth_obj.budget_reservation = None
     if skip_budget_checks:
         return
+    if general_settings.get("disable_budget_reservation") is True:
+        verbose_proxy_logger.warning(
+            "disable_budget_reservation is enabled: skipping optimistic budget "
+            "reservation. Budget enforcement is read-time only — concurrent "
+            "requests can each pass the spend check before their cost is recorded, "
+            "so a configured budget may be briefly exceeded under high concurrency. "
+            "Set disable_budget_reservation to False or remove it to restore "
+            "hard per-request budget enforcement."
+        )
+        return
 
     from litellm.proxy.spend_tracking.budget_reservation import (
         reserve_budget_for_request,
diff --git a/tests/test_litellm/proxy/auth/test_user_api_key_auth.py b/tests/test_litellm/proxy/auth/test_user_api_key_auth.py
index fa6cc8bed1b..0236646c796 100644
--- a/tests/test_litellm/proxy/auth/test_user_api_key_auth.py
+++ b/tests/test_litellm/proxy/auth/test_user_api_key_auth.py
@@ -112,11 +112,71 @@ async def test_should_clear_stale_budget_reservation_when_budget_checks_skip():
         user_api_key_cache=MagicMock(),
         proxy_logging_obj=MagicMock(),
         skip_budget_checks=True,
+        general_settings={},
     )
 
     assert user_api_key_auth_obj.budget_reservation is None
 
 
+@pytest.mark.asyncio
+async def test_disable_budget_reservation_skips_reservation():
+    """#27639: general_settings.disable_budget_reservation turns off the optimistic Redis
+    reservation so operators hit by phantom BudgetExceededError can opt out of it."""
+    user_api_key_auth_obj = UserAPIKeyAuth(token="test_token")
+
+    with patch(
+        "litellm.proxy.spend_tracking.budget_reservation.reserve_budget_for_request",
+        new=AsyncMock(return_value={"reserved_cost": 0.5, "entries": []}),
+    ) as mock_reserve:
+        await _reserve_budget_after_common_checks(
+            user_api_key_auth_obj=user_api_key_auth_obj,
+            request_data={"model": "gpt-4o"},
+            route="/v1/chat/completions",
+            llm_router=None,
+            team_object=None,
+            user_object=None,
+            prisma_client=None,
+            user_api_key_cache=MagicMock(),
+            proxy_logging_obj=MagicMock(),
+            skip_budget_checks=False,
+            general_settings={"disable_budget_reservation": True},
+        )
+
+    mock_reserve.assert_not_called()
+    assert user_api_key_auth_obj.budget_reservation is None
+
+
+@pytest.mark.asyncio
+async def test_budget_reservation_runs_when_not_disabled():
+    """Control for #27639: with the flag absent, the reservation still runs and is stored."""
+    user_api_key_auth_obj = UserAPIKeyAuth(token="test_token")
+    reservation = {
+        "reserved_cost": 0.5,
+        "entries": [{"counter_key": "spend:key:test_token"}],
+    }
+
+    with patch(
+        "litellm.proxy.spend_tracking.budget_reservation.reserve_budget_for_request",
+        new=AsyncMock(return_value=reservation),
+    ) as mock_reserve:
+        await _reserve_budget_after_common_checks(
+            user_api_key_auth_obj=user_api_key_auth_obj,
+            request_data={"model": "gpt-4o"},
+            route="/v1/chat/completions",
+            llm_router=None,
+            team_object=None,
+            user_object=None,
+            prisma_client=None,
+            user_api_key_cache=MagicMock(),
+            proxy_logging_obj=MagicMock(),
+            skip_budget_checks=False,
+            general_settings={},
+        )
+
+    mock_reserve.assert_awaited_once()
+    assert user_api_key_auth_obj.budget_reservation == reservation
+
+
 @pytest.mark.asyncio
 async def test_should_not_reuse_cached_key_object_for_request_state():
     key_cache = DualCache()